How many points do you get for each weapon type hit in Wipeout 2048 combat races?

Question: In Wipeout 2048, different weapons do different amounts of damage and earn you different amounts of points. Is there a list of how much damage/how many points each weapon type deals/earns?
Answer: Because the game is so new such charts don't seem to exsist yet, however there is a list of the weapons available in the game and thier damages in relation to eachother, "Heavy", "Light", "Mild", ect... This list can be found here. As for the points gained for each weapon used and precise damage inflicted you may have to wait it out for someone to make something up.


In lego mindstorm EV3, can you make the individual programs in a project into functions which can be used in the other programs?

Question: The bit where you program what the robot does using your computer. I'm making a project and I can't fit it all onto the screen, so if you can't do functions, is there any way to zoom out of the screen or something, so I can add more?
Answer: You can turn parts of your program into custom blocks by selecting them and choosing Tools -> My Block Builder from the menu. See https://www.youtube.com/watch?v=CApCoGQJRcU for a  tutorial on this feature.
Answer: In the MINDSTORMS EV3 software you can Zoom in and out to be able to view your complete program (there are magnifier icon and an 1:1 on the top right)
You can also create MyBlock ... select a group of block that do something specific like rotate till object detected - then click on Tool in the menu  - then on MyBlock Builder -- once a MyBlock is created they appear in the CYAN colored block tab below (the one on the far right) Now every time you want to seek for object just call that MyBlock
Answer: MyBlocks are the right way to go, but you can still end up with a program that is larger than will fit on your screen.
The canvas on which you create your programs can be scrolled left to right and up and down.
In MS Windows, hold the Alt key and click and drag on an empty space in the canvas. I don't know the equivalent key on a Mac, but I'm sure there is one. There should also be arrows you can click on at each edge of the screen if your program is not fitting in the window. If you have a scroll wheel on your mouse, you can also use that to scroll up and down.
Blocks need not be adjacent. Though not considered a best practice in EV3 programming, you can "snake" the program on screen by breaking code up into sections of blocks that will fit the width of the screen in consecutive rows. Non-adjacent blocks can be connected with a "Beam" by dragging from the exit point of one block to the entry point of the next block.
Answer: I'm not particularly familiar with Mindstorms EV3, but I know that NXT-G has the capability to call other programs from a main program using MyBlocks.
As far as I know, you can't zoom out of the EV3 and NXT programming environments, although in NXT-G there is an overall view you can get by switching the tabs in the lower left hand corner.


A word describing a 'part' of UK hospitals (departments?)

Question: What's the common name in usage in the UK for the part in the hospital that specializes in one field only (for example neurology)?
I saw two different names 'wing' and 'department'. But both are in non native English speakers country,  so I'm not sure what it reflexes and if it's correct also in context of medicine.
Comment: As a native Briton, "ward" is probably the one I've heard most, i.e. the maternity ward.
Comment: Are you asking about administrative divisions or physical areas? I know you're asking about the UK, so it's likely different there, but in the US, the "maternity department" would be the administrative aspect while the "maternity ward" would have the birthing rooms, nursery and such, and may be located on a single floor of the "north wing" of the hospital.
Answer: Wing and department are both acceptable.
"Wing" suggests a physical building that is next to the main part of the hosptial. Department suggests part of the organisational structure.
At my local hospital the buildings are named "Jubilee Centre",  "Canadian Wing", in which there are multiple "Departments", each has its own name. The Canadian wing houses the "Ross Tilley Ward",  "Day surgery" and the "Diet and Nutrition Department". It is possible for a building to house just one department. There is the "Burns and Reconstruction Department", which is both a building and a department.
There are many other words that could be used, and sometimes no word at all. One of the departments is called "Outpatients" another is called "Cardiology".
Comment: "Wing" is usually associated with part of a large building e.g. *The south wing of the museum houses dinosaur relics*. But the term is used in hospitals in the way you suggest. Sometimes you will see signs that say something like *cardiac wing*. It means that a whole section of the building is devoted to cardiac work.
Comment: @kandyman "wing" would still be acceptable in the remaining cases (either where a department took up a whole wing, or where several departments which were closely-enough-related that there was a term for them as a group took up a wing between them) but yes, in general "department" would be preferable.
Comment: @kandyman Yes, you can _imagine_ that situation. But, as someone who's worked with and been to hospitals, there are also plenty of places where the section that does e.g. emergency medicine is large enough to be its own, separate wing.
Comment: @NicHartley I'm not disputing that. But since there are also places where several departments are in one wing, I suggested that 'wing' might be too general a term and that 'department' might be more precise. Is it so hard to imagine the sentence "The radiology department is located in the south wing, next to the cardiology department" ? (or whatever departments are likely to be close together)
Comment: Surely 'wing' is too general. You could imagine a situation where there are several departments located in one wing of the building.
Comment: @kandyman Ah I see what you're saying -- it's definitely not _necessarily_ always the case. My point was that, quite frequently, the departments are all large enough to occupy their own "wing", and no one will be confused if you say "it's in the neonatal wing" -- sure, you might also have OB/GYN and prenatal in the same place, but people will understand that you meant specifically neonatal care, not the maternity ward as a whole. Normally I hear it just by the description, though -- radiology, cardiology, maternity, neonatal, etc., no additional words.
Comment: I guess it depends on the building and how the departments are housed within it.
Answer: A specialist section in a hospital is sometimes referred to as a unit as in "the maternity unit" or "the burns unit". Taking the definition from Collins:

any group or individual, esp when regarded as a basic element of a larger whole

So the maternity unit is the group within the hospital that deals specifically with maternity matters. You can see from the ngrams graph that "maternity ward" is more popular than "maternity unit", but that is more popular than "maternity department" (the order still holds if you switch the search to British English).
That said, it is also normal to simply omit the specifier (ward/department/unit) and say something like "Send these results through to cardiology".
If you attend hospital for a specific reason, you are very likely attending a "clinic". A clinic can be a specific building/location but it can also be a specific session. That is, a "baby clinic" can be held in a rural community centre, or it can be a specific part of a hospital building.
Comment: You are fairly safe with "unit". It can be anything from a button to a nuclear submarine.
Answer: Have you considered the word "ward"? That's how I've heard it mostly commonly referred to.
I am in the UK, if that helps.
Comment: I don't think ward us the term the OP is looking for. At least my understanding of the term "ward" is a room where people stay in the hospital and receive supportive care and some minor treatments. For more major treatments the patients will be transported to appropriate rooms (e.g. an operating theatre for surgery)
Comment: @PeterGreen "Ward" _can sometimes_ be used in the OP's way, but you are right that it isn't an ideal fit. Many hospitals will have some "General" wards, and a number of more special-purpose wards (e.g. "_Dementia ward_") where patients with a common complaint will be sent. However, there would often be a _department_ around that ward containing the people with the specialist skills to treat that class of patient.
Comment: @PeterGreen Also, some wards may have functions that are not related to specific "diseases". For example my nearest hospital has a group of "stepdown wards" with specialist staff to monitor patients for the first 24 or 48 hours of recovery after major surgery, irrespective of what was the *reason* for the surgery. Once it is clear that there are no high-risk complications developing, the patients are moved to the *departmental* wards dealing with their particular medical problem.
Answer: A wing is a large section of a building. It may contain multiple departments or wards but it's a term common to all buildings large enough to have them. It's not hospital specific.
A ward is a room where patients sleep in a hospital, it may or may not be bound to a specific department. A specialist ward may be, but many wards are general and will take patients from multiple departments
A department is probably the word you're looking for. This covers the likes of cardiology, physiotherapy, ophthalmology etc. They may have the word department in the name, but will often just be referred to by their 'ology' or sometimes as a unit.
Comment: @AlgyTaylor, children are special, they get more value for their choice of term :)
Comment: You can also have "Day" wards where patients wait or undergo low to mid-level medical  procedures, the patients there do not sleep over night. Although they may nap during the day, waiting can be boring.
Comment: The children's ward at our local hospital consists of several rooms, plus some supporting facilities (play room, social room, toilets, parent's room / kitchen, reception - I assume there's also a staff room somewhere).  Otherwise, this is spot on.
Answer: Generally 'department' relates to an administrative division which isn't necessarily located in one specific place while 'wing' tends to refer to a large area of a building or complex of buildings.
For example you might say that the cardiology department is based in the east wing of the hospital.
Of course if one part of the building is dedicated to one department it might well be called the cardiology (or whatever) wing.
There is also 'ward' which refers specifically to a (usually large) room or suite of rooms where patients are housed and/or treated.
You often also see the term 'unit' which tends to be used for a composite collection of staff, equipment, space and other resources intended to work together for a specific function or specialism. The implication often being that it is able to draw on a wide range of resources and coordinate them effectively.


Adding LetsEncrypt to Tomcat/Apache setup with mod_jk

Question: I have a Tomcat server running behind Apache using mod_jk (the AJP connector in Tomcat). I just tried adding SSL to this setup with LetsEncrypt, but certbot was unable to verify that I own the domain since it could not access the challenge files, as Apache was proxying all requests to Tomcat. How can I get a LetsEncrypt certificate for my server with my current setup?
Answer: You can use the JkUnMount directive to prevent a directory from being handled by Tomcat.   You can let the acme challenge directory be handled by Apache.
The configuration for that might look like:
JkUnMount /.well-known/acme-challenge/* *

The final * should unmount it from all configured workers.   If you want to exclude that directory for just one of several workers, you can use the worker name instead.
I prefer to use reverse proxy over mod_jk with Tomcat, mostly because I find it easier to debug.  Excluding directories is similar with reverse proxy.   The configuration is:
ProxyPass /.well-known/acme-challenge/ !

