The PIC12F675 is small in size and low in power. It lends itself to some really off-beat uses.
The picture at right is the completed unit with the transparent top removed. A pair of AAA nicads are hotglued in the bottom, with the motor and circuit board stacked up on top with double-stick tape in between. Rubber bands hold the whole assembly together.
To charge the batteries, the jumper is removed and a charger is connected to the battery. After that, the jumper is replaced and the top screwed back on.
One of the features of this PIC that is useful here is the 'wake up from sleep via watchdog timer'. This allows the device to draw virtually no power most of the time. Each time it wakes up, it powers up the photocell (via GP1) and takes a reading (on GP0). If the light level has dropped since the last reading, it assumes an object (cat) has passed by and activates the motor.
This device, although an engineering success, has failed to interest my cat. He would rather chew on my $40 scope probes.