Introduction: 10$ Trackable Keychain

This tutorial, serves as a sequel to the Simple & cheap, GSM controlled on/off switch, was originally published in my blog and describes one of its applications. Now, we will modify the inexpensive GSM bug and whenever we call the GSM bug, a buzzer will emit a distinctive monotonic sound that will help us locate it. If you want to read more about the hack specifics or how to purchase the small surveillance device, please visit the link or above or this article.

Materials & tools:

  • GSM bug
  • Smallest buzzer you can find. I used this 9x4.2mm buzzer.
  • NPN transistor (i.e. BC547)
  • Thin cables
  • Drill
  • Soldering iron

Cost: ~10$

Time: ~1.5 hours depending on your soldering and handcraft skills

Step 1: Drill Holes for the Buzzer and the Key Chain

First, crack the case open. Place a small screwdriver at the gap between the board and the case, as demonstrated in the photo. Push and after you hear a popping sound, you should be able to disassemble the case. After taking everything apart, use a drill to make a hole just big enough for your buzzer to pass through and some smaller ones for the key chain mount. You can see on the pictures where exactly I drilled mine. Keep in mind that you will be placing a buzzer and a transistor underneath, so make sure you choose a spot where there's enough room for both.

Step 2: Solder Cables on the Buzzer Pins

Solder two thin wires on the buzzer pins. Make sure the one connected to the positive pin is long. Pass it through either of those two holes on the board and reach to the other side. Make sure the buzzer remains on the spot it should be in the end, right below the hole made for it on the case. It might help if you mark where that is with a pen.

Step 3: Soldering Cables on the Transistor

Solder two thin cables on the base and emitter pins of the NPN transistor and pass them through either of the holes towards the back side of the board. Make sure nothing short circuits. Afterwards, connect the cable coming from the buzzer's negative pole, to the collector pin of the transistor. Make sure the buzzer and the transistor are not too far away from each other.

Step 4: Soldering the Loose Ends

At this point, you should have three unconnected cables on the rear side of the board. Connect the cable coming from the buzzer's positive pin to the positive pole of the battery. Connect the cable coming from the base of the transistor, to the salvageable point (see the pictures above) that is charged with 2 Volts during calls. Finally, connect the emitter pin of the transistor, to the battery's negative pole.

Make sure that the cables are not too long since there is limited space.

Step 5: Verify That Everything Works

After making all the connections correctly, mount a SIM card and try to call it. If successful, you should be able to hear the buzzer's distinctive sound while the phone line is active. If not, check if the transistor pins are accidentally short circuited or whether the board is powered properly. Optionally, you can chop those two microphones off, so you won't have to hear the buzzer from your phone's speaker.

Step 6: Final Touches & Thoughts

Mount the key chain and reassemble the case. The case should shut firmly and the buzzer not to wobble. Furthermore, you can drill an extra hole and add a LED light to it. Another smart thing to do is tape your phone number on it, so if someone hears and finds it they can contact you.

This device has some advantages over the typical commercial Bluetooth based products. Namely, it's much cheaper, you don't need a smartphone to use it and you don't necessarily have to be within Bluetooth range. Unfortunately, its battery can only last a couple of days without charging. Nonetheless, it’s a fun project to spend a couple of hours on and can make a great Christmas gift.