You have not said what kind of pets you have, or how smart they are.
Looking at the posts by PaulS and DRBArduino made me think about a couple of things.
You probably don't want to feed the pets every time they want fed.
However the animal needs to learn how to operate the feeder and that could be tricky if sometimes the animal gets fed and sometimes it does not.
For the animal to learn to use the feeder I think the feeder needs to indicate that it is "willing" to dispense food.
So when the feeder is prepared to dispense food it needs to have an LED, flag, sound or something, that tells the animal I will feed you if you wish. When the animal sees the indication it can then do whatever is required to trigger the food release.