Here’s a little project that I worked on a couple of weeks ago. Its from a very early issue of Make magazine. There’s also an Instructables here: Mousebot Revisited

First you need to take apart your mouse. I have a bunch of old junk mice laying around. Make sure you get a mechanical “ball” mouse, and if it has a scroll wheel, even better. Get one that is symmetrical. Here is the mouse in pieces:

You have to then trim out the insides of excess plastic, so that you have room for a battery, motors, and the circuit. In this photo you can see that I used a Dremel to remove the mouse ball’s “cage”:

I drilled holes for the motor shafts, and carved out a space up front for a microswitch (the “bump” switch). The motors got mounted with hot glue, and are sitting on a piece of Komacel (you can use anything that’s the right thickness and strength).

Here are the “eyes” of the little robot. Two IR photosensors (you can use new, like I did, or scavenge from the mouse) and a green LED. I used solid wire to give them stiffness and then put heat shrink tubing to a nicer look.

Here’s the mouse wired up. The original design uses “dead bug” wiring — where you solder directly to the pins. This seems the easier route at first, but could lead to shorting of components in such a tight space. Next time I would use a protoboard. For this one, I had to carefully tape any wires that might touch.

Here is a close-up of the front. From right to left you have: bump switch, motors, circuit (LM 386, relay, transistor, capacitor, resistor), then the battery. On the “lid” (top) of the mouse I have mounted a power switch and the eye “stalks” along with the green power LED.

Here’s the outside view of the almost-done mouse robot. You can see the little power switch I used as the “tail” toward the left of the photo:

The little brain inside the robot (actually just an amplifier) compares the light levels from the two eye stalks. It then heads in the direction of the brightest light. If it runs into something, the bump sensor will reverse the robot for a few seconds, and then it starts hunting for light again. These types of robots are often called “photovores” because the “eat” light.