Comment: Your solution almost worked, I had to change `JkUnMount /.well-known/acme-challenge/ *` to `JkUnMount /.well-known/acme-challenge/* *`. Basically, it was routing only that specific page to Apache, not the entire directory. I have edited the answer to reflect it, and will accept it when that change is made.
Comment: It's been a while since I set this up, where is the config file I need to put this in located?
Comment: You should already have an Apache configuration file with a `JkMount` in it, it should go in that same file.     Where that file is will depend on your OS distribution and whether or not you are using virtual hosts.   On my Ubuntu Linux system it would be under `/etc/apache2/sites-available/example.com.conf` for my example.com website.
Comment: I edited that in for you.   Thank you for coming back with corrections.


Do I really need resistors when controlling LEDs with Arduino?

Question: I'm just trying out Arduino Uno for the first time with 2 blinking LEDs on a breadboard. All the tutorials on the Internet seem to use a resistor. I do know the function of resistors, but does it really matter here? These LEDs are working just fine without a resistor.
Comment: That depends if you want your LED to melt or not. If you don't mind, omit the resistor. :-) .
Answer: Naughty! :-). If they say to use a resistor there's a good reason for that! Switch it off, NOW!
The resistor is there to limit the LED's current. If you omit it the current limiting has to come from the Arduino's output, and it will not like it. How do you find out what the resistor needs to be? You do know Ohm's Law? If you don't, write it down in big letters:
\$ V = I \cdot R \$
Voltage equals current times resistance. Or you could say
\$ R = \dfrac{V}{I} \$
It's the same thing. The voltage you know: Arduino runs at 5V. But not all that will go over the resistor. The LED also has a voltage drop, typically around 2V for a red LED. So there remains 3V for the resistor. A typical indicator LED will have a nominal current of 20mA, then
\$ R = \dfrac{5V - 2V}{20mA} = 150\Omega \$
The Arduino Uno uses the ATmega328 microcontroller. The datasheet says that the current for any I/O pin shouldn't exceed 40mA, what's commonly known as Absolute Maximum Ratings. Since you don't have anything to limit the current there's only the (low!) resistance of the output transistor. The current may so well be higher than 40mA, and your microcontroller will suffer damage.
edit
The following graph from the ATmega's datasheet shows what will happen if you drive the LED without current limiting resistor:

Without load the output voltage is 5V as expected. But the higher the current drawn the lower that output voltage will be, it will drop about 100mV for every extra 4mA load. That's an internal resistance of 25\$\Omega\$. Then
\$ I = \dfrac{5V - 2V}{25\Omega} = 120mA \$
The graph doesn't go that far, the resistance will rise with temperature, but the current will remain very high. Remember that the datasheet gave 40mA as Absolute Maximum Rating. You have three times that. This will definitely damage the I/O port if you do this for a long time. And probably the LED as well. A 20mA indicator LED will often have 30mA as Absolute Maximum Rating.
Comment: I lost count on how many times I have written that calculation in answers here. I should have a script for it :-)
Comment: @JohnR.Strohm I wouldn't call it an industry standard.
Comment: Not really. 3 and you won't get any light, because 3 x 2V > 5V, and 2 you'll have the same problem, only 1V drop instead of 3V. I added to my answer to switch it off now!
Comment: LEDs are designed to be operated at a certain max current. Voltage driving them means current is uncontrolled. Ports are made to provide a certain max current. Shorting them or overloading them MAY leasd to pin or whole IC destruction or just cause subtle operating problems. Or not.
Comment: @JohnR.Strohm - Like m.Alin says that's not an industry standard. Digikey lists 10mA nominal LEDs ranging from 0.4mcd to 1000 mcd, and 20mA LEDs ranging from 0.1 mcd to 54000 mcd. There's no line in it, and no guarantee that your 10mA LED will be good visible. Red LED voltage typically [ranges](http://www.oksolar.com/led/led_color_chart.htm) from 1.8V to 2.2V. For a red LED 1.6V is exceptionally low.
Comment: There are many answers on this site and many elsewhere that explain WHY a series resistor is needed and how to calculate it. There are many things in life that seem OK to do despite being told otherwise and whcih turn out to have bad consequences in many cases. This is one such. The overall combination may not be too hard on the LED or the Arduino. Or may be. LED lifetime may be shortened. Arduino lifetime may suddenly not be. But it MAY work OK for long enough for you to be happy. Or not.
Comment: I don't have any resistor at the moment. For the time being, can I just hook up a bunch of LEDs in series to mimic the resistor?
Comment: A red LED drops 1.7 V, and 10 mA is plenty of current for indoor casual use.  The industry-standard current-limiting resistor for a red LED and a 5V supply is 330 ohms.
Answer: 40plot,
I have to say that driving an LED without a resistor is NOT RECOMMENDED unless you know what you are doing. However, if you understand how an LED behaves, you can drive it without a resistor safely. As a matter of fact, driving an LED without a current limiting resistor is often better.
Why would you drive an LED without a resistor? Simple, to make your circuit more energy efficient.
Should you drive your LED with PWM set to a constant duty cycle (i.e. 5V PWM at 34% duty cycle to achieve an average voltage of 1.7V)?
Yes and no. Using PWM can work just as well as applying a specific voltage (if you are careful), but there are better ways. Things to worry about when taking the PWM approach.

The frequency of the PWM is important. When using PWM in this scenario, you are relying on the ability of your circuit's components to temporarily handle high currents. Your biggest concerns will be how the LED handles a temporary high current and how the output circuit of your chip can handle a temporarily high current. If that info is not specified in the datasheet, then the datasheet authors were lazy. BUT!!! If that information is specced on the datasheet, then you can safely take advantage of it. For example, the LED I have next to me has a max current rating of 40mA. However, it also has a "Peak Forward Current" rating of 200mA, with a note that the current cannot remain at 200mA for longer than 10us. Soooo... I can drive the LED with 1.7V (the LEDs typical forward voltage from the datasheet). With a duty cycle of 34% and a power supply of 5V (34% of 5V = 1.7V) will produce an average voltage of 1.7V, I just need to ensure my PWM on time is 10us or less. During the on-time, the current through the LED will likely rise to around 58mA (58mA = typical current draw at 1.7V of my diode divided by 34%). 58mA exceed my LEDs steady current maximum of 40mA by 18 mA. Finally... I would need a PWM frequency 33.3kHz or greater to safely drive my LED (33.3kHz = The inverse of [10us ON time divided by 34% to get the PWM period]). In REALITY, I could safely use PWM to power my LED with a slower PWM frequency. The reason is this: Datasheets usually don't specify all of the valid operating scenarios of a component. They don't spec those scenarios because the vendor doesn't want to invest time in speccing and supporting the use of their component for corner use cases. For instance, with my LED, if I can operate the LED at 40mA for ever (40mA is the steady current max rating) and I can operate the LED at 200mA for 10us. Then, I can be 99.99999% certain that I can safely operate the LED at 100mA for some period longer than 10us, probably close to 20us.

NOTE: All components can safely handle temporary current spikes above their maximum ratings as long as the duration of the current spikes are SMALL ENOUGH. Some components will be more forgiving than others, and, if you are lucky, the component's datasheet will specify how well it can handle spikes of current.

The voltage of your PWM is important. I'll demonstrate my point by example instead of via explanation. If we use the LED I was referring to earlier, we know that 34% duty cycle, at 33.3kHz, at 5V is safe. However, if our voltage was 12V, we would have to rework our calculations to keep the same amount of current flowing through the LED. Our duty cycle would need to drop to 14.167% (1.7V divided by 12V) and our minimum PWM frequency would decrease to 14.285kHz (the inverse of [10us divided by 14.167%]). HOWEVER!, this is cause for concern. In the 5V scenario we are applying 5V for 10us and in the 12V scenario we are applying 12V for 10us. We more than doubled the voltage during that 10us, there has to be some consequences. And yes, there are! My LED datasheet does not give me the data necessary to know how high of a voltage I can use for 10us before I damage my LED. Surely 1000V for 10us will fry my LED. But, how do I know if 5V at 10us will fry my LED? or 12V for 10us? If there is not a spec for it then you are taking a risk. So... 5V for 10us is risky, but most likely safe.

NOTE: You can add a capacitor to the circuit to average out the PWM and make this issue go away.

You need to be cognisant of the capabilities of the output pin you have connected your LED too. The most important parameter will be the maximum output current. For the Arduino Uno, I believe that is 40mA. You should choose a PWM duty cycle whose average voltage keeps the current going through the LED below 40mA. In order to know what voltages will produce that much current, you need to look at the LEDs IV curve (current vs. voltage plot). For a typical LED a voltage between 0.7V (typical minimum voltage needed to emit light from the LED) and 1.25V will almost certainly be safe. Why is 1.25V probably safe? Well, most LEDs won't exceed 40mA at 1.25V, even without a current limiting resistor. Another thing helping protect someone in the case they apply too much voltage, is that the digital output circuit of the Arduino will have its own output impedance, that output impedance will be low, but even a 20 ohm output impedance would provide a non-negligible amount of protection. The arduino uno has a digital output impedance around 250 ohms. Long story short, if you drove an LED using PWM at 1.0V at a high frequency, for a typical LED, there is zero chance you will damage your digital output on an Arduino Uno.

