Also, I am thirteen years old. (Mandatory mention of age for robot's challenge)
Step 1: Materials
A motor. I took one out of a broken Nerf Barricade.
A TIP31 transistor. $0.20 http://www.taydaelectronics.com/tip31c-tip31-npn-transistor-3a-100v.html
A 2N3906 transistor or similar PNP transistor. $0.02 http://www.taydaelectronics.com/2n3906-general-propose-pnp-transistor.html
A battery holder. I used one that held 3 AA's. $0.14 http://www.taydaelectronics.com/aa-battery-holder-5.html
A SFH 314 phototransistor. $0.58 http://www.mouser.com/ProductDetail/OSRAM-Opto-Semiconductors/SFH-314/?qs=K5ta8V%252bWhtbR6gV8hNfmATd6ftDkb0eH0AduyQe8MSk%3d
Perf Board. I used one I had laying around, but you should use Radioshack's miniboard, which costs around $2.19. http://www.radioshack.com/product/index.jsp?productId=2104052
A laser pointer is recommended to activate the light switch.
The total cost came to around $5, with shipping.
Must be able to solder and read schematics
Step 2: Make the Circuit.
TIP31 pinout: http://www.hobbytronics.co.uk/image/data/st/tip31.jpg
2N3906 pinout: http://www.reprise.com/host/circuits/images/to-92.gif
The emitter of the phototransistor is the longer lead. The shorter lead of the phototransistor should be soldered to the negative (black) battery holder lead.
Step 3: Enjoy Your Light Activated Motor.
Step 4: How It Works.
However, when I tried that with a 2N3904 (an NPN transistor), I found that the transistor didn't allow enough current through to power my motor. To do that, I needed a transistor that allowed more current through, like a TIP31. However, the phototransistor didn't allow quite enough current to switch the TIP31 transistor.
To allow more current to go, I used a PNP transistor. The phototransistor allowed enough negative current to go to the base of the PNP transistor, so that positive current could flow from emitter to the collector. The 2N3906 transistor allowed enough positive current to go to the base of the TIP31 transistor to switch it.
Because the positive lead of the motor is attached to the positive lead of the battery holder, when the TIP31 transistor switched, it allowed negative current to go from the battery case to the motor. The motor was then connected to the positive and negative leads of the batteries, and it begins to spin.