This is the last piece of the puzzle in my goal to replicate the keypad COMMS signals with an arduino 3.3V digital pin. The software side is working and the signal is output from one digital pin at 1.25kHz.
With my multimeter I found that COMMS pin is normally 5V, and is pulled low by the logic signal (via transistor controlled by pin RC7). It appears the transistor shorts the COMMS pin down to GND.
I'm thinking the arduino 3.3V pin could essentially replace the RC7 input in a similar circuit?
I'm just not sure about adding in resistors etc.. for safety.
Any tips much appreciated.
Here's a part circuit and pcb (I can post more but the size limit):