The PWM approach drives the LED in an open loop fashion (and so does using 1.7V power supply without PWM). You are applying an average voltage to the LED that is just the right value to turn ON the LED but not high enough to damage the LED. Unfortunately, the range of voltage from ON (and bright enough to see) to damaged LED is very small (That range on my LED is about 0.7V). There are various reasons why the 1.7V you think you are applying won't always be 1.7V...

a. Changes in the ambient temp. What if you had a motor driver, voltage regulator, etc. in a closed box that also contained the LED. It wouldn't be uncommon for those other components to raise the ambient temp inside the enclosure from 25C to 50C. This rise in temperature WILL change the behavior of your LED, your voltage regulator, etc.. Your once safe 1.7V will no longer be 1.7V and your LED that used to fry at 2.5V will now fry at 2.2V.
b. Changes in your supply voltage. What if your supply was a battery. As the battery drains, the voltage drops considerably. What if you designed your circuit to work well with a slightly used 9V battery, but then you added a fresh 9V battery. Brand new 9V lead acid batteries typically have an actual voltage of 9.5V. Depending on the circuit that is providing the 5V used for the PWM, that additional 0.5V could up your 5V PWM to 5.3V. What if you were using a rechargeable battery? They have an even larger range of voltages throughout their entire discharge cycle.
c. There are other scenarios, like induced current from EMI (motors will do this).
Having a current limiting resistor saves you from many of these problems.
Using PWM to drive an LED is not a very good solution, is there a better way that doesn't require a current limiting resistor?
Yes! Do what they do in LED lightbulbs for your home. Drive the LED with a current controller. Set the current controller to drive the current your LED is rated for.
With the proper current controller, can be increased dramatically, and you can safely drive the LED without worrying about most of the issues involved with open loop driving an LED.
The downside: You need a current controller, and you have upped the complexity of the circuit by 10x. Don't be discouraged though. You can buy current controller ICs, LED driver ICs, or make your own current controlled boost converter. It isn't that hard. Take some time out of your busy schedule and learn about boost and buck converters. Learn about switching power supplies. They are what powers your computer and they are extremely energy efficient. Then, either build one from scratch, or buy an inexpensive IC to do most of the work for you.
Of course, as with all electronic designs, there are always more things you can do to make your circuit better. Check out figure 3 in the following PDF to see how complex even a household LED lightbulb can be these days...
http://www.littelfuse.com/~/media/electronics/design_guides/led_protectors/littelfuse_led_lighting_design_guide.pdf.pdf
In summary:
You have to decide for yourself how much risk you are willing to take with your circuit. Using 5V PWM to drive your LED will likely work just fine (especially if you add a capacitor to smooth out the PWM square wave and max out your PWM frequency). Don't be too afraid to push your electronics outside of their usual operating conditions, just, be informed when you do it, know the risks you are taking.
Enjoy!
FYI: I am surprised by how many people immediately jump to the answer, "YOU MUST USE A CURRENT LIMITING RESISTOR". That is well intentioned, but overly safe advice.
Ort
Comment: I'm surprised to see how many people considered this answer to be valuable design advice. Driving a LED without current limiting using PWM is just as bad for the LED as what the OP is doing, plus it will generate EMI and VCC ripple like crasy.
Comment: @DmitryGrigoryev, I realized I completely failed to address the maximum output current of the digital output. I added a new bullet to cover that.

PWM DOES allow someone to safely drive an LED without a current limiting resistor.

EMI and VCC ripple are created anytime you drive loads with a digital signal, but this is common (ex. H Bridge, Boost Converter, Hobby Servo Control, etc.) and is hardly a reason to avoid PWM. Reasonable solutions exist to handle EMI and VCC ripple if needed. Most people won't care about the small amounts of EMI and VCC ripple created by driving an LED with PWM.
Answer: You can use the builtin pullup resistors as suggested here:

The pullup resistors provide enough current to dimly light an LED connected to a pin that has been configured as an input.
Comment: @MenelaosVergis it is safe to do this when the pin in INPUT_PULLUP mode, it is not safe to do with a pin in OUTPUT mode. This is clear in the docs if you read both sections (and from the quoted excerpt in this answer).
Comment: Not for LEDs, this can be used for buttons but for LEDs there is a danger of breaking the output
Comment: if it is not safe why the official docs say so? (I've also tried myself and it worked as described.)
Comment: Please read down to `OUTPUT`, it mentions that it needs a series resistor: "This is enough current to brightly light up an LED (don't forget the series resistor), or run many sensors, for example, but not enough current to run most relays, solenoids, or motors. "
Answer: The short answer is, yes and no, it depends on your arduino and it depends on the color of your led. For instance, a 3.3V board does not require a resistor in series with a small green LED, because the forward voltage of the LED is pretty high, see this. The internal resistance is around 25 Ohm, take (3.3 - 3)/25 = 12mA, so this is still fine, you should not go across the maximum current per pin which is 40mA for the 328p atmel processor used on the UNO boards (unless you use a derivative of the 328p where it could be a different story). However, for an arduino running at 5V trouble will arise with an infrared LED which has a much lower forward voltage, typically 1.2V, (5-1.2)/25 = 150mA, and this is definitely too much, so use a current limiter such as a resistor to drive those type of LEDs. Pin 13 on the Arduino boards (or another pin on variants) already has a led and a resistor in series. Also, the power supply to the board has a maximum rating, typically 200mA, and you must stay under this level, and you can not draw more than a certain amount of mA per group of pins, this is explained here. If you want to drive many LEDs, then consider to use a matrix LED driver that does the multiplexing for you, see for instance my youtube area where I demonstrate the MAX7219CNG driver. But also Arduino Uno's can do the multiplexing for you, see my IR thermometer with 4 seven segment LEDs on youtube. Happy hacking.
Answer: The answer from stevenvh explains what you need to do, but you also must calculate the power dissipation across the LED so that you don't burn out the voltage drop resistor. For example, if the supply voltage is 5V and the forward voltage of the resistor is 1.0V then you'll be dropping 4V. Using a 220 ohm resistor will result in a current of (I=V/R) of 18mA and a power dissipation (P=IV) of 72mW.
0402 Imperial sized (1005 Metric) resistor are generally 1/16W, which is 62.5mW. So in this case this would not work; it would overheat the resistor and shorten its operating life. So you would need to change to an 0402 resistor with 1/10W rating, or a larger 0603 resistor.
Whenever you do calculations such as these, add them to the schematic, so that the reviewer can double-check your work easily.
Note that forward voltage (and hence, resistor value) is a function of the LED, and different colors of LEDs will have different values. Blue LEDs in particular have a high forward voltage (~3.0V typ). So if you are trying to get four different LEDs to have the same brightness then you'll need to repeat the calculations for each LED. To really do it right, look at the optical characteristics of each LED at its rated current, and adjust accordingly.
Answer: YES! It can be done.
Even though what has been said is correct.... there's another way. A more energy efficient way of driving LEDs with 5v.
This is a bit undocumented and it is unknown if the solution will wear off the LEDs, but it can be done. I'm actually doing it.
Using PWM by hardware: Here's an example:
#include <avr/io.h>
#include <util/delay.h>
void pwm_init()
{
    // initialize TCCR0 as per requirement, say as follows
    TCCR0 |= (1<<WGM00)|(1<<COM01)|(1<<WGM01)|(1<<CS00);

    // make sure to make OC0 pin (pin PB3 for atmega32) as output pin
    DDRB |= (1<<PB3);
}

void main()
{
    uint8_t duty;
    duty = 1;       // duty cycle = 0.39% of the time (depends on the oscillator.)

    // initialize timer in PWM mode
    pwm_init();

    // run forever
    while(1)
    {
        OCR0 = duty;
    }
}

PWM can also be simulated using software and the avrs timers. You can find an example inside the lufa library, called LEDNotifier.c.
My conclusion: It is possible to drive a led on 5V.
PROS: No need for a resistor. Some energy saving too (~50%)
CONS: I don't know if the component is stressed and about its lifetime reduction.
There's a guy who also did this experiment at Stanford and posted some info at his site.
Comment: This just doesn't seem like a good idea.  You're probably sourcing more current from the controller than it's rated for, even if you're not doing it for very long.
Comment: As said this is undocummented. The Arduino UNO output IO can drive around 40ma-50ma. That is constant. I can for sure handle very short pulses with more current. Please have a look at [wikipedia](http://en.wikipedia.org/wiki/LED_circuit#Pulsed_LED_operation).
Comment: @Ivanzinho  it's generally not a good idea to do stuff that's undocumented in the data sheet, for a variety of reasons.  Undocumented behavior is not guaranteed -- not for existing chips, not for any engineering changes to the chips.  In this case, not for every LED type.  We tend to design to what the data sheets tell us, because doing that will yield designs that reliably work, which generally uses less resources (time, money, aggravation) than doing things in a marginal way that will work in some cases and not in others.
Comment: In the Arduino environment, one can get PWM more easily with `analogWrite()` on the corresponding pin. Still not sure that this is a good idea, but at least for IR LEDs, it's common for the data sheets to allow considerably higher peak currents for duty cycles less than 100%.
Comment: I'm pretty sure energy savings will be **negative** compared to a resistor-based solution, because the LED efficiency becomes lower as the current increases.
Comment: @ScottSeidman Why isn't that a good idea? just because most people believe it is not recommended to use an LED without a resistor? this answer proved investigation, a very respectful source like Stanford University and from what I can experiment, it works. I definitely love downvoted posts because they tell those who voted down are just biased. What a paradox...


