Would it be a problem using a PIR on pin13? I know an extra resistor for the LED pulls down the voltage. I got one connected that tends to trigger randomly. I don't know if it's triggering from motion or something else coming from the Uno.
"I know an extra resistor for the LED pulls down the voltage."
D13 connects to the input of a comparator; the comparator output drives the "L" LED.
Reading the output of a PIR should not cause an issue. If your PIR output is active low, then you can turn on the internal pullup resistor with
pinMode (13, INPUT_PULLUP);
to hold it high when the PIR is inactive.
If the PIR output goes high when it is active, then a 10K or 20K resistor from D13 to Gnd will keep it low when the PIR is inactive.
It's active high so I did not use a pull-up. No pull-down either, I was hoping the PIR had one on it's output.
It's expoxied in so I can't change the darn sensitivity without a ton of work. I wanted to find another cause, although i'm pretty sure the sens is so high it's picking up IR noise. Filtering the signal in C++ to my ability did nothing to reduce falsing. Time to get out the dremel and hope it looks great once I put it back in!