Step 1: Materials
- 1 LED
- Alligator Wires
- Set of Infrared Emitter and Detector (Radioshack #276-0142)
- 1 Small Enclosure/Case (About 3.5"x2.5"x1")
- 2 3V Coin Cells
- 1 PNP Transistor
Step 2: Emitter
Grab the dark LED from the infrared pair- this is the emitter. Pinch the two prongs together, but make sure they don't touch. This will let the coin cell fit firmly. Slide the coin cell between the two prongs. The longer prong should touch the positive side of the battery, and the shorter side should touch the negative side. You can do this all later to save battery power.
Step 3: Detector
Now take the clear LED out of the infrared pair- this is the detector (also called a phototransistor). Connect the negative terminal of the 3V Battery to the negative LED prong using an alligator clip. To make the clip stay on the battery, I used a Neodymium magnet, but you can just tape it. Connect the positive LED lead to the emitter of the PNP transistor. Connect the collector of the transistor to the positive side of the battery. Connect the base of the transistor to the negative detector lead. Then connect the wire coming from the emitter of the PNP transistor to the positive lead of the detector. Now, carefully, put the whole circuit in the small container, leaving the detector easily visible.
Step 4: Using It
First, you lay out the two circuits so that the emitter is directly across from the detector. Leave about an inch gap between them. Swipe your hand through the gap. The LED will blink!! Now, move the emitter back a little more, and swipe your hand again. Keep doing this until you can get a very large gap.
Step 5: Circuit Diagram