Would a 100KΩ resistor connected to ground pull an open analog line down to 0 I have a project that has a photoresistor wired into a voltage divider and connected through a 3.5mm “stereo mini plug”. When nothing is in the mini plug, the analog line is an open circuit, and floats, creating jittery spurious readings.

I’d like to wire a pull-down resistor into the project box to tie it to ground so I get a zero reading when the light sensor is not connected, but want to alter the voltage reading from my photo sensor voltage divider circuit as little as possible.

If I connect a 100KΩ resistor between my analog input and ground, that would give me zero readings on the analog pin when the photo sensor is not plugged in, wouldn’t it? I can’t see how &lt; 20 CM of 26 gauge wire would pick up enough interference to pull my analog pin up from 0 through a 100kΩ resistor.

What is the effective range of an analog pin on an Arduino? (A Mega 2560, if that matters) What’s the lowest voltage that will register above 0?

(The project box is getting crowded, so experimenting is getting tricky. I’d like to make a single change rather than trying different resistor values.)
With a 100 kΩ resistor to GND, it is not guaranteed that the value will be zero. If a (servo)motor is connected, it might get some noise.

The pull down resistor can be any value, because it is possible to calculate the value of the photoresistor.

There is an alternative solution. I think that all the analog pins of the Arduino Mega 2560 are digital pins as well. You can do a pinMode(A0,INPUT_PULLUP); to pull the input high. When a photoresistor is connected, you can turn the internal pullup resistor off with pinMode(A0,INPUT);.