Elasticity of Substitution (CES)

Question: This appears to be a nice forum. I just registered since I have a question...
I have a CES aggregator-function
$$
f(c,q) = (r c^{a} + b q^{a})^{1/a}.
$$
It is postulated that it can be rewritten as
$$
f(c,q) = c \left[ r + b \left( \frac{q}{c} \right)^{a} \right]^{\frac{1}{a}}.
$$
I don't see how, and neither does Maple. Can anyone help? I'm confused and I don't know what I have missed. I bet that there is insufficient information.
Thanks for the edit; I will write the problem in the correct language now.
Comment: This is valid if $c > 0$.
Comment: It was enough! thank you!
Answer: Hint: $c = (c^a) ^{\frac {1}{a}}$ if $c > 0$.

Elasticity of Substitution (CES)


How do I remove bitterness from zucchini?

Question: I have thin-sliced zucchini and steamed it for use in my soup. One of the zucchini turns out to be bitter. I have saved half of it for use on the next day. Is there a cooking method that will remove the bitterness from that same zucchini, preferably with a presentation that is similar to steaming?
Answer:
Mild bitterness in zucchini, like that found in cucumber, may be
  result from environmental factors such as high temperature, low
  moisture, low soil nutrients, etc. The bitterness is caused by
  compounds called cucurbitacins. There is also a rare condition which
  can cause extreme bitterness in zucchini. A compound called
  Cucurbitacin E is found in wild species of squash, but is extremely
  rare in cultivated species. (Univ. of Arizona)

Having bounced around to the various other sites I basically had what I've heard anecdotally before confirmed; salt and lay out to drain liquid. I suppose the implication is that by dessicating the zuke a little bit, it can remove the Cucurbitacin (which is a steroid developed to ward off herbivores). Putting the zukes in salt water is also recommended by some.
It appears that people can become ill from incredibly bitter zukes, so be wary of eating extremely bitter ones. In this case, you may want to follow the wisdom of your taste buds and drop the zucchini in the trash.
Comment: Yep. There are known cases where people ate bitter squashes from plants that accidentally got cross pollinated by other cucurbitae species, and poisoned themselves.
Answer: I learned a nice tip from a Sri Lankan friend which seems to make them taste less bitter:
1) Chop off the stem end of the courgette with a sharp knife (not serrated)
2) Press the stub / courgette surfaces back together and rotate the stub end against the courgette continually for a couple of minutes.
3) As you do so you will find a thick, white substance leeches out of the courgette through the cut end (it might continue for half a minute or for a few minutes)
4) Once it's stopped leeching out (or you're bored!) wipe off the white stuff and discard the stub.
5) Continue to prepare / cook / eat as planned.  It should taste better!
Comment: Best explained by a video.
Comment: This was the best tip - rubbing the stem end against the courgette, I do the same with cucumbers but thank you for this tip which I will use on my courgettes also.
Comment: I don't understand step 2. What exactly is the stub, and what are the "courgette surfaces"? Do I rub the stem-end all over the courgette, or just in one spot?
Comment: Your Sri Lankan friend was right. Here in India we do the exact same thing for cucumbers, which is from the same family as zuchinni. Taking out the white stuff makes it less bitter. In addition, before rubbing the two surfaces together, you should lightly poke the two surfaces three to four times with your knife. This helps in taking out the white stuff better.
Answer: When I cook zucchuni I slightly scrape the skin with help of a knife to reduce the bitterness.
Also for some meals prepared by mashing, I peel the skin off.
Comment: After experimenting, removing the skin is found to be most effective in removing the bitter taste. (This is also mentioned in one of the links in @mfg's [answer](http://cooking.stackexchange.com/a/19596/6924). Cooking in water removes some, but is not as effective as removing the skin. Salt only masks the taste, but is ineffective for zucchinis that are particularly bitter.
Comment: @rwong Salt does not simply "mask the taste". It also reduces perceived bitterness. You can confirm this yourself with any number of simple experiments. Try adding a small pinch of salt to your coffee. Bitterness is reduced without any noticeable saltiness.
Answer: Also, salt masks bitter flavors; it's one of the reasons that, for example, bacon goes so well with cooked greens.
Answer: Cutting off the tip of the zucchini with the stem part and rubbing the two cut ends together, is also what a Californian friend of mine recommended to me years ago. But after rubbing and letting the white liquid come out, I cut another slice of the zucchini off, to be sure none of the white liquid is still on there. This method works most of the times for the bitterness to be gone. From personal experience though, I have noticed that whenever the peel is very thick and dark it is best to peel the zucchini, because then its the only way to get rid of the bitterness, which goes  as well for cucumbers.
Answer: My mom always taught me to rub the two cut surfaces of the zucchini together to get the bitterness out. I also do this for cucumbers. (https://i.stack.imgur.com/KOq5E.jpg)
Answer: It's called milking the vegetables. It's not milk, but looks like it. I milk my zucchini and squash. Then salt it, wait a few minutes, then rinse. Now I'm free to do what I want with it. Milking helps, also salting.
Comment: Did you intend to post this as a comment on anja's answer, rather than as a separate answer?


What meaning does やって add to the following main verbs?

Question: What meaning does the prefix やって give to the main verbs?
Is it a constant one can do with some other verbs as well?
For example
やって来る
やって見る
やって退ける
やって行く
Comment: Okay, I guess maybe the question could be reworded to "what does やる mean in these various expressions that use it" or something like that, since やる does have a lot of definitions. The framing of the question initially confused me. The auxiliaries that attach to やる all basically have their typical meanings as auxiliaries.
Comment: What do you mean by やって being the prefix to the main verb. In something like やってみる clearly みる is an auxiliary verb to the main verb やる, no?
Comment: Sorry for the confusion and I appreciate your suggestion in rewording the question.
Answer: やって is basically just the te-form of the verb やる, which has various meanings. As @Leebo points out, most expressions in the form やって + another verb can be understood using the ordinary grammar of subsidiary verbs such as (て)みる or (て)くる.

やってみる: to try doing (it). See: What is the difference between "verb+て+みる" and "verb+(よ)う+とする"?
やっていく: to do something and go; to do something before leaving. See: Difference between -ていく and -てくる

宿題をやっていく。
  I'll leave home after finishing my homework.

やってくる: to have been doing something over time; to keep doing something up until now; to do something before coming. See: Difference between -ていく and -てくる

この仕事を3年やってきた。
  I have been doing this job for 3 years.

...and so on and on. In the examples above, やって means just "to do".
However, there are a few verbs that have (additional) fixed meanings:

やってくる: to come along; to come all the way

サンタクロースが町にやってくる。
  Santa Claus is coming to town.

やってのける: to succeed in (a difficult thing)
やっていく: to make a living; to make ends meet

These やって are still loosely related to the original meaning of やる; for example, I think やって in やってくる originally means "to bring/send". But it's probably best to memorize these as special cases. Aside from them, you can use the standard grammar to make sense of the phrase.
Comment: At first glance, I would have put やってのける in the first category, of just a て form and an auxiliary. But I don't know if there's some nuance between the idea of やって + のける and やってのける that I'm just unaware of.
Comment: @Leebo Ah, you're right, we can say 言ってのける or 笑ってのける, so it can be categorized as a subsidiary verb (although there is some restriction)! I'll edit the answer later.
Comment: Thanks a lot!

It looks like やって simply means "to do" as the te-form of the verb やる except that of やってくる and やっていく.

While やっていく means "to make a living; to make ends meet", it could also mean "to do and then go" in view of the example of 宿題をやっていく.

Could やってくる mean "to do and then come" too?  I'm not sure how it gets to mean "to come along or to come all the way".  I would just take it as an idiom.


Is MS Access on-topic?

Question: I thought about asking a question regarding triggers in MS Access but wasn't sure if that belonged here or not.
Answer: I believe MS Access is on-topic as long as it is not about using the UI of Access.  If you have a question about designing a DB in Access that would be fine.
I remember in college (I sound so old) that the first week or two of my first db class were taught using Access to get some of the basic concepts down before we dived into MySQL.
Besides dba.stackexchange.com is for all databases, not just the big three.  There are some small businesses/projects that depend on MS Access.
Comment: I think the key around here is not to get hung up on the word DBA
Comment: I believe so, there are small companies that rely on MS Access for their client database and product info and whoever admins that data is their DBA. Granted I believe most people on the *.stackexchange.com network don't think much of that, but it still exists)
Comment: I think the key is that its DBA.stackexchange - is there such a thing as an MS Access DBA?
Comment: Agree with the others. Access is a DB system and demands managing. Think network access, access control, replication, fallback, migrations etc.. even it's file based.
Comment: /@Sebastian: Fair enough.
Answer: On-topic.

Microsoft Office Access, previously
  known as Microsoft Access, is a
  pseudo-relational database management
  system from Microsoft that combines
  the relational Microsoft Jet Database
  Engine with a graphical user interface
  and software-development tools.

