Flashlight tag is a beloved kid's game, and I always love to throw a new spin on an old classic. So just for fun, I decided to add electronics to the game and I designed some pin-on badges that beep whenever you shine light on them. This gives sort of a laser tag feel to the game, and also resolves any debates over whether a person has been tagged or not. Here's how to make it.
Step 1: Materials
Printed circuit board (Radio Shack part #276-159)
Three 1.5V button cell batteries
555 timer IC (Radio Shack #276-1723)
1μF capacitor (4.5V or higher)
3-12V piezo buzzer (Radio Shack #273-074)
Heat shrink tubing
Large paper clip
Clear plastic sheet or case
Hot glue gun
Step 2: Circuit
There are a wide variety of the CdS photoresistors and the manufacturing tolerances vary wildly. The one that I used had a range of 3KΩ when well lit to 30KΩ when dark. Depending on the photoresistor that you use, you may need to change the value of the resistor between pins 2 and 4 (the 10KΩ resistor). In order to activate the timer, this resistor must have a resistance at least two times larger than the photoresistor when it is illuminated. Adjusting the value of this resistor will also change how sensitive the sensor is. I recommend trying out several values before soldering the circuit together.