Parts and Tools:
1) Two small DC motors, with fairly long shafts.
2) Three popsicle sticks
4) Two potentiometers, around 10K
5) Two (matched) CdS cells
6) Insulated wire
7) Two N-Channel MOSFETs
8) Two diodes, preferably fast switching type
9) Tube or shrink-tubing to use for wheels
10) [optional] 1/2 ping pong ball, or metal thumb-tack, to use for rear skid.
11) Hot glue, and hot glue gun
12) Solder, soldering iron
13) Multimeter (only needed if you need to select CdS cells)
14) 9v battery plug
15) 9v battery
On-line surplus electronics stores often sell large bags of Cadmium Sulfide (CdS) light sensor cells, which are Light Dependant Resistors (LDRs) for reasonable prices. When using cells from such a grab-bag, you should be careful to select two matched cells with a similar resistance under both low and bright light. The values of these cells can vary significantly. Matching to within 10-20% is sufficient.
This robot is based off of two mirror-image motor driver circuits as shown here. The photocell on the left side drives the motor on the right, and vica-versa.
Step 1: Select Matching CdS cells
Measure the resistance of cell in dim, ambient light. Repeat this measurement pointing toward a lamp. The second measurement should be significantly lower.
Repeat this measurement on several cells until you find two which have similar values both under dim light and when pointed toward a lamp.