Source: Wikipedia
Just think about it, it has tables, triggers, views, etc. and you can use SQL on it. Sure, it's not a full-fledged, powerful DBMS, but as long as the questions are related to those database elements, of course it is on-topic.
Answer: on-topic. Just like a VBA question isn't really off-topic on SO. Just because we don't encourage the thought of Access as a "real" db it doesn't mean that it isn't valid in some limited areas.
Answer: I think they are on-topic. Here are two questions I asked on SO that now, I would ask here. I don't think they would have been on-topic for SU as @bigown suggests.
Help with Query design in MS-Access
How to make a drop down list (list box)in an MS Acess Query with values from two different tables.
Answer: Even though putting the words Access and database together makes me want to puke I'd have to agree that it's on topic. If nothing else maybe we can help a few people with the step up to a real database platform. :)
Answer: Off-topic and the OP's non-meta question is a good example of why.
As @Gan's answer quotes:

combines the [pseudo-]relational Microsoft Jet Database Engine with a
  graphical user interface and software-development tools

I would say that questions about the Microsoft Jet Database Engine are on-topic but those about the graphical user interface and other tools are off-topic (and on-topic for SU). The problem is that even experienced users of Access have problems seeing where the boundary between the two lies. For example, a simple question about a Jet SQL query may get a SQL answer which resembles a Jet SQL query but because it includes the NZ() function it will error unless run within the Access user interface.
Going back to the OP's non-meta question about triggers, Access triggers are a feature of the Access user interface (or is it the other tools?) and are not part of Jet. The rule of thumb should be: can I create a trigger using SQL DML? The answer in this case is no, I need the Access UI/tool for this, therefore is off-topic for DBA.

Is MS Access on-topic?


How is a glider recovered and returned to an airport after landing in a field?

Question: If a glider has a safe normal landing in a cow pasture or other farm field and has no damage, how is it typically recovered to be returned to an airport for a future flight?  Are the wings designed for quick/easy removal and the wings and fuselage are just hand-carried to a long trailer waiting on a nearby road?  That's kind of how it's done (I believe) with balloon landings, but not sure what the process is for gliders.
Comment: @TylerDurden For XC flights field landings are very common. Yesterday eleven gliders took off at my airfield and seven of them had field landings. We don't plan to be in airport range but in "field range". In regions like the Alps there are even "field landing catalogs" which people use to plan their flights.
Comment: @TylerDurden: there are of course places where outlandings are complicated, and maybe New England is one of those. But in the vast majority of places where gliders are flown, outlandings are possible, and common. Definitely not rare. I have personally outlanded many times, and have done even more retrieves of other pilots; both in club flying, and in competitions. Of my 30+ outlandings, as far as I can remember only one was at an airport.
Comment: FYI Usually glider pilots plan their flights so that they are always in range of an airport. Field landings are pretty rare.
Comment: @Gypaets Maybe in rural Switzerland that works, but landing outfield in New England 70% of the time would be a life-limiting course of action.
Answer: All gliders can be disassembled to take off wings and elevator.
For each glider type there is a car trailer into which the glider fits after disassembling.

Here you see the "reverse" process (assembly):
Comment: [Even _Matchbox_ gliders can be recovered](https://www.toymart.com/photos/77150473_98009496_88268056.jpg).
Comment: In our club, before every cross-country flight, a "pick-up" party is appointed and "paid" in the local bar  if the service is needed
Answer: Trailering is the most common, however at some of our club's select off airport spots, we had landowner's permission, and I would land the Pawnee and the crew with the trailer would show up for a launch.  It saves the time of disassembly and reassembly.  But for most glider pilots the norm for an off-airport landing is to trailer the craft to the airport.


What kind of technologies are available for sending text messages?

Question: I'm looking into sending regular automated text-messages to a list of subscribed users. Having played with Windows Mobile devices, I could easily implement this using the compact .Net framework + a device hooked up to usb and send the messages through this. I would like to explore other solutions like having a server or something similar to do this. I just have no idea what is involved in such a system.
Answer: It really all depends on how many text messages you intend to send and how critical it is that the message arrives on time (and, actually arrives).
SMS Aggregators
For larger volume and good reliability, you will want to go with an SMS aggregator.  These aggregators have web service API's (or SMPP) that you can use to send your message and find out whether your message was delivered over time.  Some examples of aggregators with whom I have experience are Air2Web, mBlox, etc.
The nice thing about working with an aggregator is that they can guide you through what it takes to send effective messages.  For example, if you want your own, distinct, shortcode they can navigate the process with the carriers to secure that shortcode.
They can also make sure that you are in compliance with any rules regarding using SMS.  Carriers will flat shut you off if you don't respect the use of SMS and only use SMS within the bounds of what you agreed to when you started to use the aggregator.  If you overstep your bounds, they have the aggregator relationships to prevent any service interruptions.
You'll pay per message and may have a baseline service fee.  All if this is determined by your volume.
SMTP to SMS
If you want an unreliable, low-rent solution to a low number of known addresses, you can use an SMTP to SMS solution.  In this case you simply find out the mobile provider for the recipient and use their mobile provider's e-mail scheme to send the message.  An example of this is email@example.com.
In this scenario, you send the message and it is gone and you hope that it gets there.  You really don't know if it is making it.  Also, some providers limit how messages come in via their SMTP to SMS gateway to limit SMS spam.
But, that scenario is the very easiest to use from virtually any programming language.  There are a million C# examples of how to send e-mail and this way would be no different.
This is the most cost-effective solution (i.e. free) until you get a large volume of messages.  When you start doing too much of this, the carriers might step in when they find that you are sending a ton of messages through their SMTP to SMS gateway.
Effective Texting
In many cases you have to make sure that recipients have properly opted-in to your service.  This is only a big deal if your texts are going to a really large population.
You'll want to remember that text messages are short (keep it to less than 140 to 160 characters).  When you program things you'll want to bake that in or you might accidentally send multipart messages.
Don't forget that you will want to make sure that your recipients realize they might have to pay for the incoming text messages.  In a world of unlimited text plans this is less and less of a concern.
Comment: Anyone that is interested in the SMTP to SMS option I found a page that has a nice list. http://tips.slaw.ca/2011/technology/send-a-text-message-to-a-mobile-phone-via-email/   (Canada/US only)
Answer: You can usually get an account with an sms service provider and send messages using an API (SOAP, resful http, smpp ....)
A google search for sms service provider yeilds many results with varying costs.
Here is an informative article How to Choose an SMS Service Provider
Answer: I use AQL who provide gateways to send SMS messages, voice push messages, inbound SMS -> HTTP POST gateways and other stuff.
For Perl there's my SMS::AQL module to interface with them; whipping up something in C# should be pretty easy.
Answer: You could always try a third-party gateway service for this. Somebody like clickatell provide a number of services and APIs to make this work in a variety of countries. This isn't an ad! I only used their services for a technology pilot. There are quite a few of these around.
Answer: Another technology to send sms messages is text2land.com text to speech technology to send sms messages to landline phones.


How do I get the feature importace for a MLPClassifier?

Question: I use the MLPClassifier from scikit learn. I have about 20 features. Is there a scikit method to get the feature importance? I found
clf.feature_importances_
but it seems that it only exists for decision trees.
Answer: The short answer is that there is not a method in scikit-learn to obtain MLP feature importance - you're coming up against the classic problem of interpreting how model weights contribute towards classification decisions.
However, there are a couple of great python libraries out there that aim to address this problem - LIME, ELI5 and Yellowbrick:
LIME (or Local Interpretable Model-agnostic Explanations, blog post here, arxiv paper here) which "explains the predictions of any classifier in an interpretable and faithful manner, by learning an interpretable model locally around the prediction", or basically to explain model outputs by approximating the results of a classifier with a model that can be explained. It's on pypi and they have examples on their github page - very straightforward to get started.
ELI5 (or Explain Like I'm 5) is a "Python library which allows to visualize and debug various Machine Learning models using unified API". Although not all scikit-learn integration is present when using ELI5 on an MLP, Permutation Importance is a method that "...provides a way to compute feature importances for any black-box estimator by measuring how score decreases when a feature is not available", which saves you from trying to implement it yourself.
Yellowbrick is "a suite of visual diagnostic tools called "Visualizers" that extend the Scikit-Learn API to allow human steering of the model selection process" and it's designed to feel familiar to scikit-learn users. Compared to the other two libraries here it doesn't offer as much in the way for diagnosing feature importance, but it's still worth mentioning for more general use cases.
Answer: The Multi-Layer Perceptron does not have an intrinsic feature importance, such as Decision Trees and Random Forests do. Neural Networks rely on complex co-adaptations of weights during the training phase instead of measuring and comparing quality of splits.
A simpler approach for getting feature importance within Scikit can be easily achieved with the Perceptron, which is a 1-layer-only Neural Network.
<code>from sklearn.datasets import load_breast_cancer
from sklearn.linear_model import Perceptron
X, y = load_breast_cancer(return_X_y=True)
clf = Perceptron(tol=1e-3, random_state=0)
clf.fit(X, y)

coeffs = clf.coef_
</code>
The coefficients will return an array for binary classification or a matrix [n_classes, n_features] for a multi-class classification.


