How can I construct a basic circuit that will detect the movement of a small animal, such as a mouse. I am thinking along the lines of the motion, or infrared detecting devices that are used in "no touch" towel dispensers, hand dryers, and a number of other devices these days. I would like the circuit to control enough volt*amps to operate a relay which would then control other devices. The circuit should be powered by D cell batteries so as to be completely portable. Use as many batteries as necessary.

LEGO Shop - LEGO® MINDSTORMS® NXT (http://shop.lego.com/Product/?p=8527)

This Lego robot is probably overkill for what you want, but thought you might be interested. My son has one, and they are very versatile. They have motion sensors and other sensors that they can be programmed for as well.

LEGO Shop - LEGO® MINDSTORMS® NXT (http://shop.lego.com/Product/?p=8527)

This Lego robot is probably overkill for what you want,...

You are absolutely right. This is extremely interesting, but overkill. My emphasis is on doing this as economically as possible, because I may be making 10 to 20 of these. :)

I would use an IR emitter and detector, and may use 2 pairs in an X pattern, crossing at the center so a Mouse? Would trigger a solenoid release only when he is in the center of the X, not just break 1 set of beams?
One of my security companies sell these as pairs. I think you could detect a cock roach, may have to have the emitter in a blackened straw or tube.