@Alec
It looks like the feed "hook" is negative edge triggered which I guess is probably fine. Any reason to include programmable (rise, fall via jumper) edge detection there?
I could envision an I/O module connected to the output of an AC timer, and it's output would be open collector. If there was some DC powered gizmo then an Opto could be used, so probably the negative edge trigger makes sense.
Joe, for the sake of argument say you purchased this fish feeder:
http://www.aquariumguys.com/current-aquachef-automatic-feeder.html
Note, it's really cheap, but would not have the required output. Let's just assume it has a motor to rotate the gizmo when it feeds or some solenoid release mechanism. So, you would have to find the voltage to the motor OR the voltage to the solenoid.
Armed with that information, one could add two components, a optoisolator (a 6 pin IC) and a resistor and trigger the pump feed mode. The timer would be the timer in the fish feeder. So, that's the plan.
You would essentially be building a version of this:
https://www.electro-tech-online.com/custompdfs/2012/12/MR-BusIO-DCIN-Manual.pdf or even buy this, and change one resistor. I bought a few to play with. It expects 5V, not 12V for a logic supply. Gravitech has an AC voltage detection system too.
A alec said, you have to make a motor speed controller since one of your power supplies won;t go low enough in voltage.