Libraries to simulate path planning in a dynamic environment for a 2D space

Question: I need to simulate a path planning approach for a 2D space where the obstacles are moving. Initially, I need to implement probabilistic road map, D* lite, A* and Bug2 algorithm. The robot needs to be able to follow the wall of the obstacles.
I do not know which library is most suitable for these algorithms. I have a tight schedule, I need to finish the simulation within 1 month. I found ROS, matlab and Player. Which one can handle the simulation of dynamic obstacles and has the algorithms that I need. Any other suggestions are welcome.
Comment: What do you mean by simulate? Do you need a simulated robot which actually moves on screen or do you need a plot of the results of the algorithm?
Comment: I need a robot that moves on the screen. The robot needs to reach a goal and avoid moving obstacles at the same time.
Answer: Generally you will need to combine more tools to achive your goals. You will need a path planner library and script, you need to move things around and you need visualisation.
An open source 3D simulation environment which supports dynamic obstacles is Gazebo. You mention Player, as i understand that is also somthing similar, but I have never seen it in use and it seems that Gazebo grew out of the Player Project. Gazebo would take care about moving things around and visualisation. You will need to add a pathplanner.
As for path panning libraries, probably the best known ones are OPML and MoveIt! (MoveIt! is based partially on OPML). MoveIt! is a ROS library and there are tutorials how to couple this library with Gazebo.  However, OMPL and MoveIt does not include all the algorithms you have mentioned out of the box. This means that you have to search for plugins or implement the algorithms yourself.
Matlab probably has most of the algorithms you need, either in the Robotics Toolbox or on Matlab file exchange, or they can be implemented. However, rendered 3D visualisation is not as easily done as with Gazebo. If 3D plots and graphs would suffice Matlab alone is also a viable option. If not, you can connect Gazebo to Matlab by following this tutorial.
Comment: Thanks a lot. Your answer is really helpful.
Answer: The most compact pathplanner in a 2d world is "rrt.py lavalle" The code which can be found in the Internet, includes the documentation and has only 64 LoC. It draws a wonderful randomized tree to the screen with the help of pygame. It works under all operating systems and can be adapted to new constraints.
What i can't recommend is to combine RRT with Fuzzy logic to inject expert knowledge in the pathplanner. In theory, this would allow to handle dynamic obstacles and classifies different maps. The problem here is, that it is not sure, how to falsify this kind of things, so the question is, if it's still mathematics or something else.


Difference between an average "grey" using AV and Manual Modes on Canon 5D Mark II

Question: I have a Canon 5D Mark II. I notice that using AV or TV mode that the meter sets itself to an "average" grey automatically smack in the middle. But using Manual mode, it fluctuates alot no matter what metering i.e. evaluative, spot etc, setting i use. Is using AV or TV mode more dependable on getting a "perfect" average grey, since it sticks very well right in the middle of the meter? Of course i would only use that as a starting point for really bright or dark subjects.
Comment: Do you know what [exposure compensation](http://photo.stackexchange.com/questions/4578/what-is-exposure-compensation) is?
Comment: Do you use different metering mode for Manual and Av/Tv?
Comment: Romeo, no, i usually stick to the evaluative mode for Manual or TV.etc (though i have been exclusively using manual mode the past 3 years).
Comment: If the meter is fluctuating in **M** mode when the camera is moved with respect to a particular scene, then the Tv, Av, or ISO are automatically fluctuating when the camera is set to one of the automatic modes and the camera movements are the same with respect to a particular scene.
Answer: In automatic modes the scale is not displaying exposure. It's instead showing you the amount of exposure compensation you have dialled in. Only in manual mode does it actually work like an exposure meter.
So if your exposure compensation is set to zero the meter will not move at all.
The meter reading is fluctuating in the automatic modes. It's just not being displayed, except that you can see the shutter speed and/or aperture changing.
Comment: I understand, in auto mode, their is no exposure meter, it's just averaging out in the truest sense..an "middle grey".
Comment: In manual mode the metering system is doing the same thing as in the automatic modes. But it shows you in the view finder which way you need to adjust the controls to match that reading. In automatic mode it doesn't need to show you the meter reading. It shows you the settings instead. The metering is the same: it's how the settings and made that changes. I feel I am not understanding your point.
Answer:
I notice that using AV or TV mode that the meter sets itself to an "average" grey automatically smack in the middle. But using Manual mode, it fluctuates alot no matter what metering i.e. evaluative, spot etc, setting i use.

With Av or Tv modes, the camera adjusts shutter speed or aperture, respectively, to achieve a good exposure based on the metering mode you've chosen, so the meter normally shows a reading right in the middle of its range. With M mode, you're in control of all the parameters (shutter speed, aperture, and sensitivity), the camera tells you what the meter says without trying to change any settings to compensate. You're free to adjust any of those parameters to get the meter to read +/-0EV if you want to, just as the camera would do in one of the automatic modes, but you're obviously also free to use your own choices for those parameters even if the meter tells you that it's reading more or less light.
Your camera provides a number of tools for working with the metering system. As described above, manual (M) mode puts you in total control. In Av, Tv, or P modes, you can use the AE-lock function to get the camera to pick settings based on the current metering mode, and then lock those settings in so that you can reframe without having the settings change even if reframing the image makes the meter read something else. You can also use exposure compensation to make the camera choose settings that make the image darker or lighter than it would otherwise, basically telling the camera "yes, I really do want the image that bright."

Is using AV or TV mode more dependable on getting a "perfect" average grey, since it sticks very well right in the middle of the meter?

Av and Tv are more dependable if what you want is a reasonable exposure, and those modes are more effective when you get the hang of using AE-lock and exposure compensation because those tools give you a greater ability to tell the camera what you want while still letting it adjust for changing lighting conditions. M is more reliable if what you want is to not adjust for changing lighting conditions, or if you're working in conditions that would confuse the auto exposure system.
Comment: The real reason for the question is because my exposures vary so much because the LCD outdoors is hard to judge well, even though i always end up with no highlights being blown in final shots, but i prefer to never get my photos TOO dark if i can help it. I'm discovering using the histogram in Live View to see where i can get the exposures a bright as possible without blowing the highlights, which is the best possible exposure...i won't have to depend on visually looking at the image itself as much.
Comment: Using manual, even the slightest movement of the camera causes the meter to fluctuate (even using evaluative).
Comment: @PatMcMullen Your results will vary depending on the metering mode and scene, but in general it's not surprising that the meter changes when you move the camera because the scene changes. If you're using spot metering, for example, a slight movement could easily change the light reaching the meter's central spot.
Comment: The histogram really is the best tool for judging exposure levels -- even indoors, you can't really tell. Another useful feature along the same lines is the highlight alert that causes any blown out areas to blink.
Answer: When in manual mode, the meter changes if the amount of light being metered changes. This can be due to camera movement that includes more bright or more dark areas in the part of the scene being metered. To compensate you need to manually change either the shutter speed. aperture value, or ISO. When you have modified the exposure settings to allow proper exposure, then the meter will indicate such by returning to the middle.
When in an automatic mode, the meter indicates your instructions to the camera regarding exposure. If the amount of metered light changes, the camera automatically alters one of the exposure parameters (Tv, Av, or ISO) to compensate in order to maintain proper exposure.
If you are in Tv mode, you have selected a particular shutter speed and the camera will adjust the aperture and/or the ISO (if the aperture is at maximum, either Auto ISO or safety shift is enabled, and exposure is still too dim). If you are in Av mode the aperture will remain at the setting you have selected but the camera will alter shutter speed and/or ISO to compensate.
Note that even when shooting in an automatic mode, if the scene becomes so bright or so dark that the camera can't compensate enough to maintain proper exposure, the meter will move to indicate over- or under- exposure.
Suppose you are in Av mode and have selected f/1.4 using your EF 50mm f/1.4 lens in bright, direct sunlight. Even at your camera's minimum shutter speed of 1/4000 second using the camera's minimum ISO 100 the scene will probably still be too bright. In such a case the meter will move to show overexposure.
Or suppose you are in Tv mode and have selected a shutter speed of 1/1000 second but the light is so dim that even at your EF 70-300mm f/4-5.6 lens' maximum aperture of f/5.6 at the selected 300mm focal length and the selected sensitivity of ISO 400 (with Auto ISO and safety shift disabled) the metered scene will still be underexposed. In that case the light meter in your viewfinder will move to indicate the underexposure.
Answer: Using manual, even the slightest movement of the camera causes the meter to fluctuate (even using evaluative). This isn't as dependable as using one of the "auto" modes as AV. Because it can easily be thrown off by a bright patch of light in one small area of the scene. So, a truer middle grey is only achieved using the T.V. or A.V. modes since all of the darks, lights are distilled down to grey. The meter shows that. I understand having more control and freedom using manual mode, it's more like using a spot meter to find your middle grey in a jumble of tones though. I still like manual though because it locks the aperture, shutter speed once you do get the right exposure.
Comment: All automatic modes can be thrown off by a bright patch of light in one area to some degree, just like manual, the metering system is the same. For indoor sports I use manual mode when the light is even. That way the exposure is not affected by a bright light source entering the scene. It's more dependable, not less.


Why did David help on-board of the Covenant?

