So I've found a basic composite video "synth" which I'm trying to manipulate with an arduino. The device was originally controlled by NO tact switches between pins on the CMOS MCU, a Signetics SC80C51B (datasheet below).
I've replaced some of the tact buttons with 2N7000G N-channel signal mosfets, which are driven directly from the pins of the arduino UNO (5v).
So far, I've had some success. The arduino can reliably emulate most of the buttons by sending a momentary (~10ms) HIGH to the gates of the fets. However, some of the buttons behave erratically, triggering other buttons, even those which have not been replaced by mosfets.
EDIT: removed confusing example, made unnecessary by schematic.
Now, this leads me to think that the buttons are multiplexed, which makes sense. However, I can't figure why some of them work completely fine and others don't. Logic leads me to think that if the multiplexing were the problem, all of the buttons would behave erratically.
Unfortunately, I don't have a digital scope to capture any of the pulses and investigate further. I suppose I'm just hoping that someone can explain to me how I can better interface with the Signetics chip, or how to better emulate the button presses.
If you need further informations (pin #s, etc), just ask, I'm just not sure what's relevant.
I've attatched a photo of the pins as they're hooked up. FETS A, B, C, and "OUT" all work correctly, "REV" and "WIPE" trigger other buttons as well as themselves. FET A's drain does not pass a continuity test between any of the pins on the MCU, but works correctly, don't know what to make of that.