Herbie line follower
From BEAM Robotics Wiki
It's pretty easy to hook up a few LED's facing downwards and turn the photocells downwards, put some black tape on a white floor, and make Herbie into a line follower (finished dead last the timed light following contest at the Seattle Robotics Society Robothon in April of 1996.
|2 low-current (< 100 ma?), geared motors (there are sometimes hard to find).|
|2 CDS photocells|
|1 10K Trimmer|
|1 DPDT slide switch|
|1 LM386 audio amplifier|
Note: To increase the gain on the amplifier from 20 to 200, you can connect pins 1 and 8 of the amplifier together (with just a wire -- the app notes for the chip say use a capacitor, but that's only if you want to drive a real audio load). This makes the bot respond a little more crisply to changes in light.