Question: Why did David (in Walter's body) help kill the Xenomorph onboard the Covenant?
Couldn't he have it kill the rest of the crew and then trap it in one of the chambers?
Answer: First of all, it's more likely that David simply disguised himself as Walter rather than swapping bodies.
The reason that David went along with the humans' plan to kill the Xenomorph was so that the humans would go into hypersleep, trusting him to actually be Walter instead of David. That way, he has free reign over the thousands of people in stasis on board to continue his experiments without putting himself or two of his other test subjects at risk.
Answer: I think David saw it as a test for the Xenomorph.
The only way to gauge how close to "perfect" his creation is, is to test it.  He was in a good position to watch the Xenomorph and see how it reacted and tried to kill the remaining crew members. With any type of creation, organic or machine, the creators will always want to test it to see what needs to be improved.
I don't think David was overly concerned with pretending to be Walter at this point in the movie.  If the Xenomorph killed the remaining crew then he gains control of the ship, if the remaining crew kill the Xenomorph then he "helped" and has their trust.  Either way, he comes out of it with control of the ship.
David didn't seem to believe that the Xenomorph was a threat to him so being left along with it on the ship wouldn't be something he was concerned about had it successfully killed the crew.
Comment: I do wonder how xenomorphs perceive androids in general. I know in *Aliens* the Queen almost destroys Bishop but that is our only example across six movies in the franchise, I think.
Answer: There's no evidence that David had any control over that xenomorph.  He could have wanted it off the ship just as much as the others.  The xenomorph could have ruined the ship or him as well. He wanted to live and wanted the ship and human cargo intact for himself.
Answer: David didn't switch bodies, you can tell because he still has the damage to his face. I'm surprised he managed to short and dye his hair in such a short time. Equally perplexing is why Daniels didn't notice that David wasn't Walter, especially while she was helping him staple his face back together; You'd think she'd notice that he wasn't self-repairing like he should.... Also worth noting is that David was missing the nail hole under his chin.
In my opinion, it was way to obvious that David had replaced Walter especially the way they cut away from the fight...
Comment: Any of this might be true, but it doesn't answer the question of why David helped kill the Xenomorph.
Comment: `In my opinion, it was way to obvious that David had replaced Walter especially the way they cut away from the fight` The film's characters might not have been aware of the dramatics choice made in the films editing phase.
Answer: David's ultimate goal was to create life. Specifically, life that is "better" by some subjective measure than his own creators or their creators (Engineers). Everything else is secondary to that goal. He even cut off his own hand to impersonate Walter, that is how much nothing else mattered to him.
Think back to the scene where he shows the captain his experiments. He tells him he really wants to show him his successes, downstairs: he then says the eggs are waiting... for what? A host, obviously, as we all know from previous movies. He started out with a single host, Elizabeth Shaw, who was dead long before his creation was perfected. He needed another human to be the host to the facehugger to see what it would do. This is why he broadcast that song, to lure spaceships to the planet so he would have more hosts.
He learns that the ship is a colony ship, with hundreds of humans in cryosleep. Jackpot. Practically infinite fodder (hosts) for breeding the perfect organism. Keep in mind that so far, he only had two human hosts to work with and had great success: the ship has a hundred times as many.
Helping the crew is a means to an end: gaining control of the passengers on the colony ship. He could have revealed his secret early, except the crew already figured out that the xenomorphs use humans as incubators. They saw an alien burst from one of their own (the second one to be infected early in the movie, I do not remember his name), and Daniels saw his "artwork" showing aliens being born out of humans. It is not a great leap for them to suspect that David wanted the sleeping colonists for experimentation, or that he meant them harm at the very least. They may have even sacrificed themselves to stop him, or instructed Mother to lock David out of being able to control the ship, jeopardizing his greater plans.
Helping the crew gained their trust, even if it meant killing his creation. That is okay, however, because their trust is crucial to his plans: get the crew to go to cryosleep willingly, rendering them incapable of interfering. Once that is accomplished, he has hundreds of unwilling hosts for his xenomorphs as well as alien embryos representing the successful culmination of years of experimenting. Why sacrifice everything for a single alien when he could have hundreds of aliens? Maybe even better aliens, given enough time.
Sometimes it is better to lose the battle in order to win the war.
Answer: I understood it in such a way that if he disobeyed the remaining crew, or lied to them, and they somehow didn't get killed too quickly, Daniels (or Tennessee in case of the former's death) would understand what is happening and, as the captain, could inform Mother (the computer system of the Covenant) that David is a threat. So, David would be risking getting locked in where he was without any authority over the ship, which wouldn't be very good for him even if the crew died. So he chose to sacrifice the pet in order to be able to go on with his experiments witn higher probability.


Can a Sheitel be made out of one's own hair?

Question: Suppose a married woman is able to create a Sheitel that is entirely made up of her own hair (detached, of course). Does that satisfy the requirement for covering her hair?
Answer: This is a matter of disagreement among the poskim. Those who say this is an acceptable type of hair covering include Shiltei Giborim (Shabbos 29a in dapei haRif) and P'ri M'gadim (Eshel Avraham OC 75:5).
Those who say it is unacceptable, and differentiate between a regular wig and a wig made out of a woman's own hair, include Ateres Z'keinim (OC 75:3), Be'er Heiteiv (OC 75:8, quoting Ateres Z'keinim), and Magein Giborim (75:8).
CYLOR for a practical ruling.
Comment: The debate is whether that's called "pe'ah nochrit" -- *foreign* hair. For those who say it has to be foreign to you, I heard a legend about two identical twins who traded hair for their sheitels! (Personally? Eww....)
Comment: @shalom, I know twins who did it.
Comment: @Shalom Correct, this is based on the *mishna* in *Shabbos* (64b, יוצאה אשה... ובפאה נכרית). Interesting about the twins.
Comment: @Yishai they're likely the ones I heard about via a friend of a friend
Answer: Mishna Berura (סימן עה, ס' ג, ס'ק טו):

וכתב הפמ"ג דבמדינות שיוצאין הנשים בפיאה נכרית מגולה יש להם לסמוך על
  השו"ע ומשמע מיניה שם דאפילו שער של עצמה שנחתך ואח"כ חברה לראשה ג"כ יש
  להקל ובספר מגן גבורים החמיר בזה עי"ש.
The Peri Megadim writes that in those countries were women wear wigs,
  they can rely on the Shulchan Aruch. One can infer from this, that
  even one's own hair that was cut off, and then later reattached, one
  can be lenient (and this counts as a hair-covering). The Magen Giborim
  was strict in this matter.

(free translation; text copied from here)
Answer: I heard that it is okay.
Because once it is no longer attached to your head it is no longer considered your hair.


Do generational curses exist in Hinduism?

Question: The Bible talks about generational curses. Does Hinduism have generational curses? If so, please include some verses describing them.
Comment: @moonstar2001 actually, no one can curse Kasi city or residents though Visveswara and Annapurna Devi made vyasa to realize the greatness of Kasi.
Comment: The Atharva Veda contains  many of the mantras for protection from many various afflictions of life, gaining various things in life, and mantras against enemies and other opponents in life. A brief perusal showed no generational curses.
Comment: I'm not sure why this is tagged [tag:christianity]. It *mentions* the Bible, but the question isn't *about* Christianity or the Bible.
Comment: @TRiG it mentions the bible and the post requires at least one tag so I choose it.  There is no generational tag and nothing else seemed to fit.
Comment: Vyasa tried to curse 3 generations of Kasi residents but was preempted by Annapurna devi.
Answer: Yes, generational curse do exists. They are of 2 types:
1. External
Such kind of curse may come from a strong and austere personality like a Brahman or a saintly person. There are couple of references in AnushAsanika Parva as footnotes:

The thin bamboo rod in the hand of the Brahmana is mightier than the thunderbolt of Indra. The thunder scorches all existing objects upon which it falls. The Brahmana's rod which symbolizes the Brahmana's might in the form of his curse blasts even unborn generations. The might of the rod is derived from Mahadeva.

Here is a small instance:

Once Brahma asked Surabhi to bear evidence before Vishnu to the statement that Brahma has seen the foremost part of Siva. Surabhi having given false evidence out of fear for Brahma was cursed by Siva that her offspring will eat unholy substances.

From this answer, a very [in]famous curse was given by GAndhAri to lord Krishna, which was so sharp that, the whole Yadu race met with rather a too quick annihilation to be able to witness further generational curse.
2. Ancestral
That comes from the ancestors of the same family itself, also known as Pitru runa or Pitru dosha. Usually, our ancestors live in subtle forms with their mental desires still unfulfilled. If such desires are not fulfilled by subsequent generations, then these desires are gradually transformed into curse and create various obstacles for ongoing generations. One reason for such Pitru dosha is shortly discussed in Gita:

BG 1.42 And the hybridisation in the family leads the ruiners of the family into hell only. The forefathers of these fall down (into hell) because of being deprived of the offerings of food and water.

Personally I know a very close friend, whose family was diagnosed with a curse, where no male child was getting born. Even if born, then it won't survive. They visited PrAchi, Gujarat, India for an intense ritual to lower down its effect.

We may wonder, why some misdeed of our forefathers have to be paid by us? That's because, a major [material] part of our father gets born as our own self. There are numerous references in ancient scriptures, which suggest that the father is born as a son/daughter. Hence, in a way we can assume that our great-great forefather's [material] part has born our own selves.
Here is one such reference by Bhishma while discussing fair property distribution among son & daughter:

"Bhishma said, 'The son is even as one's own self, and the daughter is like unto the son. How, therefore, can another take the wealth when one lives in one's own self in the form of one's daughter?

Traditionally in earlier times, the daughters would change the family after marriage. Hence, any forefather effect could be subsidising in their case.

Common people giving curse should be ignored & the receiver of the curse should grant them a pardon. Usually such curses will go in below category:
What do scriptures say about using swear or abusive words for cursing people?
Comment: I believe this is possible ... I'm doing prayers, trying to create beautiful things and hopefully bad deeds from past will be forgiven ... even Christian pastor Joel Osteen (many call him motivational speaker) says generational curses exist but we can break then if we ask. God for help and be our very best.
Comment: @Rhonda, to bring in positivity, there are immediate measures also available in Indian culture. 1 of them is **"De-cluttering"** the house. This is more related to "cleanliness". But often ppl relate "cleanliness" with garbage or dust. In "De-cluttering", one should dispose all the unwanted & unused items completely out of the house. Such practices increase the *sattva* (goodness) & suppresses *rajo* (passion) & *tamo* (ignorance) modes. If you have a luxury to build new house, then build according to *[Vastu](https://en.wikipedia.org/wiki/Vastu_shastra)*. That nearly nullifies *Pitru dosha*.
Comment: Interesting you say that. I get urges to clean and declutter my house whenever possible. One day I shall have luxury to build new house ...
Comment: Abt Bhishma Pitamaha and property. True if curse is also property.
Answer: Yes, generational curses do exist in Hinduism. There is a famous story described in Aitareya Brahmana (7.13-18) where Sage Vishvamitra cursed his own sons that their descendants will be born into the lowest castes.

