Our house was overrun with mice -- smart mice. They could remove the bait from snap-traps without triggering them. They would treat corral-traps as feeding stations, slipping out as easily as they entered. When my patience ran out, I built the proverbial better mouse trap.
The body of the trap is made from a shortbread tin. Mice enter through a hole cut in the lower-right side of the tin. Once inside, they are forced to walk though a toilet paper tube to reach the bait. On the bottom of the tube is an infra-red LED which is aimed at a photo transistor at the top. These components are taken from the write-protect sensors of a 5.25" disk drive. When the light beam is interrupted, the floppy drive's main motor starts to spin. This instantly retracts a piece of coat hanger wire that was holding the door open. The door (which is made from shielding from a television set and is hinged with a rod from a clothes drier and a drinking straw) falls closed, and is secured by five fridge magnets. A switch from a vending machine detects that the door has shut, and cuts power to the motor. A computer power supply provides 5v for the sensor and 12v for the motor. Only minor modifications had to be made to the floppy drive's controller board to get it to spin the motor when the infra-red sensor was tripped.
I am happy to say that the trap works perfectly. Several mice have been caught and released with this device.
Update: It turns out that this mouse trap has an Achilles heel. Because it is made out of junk, it looks like junk. As a result it was thrown out by a relative who took it upon herself to do some cleaning. Grrr...
Featured in: Makers, O'Reilly, 2006.
Comment from Liam Morland: "What? It doesn't send an email telling you it has caught a mouse?"