How does this flasher circuit work?
Re


I understand but I do not not how C1's current flows and comes from where and goes to where. But here, I think the voltage at "out" and Q3:C which is supply voltage, adds up.


The output if driving an LED would depend on Vsupply being &lt; Vf_LED_On but > VfLEDOn/2. When Q3 is off C1 charges to vsupply via Vsupply-R3-C1-R7//R4//Q1_be.  

When Q3 turns on the left side of C1 goes from 0 to Vsupply so the right side of C1 goes from Vsupply to ~= 2 x Vsupply *(as voltage across a capacitor cannot change instantaneously and left end was raised by Vsupply so right side must be too.

2 x Vsupply acrosss D1 turns it on if VF_D! &lt; 2 x Vsupply.

This is a bad wayto drive D1 as it provides large chort current pulses. IF the circuit is driving D1 at about mean rated current then current peakswill be >> I rated. Moderm LEDs only allow a small margin - which this will exceed if LED is near mean rated current.

BUT cap C1 is charged vi R3 + R7 etc. If Vsupply = 1.5V then Icap charge = 1.5/27k = very very very little. .

_______________________________________. 

Ocsillator action:

Q1 Q2 Q3 are effectively inverters.

Remove C2 temporarily

Force Q1b low -> Q1C high 
Q1C high -> Q2b high -> Q2 on.
q2 on -> qQ2clow -> Q3 on -> Q3C high.

So the 3 Q's are high low high

And Force Q1b high -> Q1C low -> Q2B low -> Q2 off
Q2 off-> Q2C high -> Q3 off -> on Q3C low

So the three Q's are low high low 

This like 3 CMOS inverters in a row with output connected to input.
 As their are 3 you get net inversion.
 Connect out to in as here and it tries to "chase its tail"  With pure R biasong it will settle dowm to some stable point/

BUT R5 + C2 connect across an inverting point - when one end is high it is driven by a low input and when low it is drive by a high input. This provides the negative feedaback to ensute ris system is unstable and will always chase its tail.



This is not a very "exciting" circuit. It id not obvious that it does anything overly well. But, it may.

Consider this circuit:

Try simulating that.
Make L1say 1 mH to start. Can be say 100 uH or whatever.
 Play with R1 &amp; C.

Does it oscillate.
 Note this is for 1 cell.
 Q2 grounds Q1's base - noyt good for high Vsupply

Components may be re-added as desired - this was a stripdown attempt AFAIR.

If R2 C1 time constant is long - say 0.5 second, you have a LED flasher. If R2 Ca time constant is short you have a boost converter and steady LED driver (it seems visually).

I "invented" this circuit in about 2000 and published it on-web in ?2001?.
  I say "invent" as Q1 Q2 oscillator was used by Noah to power a deck light. I simply added L1 which was wholly obvious -and very successful. I would be completely non-surprised if this had been invented by 3,456 people, 10 years before I suggested it, BUT I've never seen any earlier versions. Several close version turned up inside 6 months of my publishing the original. No doubt a fluke - they didn't mention me :-).