This Risi Visvamitra had a hundred sons, fifty of them were older than Madhuchhandas, and fifty were younger than he. The older ones were not pleased with (the installation of Sunahsepa to the primogeniture). VisvAmitra then pronounced against them the curse, " You shall have the lowest castes for your descendants." Therefore are many of the most degraded classes of men, the rabble for the most part, such as the Andhras, Pundras, Sabaras, Palindas, and Mutibas, descendants of Visvamitra. But Madhuchhandas, with the fifty younger sons, said, " What our father approves of, by that we abide ; we all accord to thee (Sunahsepa) the first rank, and we will come after thee ! " Visvamitra, (leligated 'at this answer) then praised these sons with the following verses :
" Ye my sons will have abundance of cattle and children, for you have made me rich in children by consenting to my wish."
Comment: Great reference to the Vishwamitras. Also you can curse your own descendants
Answer: The answer is yes. But one thing you need to understand : curses are our own doing (actions of past lives or present one) . They only manifest when their time comes like fruits on a tree. When the sins are eradicated by suffering or by pious actions, the effects of curses vanish. As for generational curses, if one is born in a cursed lineage, it is because one's sins are synonymous to the curse. Even if he was not born in an cursed family, he still will have to suffer the sins like a curse , but the curse or suffering will be his own and not directly of the family.
It is a case of काहु न कोउ सुख दुख कर दाता। निज कृत करम भोग सबु भ्राता॥-Ramcharitmanas.If people of similar crimes gather in one place like a family , then it appears as a generational curse like the case of Sage Vishwamitra's sons. In simple way, one can understand by comparing the analogy with suffering of prisoners in a prison for crimes.


Detect correct resolution dual monitor VGA 9800gt Ubuntu 12.10

Question: I connected a second monitor to my 9800gt on the VGA port. The first monitor is on DVI. Both monitors have a native resolution of 1920x1080 but the second monitor doesn't get past 1360x768 and appears as "Unknown" in the display settings.
I tried this guide on how to add undetected resolutions but it didn't work.
First I create the new mode with xrand --newmode "1920x1080_60.00"  173.00  1920 2048 2248 2576  1080 1083 1088 1120 -hsync +vsync and then when I try to add the new mode to my second monitor like randr --addmode VGA-0 1920x1080 I get this error:
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  18 (RRAddOutputMode)
  Serial number of failed request:  31
  Current serial number in output stream:  32

I tried using the nvidia-settings panel to generate a xorg.conf:
Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0" 0 0
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
    Option         "Xinerama" "0"
EndSection

Section "Files"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/psaux"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Keyboard0"
    Driver         "kbd"
EndSection

Section "Monitor"
    # HorizSync source: builtin, VertRefresh source: builtin
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "CRT-1"
    HorizSync       28.0 - 55.0
    VertRefresh     43.0 - 72.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce 9800 GT"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "Stereo" "0"
    Option         "nvidiaXineramaInfoOrder" "DFP-0"
    Option         "metamodes" "CRT: 1360x768 +0+0, DFP: nvidia-auto-select +1360+0; CRT: nvidia-auto-select @1920x1080 +0+0, DFP: nvidia-auto-select +1360+0"
                                    ____^____ I tried changing this to 1920x1080 but no luck
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Here's my xrandr output in case it helps:
Screen 0: minimum 8 x 8, current 3280 x 1080, maximum 8192 x 8192
DVI-I-0 disconnected (normal left inverted right x axis y axis)
VGA-0 connected 1360x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   1024x768       60.0 +
   1360x768       60.0*    59.8
   1152x864       60.0
   800x600        72.2     60.3     56.2
   680x384       119.9    119.6
   640x480        59.9
   512x384       120.0
   400x300       144.4
   320x240       120.1
DVI-I-1 connected 1920x1080+1360+0 (normal left inverted right x axis y axis) 510mm x 290mm
   1920x1080      60.0*+   50.0
   1680x1050      60.0
   1600x1200      60.0
   1440x900       59.9
   1280x1024      60.0
   1280x960       60.0
   1280x800       59.8
   1280x720       60.0     50.0
   1024x768       60.0
   800x600        60.3     56.2
   720x576        50.0
   720x480        59.9
   640x480        59.9
HDMI-0 disconnected (normal left inverted right x axis y axis)

Any ideas on what to do next? I've been trying for hours with no luck, it works fine in Windows so it can't be the cable or monitor...
Answer: I think what you need to do is disable EDID probing. Methods vary depending on graphics chipset/driver. For example, with nvidia's binary blob driver, you use "Option "IgnoreEDID" "false"". This goes in your monitor configuration section. YMMV.


Simple ways to encrypt a message

Question: I am planning to organize a sort of treasure hunt and I would like to know some easy ways to encrypt a message. I'd say something in between Ceasar's Cipher -shift each letter of a word of a fixed position- and Vigenere's Cipher, which I suppose is too difficult for an average person who has no knowledge of Cryptography. Can you suggest me some easy ways (easy for a person who has no specific knowledge) to encrypt sentences or words? I need two different methods as long as I have three riddles to be solved and for one of them I'll use Ceasar's. Any help will be appreciated! Thank you very much!
Comment: Should all encryptions be solvable without using computers?
Comment: If you mean computationally heavy and too much difficult for a human the answer is no. I you mean google than yes. I could give hints on what to search on google for example if use morse code to encrypt a message I could say something like "MOst difficult Riddle ever SEnd" and than the encrypted message. Thanks!
Answer: Here are some examples of simple ciphers you could use:

Base64 - a very common encoding (i.e Y2lwaGVy => cipher)
Caesar Cipher, or, you can specifically use ROT13
Morse Code (i.e .... . .-.. .-.. --- => Hello)
Hexadecimal (i.e 8 5 12 12 15 => Hello)
Braille (⠉⠊⠏⠓⠑⠗ => cipher)
A1Z26 - each letter is represented by its order number
Atbash cipher - where a=z, b=y etc.
xor with a keyword (iczbox ^ a => cipher)
Binary code  (i.e 01100011 01101001 01110000 01101000 01100101 01110010 => cipher)
Use the periodic table to encode a message (i.e 6 53 15 1 68 => CIPHER)
Ascii numbers (i.e 072 101 108 108 111 => Hello)
Vigenere Cipher with a keyword
Answer: To add to pirate's answer -
A mirror cipher may be simple enough - you know, where the ciphered text would read:
:daer dluow txet derehpic eht erehw ,wonk uoy - hguone elpmis eb yam rehpic rorrim A
Or perhaps an ascii cipher, where you spell out words with letters... of course, you run the risk of someone misreading what you write, but that's up to you to make sure your message is clear.
Per Caesar ciphers, I always thought it might be a clever clue to give the name of a Caesar alongside the encrypted text - so that the nth number to shift in the cipher would be given by the nth Caesar.
Comment: thanks for your answer! your clue for the Caesar cipher is really clever
Answer: You could also use a substitution, so that every letter gets replaced by another letter but not all with the same distance (like Caesar's rotation).
It works like this: you choose a key (word or sentence), remove all duplicate letters and add the letters of the alphabet that weren't used. Then put the regular alphabet above it. Every letter of your message then gets replaced by the letter of the alphabet you just constructed.

Example
Key: some nice little message
Message: you found it
<code>abcdefghijklmnopqrstuvwxyz
||||||||||||||||||||||||||
vvvvvvvvvvvvvvvvvvvvvvvvvv
somenicltagbdfhjkpqruvwxyz
</code>
Encrypted message: yhu ihufe tr
Comment: The weakness of this method is that letters "higher" than the "highest" in the key don't get encrypted, in this case the last 6 letters of the alphabet.
Comment: @WeatherVane I know, but it's a simple method and the people he's making the treasure hunt for probably don't know this. And if it's really an issue, then just pick a long key that includes all letters
Answer: Feel free to choose some cipher methods from the links posted under "cipher tool" in the post from Meta:

A collection of Puzzling Tools

This link provides multiple sublinks, and for me this has been the most useful with short descriptions beside each cipher method:

 Rumkin
Answer: These are all really great suggestions. My personal favourite hand cipher is the Playfair cipher. It's part of a larger group of ciphers known as "polygrams". Essentially how they work is by encoding a set of letters at a time in a specific fashion (usually involcing making something called a polybius square which is a 5 x 5 textbox containing the alphabet).
There's really good directions here that'll better visualize it for you because I suck at explaining things.
One thing to note in decoding Playfair ciphers is that you're most likely want to give the key to solving it.
Yes, it is possible to solve it without a key in a method called cribbing, but that's kind of tedious and takes a lot of time if you're new at it.
There's some other variations of Playfair ciphers, my other favourite being the foursquare variant.
Playfair ciphers are more challenging in general than the mono/poly-alphabetic cipher, since they don't follow traditional frequent letter analysis techniques.
Polyalphabetic ciphers are nice alternative as well, since it's possible to break without a key but takes less time than polygram ciphers.
Monoalphabetic is only used for encoding really simple texts, and in Puzzling SE, rot13 is commonly used when you don't want to spoil something in the comments. However, I would advise against using a Caeser cipher, since those can be broken really quickly, especially because of the software.
If you want, you can try using a plain old keyword substitution cipher, except not reveal the cipher, so people actually have to solve it by hand.
Hope this helped!
Answer: After checking the other answers, looks like pigpen wasn't mentioned yet, it's quite popular on this site at least.

Simple ways to encrypt a message