This instructible will show you how to make a light following "robot" using only a couple batteries, light dependent resistors, and a pair of motors. Robot is a bit of an exaggeration, as this is quite a simple beast, but it reacts to stimuli, so that's close enough!

Video of the process can be found here, and I'll embed it at the end of the tutorial as well.

Step 1: Materials Needed

(2) Gearmotors
(6) LDRs (light-dependent resistors)
Wire
(2) Coin cell batteries
Perfboard
1/2" MDF
Toy car or other source of wheels

This is what I used, but nearly everything on this list can be substituted.

Step 2: Cut Out Frame

This can be made out of nearly anything you can think of, but I decided to cut mine out of a piece of 1/2 inch MDF. A 1/2 inch slot was first cut down the middle of the frame in order to accommodate motors on either side, then the body was cut out with a 2 3/4 inch hole saw.

An area was also cut out in the back for the coin-cell batteries.

Step 3: Source Wheels

Wheels were sourced from a broken toy truck. Lock the axle in a vise, then pry with pliers.

Step 4: Install Batteries

Like everything else in this project, you could substitute something different (like a LiPo pack or two), but after carving out space for my coin-cell batteries, I stripped a piece of wire then wrapped it around, connecting it to the positive battery terminal on the bottom. The top negative side was connected by the ends of a wire run through the PCB and sticking out the bottom slightly.

Technique is somewhat similar to these shrink wrap battery packs outlined here.

Step 5: Electrical Connections

Electronics are quite simple. Positive from the batteries goes to the LDRs as shown, then the bank of LDRs on the right feeds the motor on the left and vice versa. Three LDRs in parallel were used here to allow more current to flow to each motor.

Step 6: Assemble

Assemble the motors with hot glue, along with the circuit board that forms the top. And that's it, you now have a little robot that will follow a light around!