I am currently working on an industrial Digital input/output board controlled via an atmega32u4. I used a simple circuit with 4n35 (with pull-down) driving 12v relays via ULN2803 for outputs and 4n35 to pull-up in input, so i can isolate the 12v/24v from 3.3v of the atmefa32u4. But i am encountering a weird problem, whenever i put my finger on the 4n3t output side (5 & 6 pin), the relay is triggered, the same for the inputs, same when putting it on a surface, but work very well when not touching anything.
Can someone please explain this behavior?
Thanks
One of the possible reasons is the values of pull-down resistors. If it is too big, your finger could induce interference on the ULN2803 input.
On the most applications it is enough to provide 1 mA current on the detector side of the optocoupler, so you could calculate the proper values using the following formula: R = U / 0.001 or R = 1000 * U, where U is a wokring voltage of the detecor side (3.3V, 12V or 24V in your case).
If it not enough to get rid of your problem try to decrease resistance by 5 times.