Flashlight Tag Light Sensitive Badges

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)
CdS photoresistor
1μF capacitor (4.5V or higher)
3-12V piezo buzzer (Radio Shack #273-074)
1MΩ resistor
10KΩ resistor
Heat shrink tubing
Safety pin
Large paper clip
Switch (optional)
Jumper wires
Clear plastic sheet or case
Tape

Additional tools:
Heat gun
Hot glue gun
Soldering iron
Knife

Step 2: Circuit

Here is the circuit that I designed. It is a basic 555 timer circuit in monostable mode. In this configuration the IC sends one pulse to the buzzer every time pin 2 is brought low (below 1/3 the supply voltage). The length of that pulse is determined by the values of the resistor between pins 7 and 8 (in this case, the 1MΩ resistor) and the capacitor. Increasing the value of either of these components will increase the length of the pulse and reducing either value will shorten the pulse.

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.

Step 3: Button Cell Battery Pack

To make the battery pack, take the three button cell batteries and wrap them in tape. This will help hold them in place while you heat shrink them. Then insert the batteries into the center of the heat shrink tubing. Shrink the ends first. This will squeeze the batteries tightly together and make good contact. Then just trim off the excess tubing. You can use more button cells if you want. Three is just the minimum because the timer IC requires at least 4.5V to function.

Step 4: Battery Holder

To make the battery holder, cut off two pieces of the paper clip that are about an inch long. Fold each of them in half. The bent end is where they will contact with the battery. The cut ends will be soldered to the board. When positioning the battery connectors on the circuit board, choose pin holes that are spaced slightly further apart than the length of the battery pack. When soldering them, try to align the connectors so that they are leaning slightly towards each other. You want the tips of the connector to be slightly narrower than the battery so that it will make a tight fit with the battery pack. It is important to get the connectors close to their final position before soldering. You can bend them a little after soldering but putting too much pressure on the solder joint can break it.

Step 5: Assembly and Modifications

From here on, it is a pretty straight forward soldering job. If you are using the same PCB, you can just copy my layout.  Try to keep all the components as low to the board as possible. This will make it a lot easier to make a cover for the circuit later. Once everything is soldered, attach the battery and test out the circuit to make sure it is working properly.
Note: In my first version of this circuit, I just removed the battery to turn it off, but that got a little annoying. So I went back and added the power switch (retroactive engineering!) To do that, I had to sever one of the lines between the battery terminal and the rest of the circuit and solder in the switch in between the two points. If you want to include the switch from the beginning, I recommend moving the IC down a few holes to make room.

Step 6: Add the Safety Pin

To add the safety pin, just attach  to the edge of the board with hot glue. Be careful to avoid making contact with the circuit. This can create shorts.

Step 7:

To make the cover, I used a thin piece of clear plastic. This will let the light through and show off your circuitry. Start by folding it in half. Then, using a sharp knife cut a slit in the folded end that is the same length as the safety pin. Slide the safety pin through the slit and fold the plastic over the circuit board. This will give you an idea of how wide it needs to be to enclose the circuit. If you trim the plastic, leave it a little wider than you think that you will need. You don’t want it to be too tight of a fit. Then tape the sides together. I recommend using a strong tape. With all the running around in tag, you don’t want it to tear open. Then to close the bottom, I folded one end over and cut a slit in it. Then I made a notch in the other side and fit the two together. This did a good enough job holding it together. However there are lots of other ways that you could cover your circuit.

Step 8: Finished Badge

Then just slide your circuit board into the cover and you have the finished flashlight tag badge. There are of course, any number of other applications for a light sensitive buzzer--intruder alert for a room, cookie jar security alarm--use your imagination and have fun with it.