Simple & Cheap, GSM Controlled On/off Switch

This tutorial will teach you how to modify an inexpensive GSM surveillance bug, into a simple on/off switch, controlled by your cell phone.

It was first published in my blog where you can find some more details about it.

The minimum tools and components you will need:

• GSM surveillance bug
• Soldering iron
• NPN transistor (i.e. BC547)
• Cables
• Scissors
• Small screwdriver

Time: ~1.5 hours, depending on your soldering skills

Cost: 12$

Step 1: Buy the GSM Surveillance Bug

First things first. Get your hands on one of those little gadgets. They shouldn't cost more than 10$ and you can get them from Chinese sellers found at Ebay and AliExpress. Keywords like "gsm surveillance ear spy bug" should do the trick. This tutorial is for the specific model that's depicted on the photos above, however they all should be more or less the same.

They are based on the MT6223DA chip by MediaTek, made for ultra low cost phones. Their main function is to automatically answer a call and let their caller listen from the on-board microphone. You can use them to "spy" on others since they don't emit any sounds and they are really small, so they can be easily concealed.

Step 2: Find a Salvagable Signal Point

Carefully crack the case open with a small screwdriver. Don't worry, you will not damage the interiors unless you really try to. Inside you will find a battery connected to a board. The board is what we will work with.

Now, we need to find a way to extract a useful signal, while we are calling the GSM bug. After poking around with the multimeter, I found a particular spot that is charged with 2 Volts during calls. You can see it on the photos above, where the green cable is located. Be aware that it might be different on other models, so if you are not sure you have the same board, test it first with the multimeter.

Then all you have to do is solder a cable on that specific side of what seems to be a capacitor. Solder another cable at the negative pole (ground) of the lithium battery that powers up the GSM bug. Make sure the cables are not too thick either since the space is limited as you might have already noticed.

Step 3: Reassembling the GSM Bug

After you have soldered the cables it's time to reassemble the GSM bug and pull those cables outside. The easiest (and possibly the only) way to do this, is if you pass them through those two holes that happen to be conveniently placed on the board. Then you can put the case back together, which shouldn't be a problem if the cables are not in the way.

Afterwards, cut the lid's corner with some scissors, in order to create space for your two cables and pass them through as demonstrated on the photos.

Step 4: Make Sure Everything Works

This is an optional step but always useful in order to avoid problems. Hook your two cables to a multimeter and verify that you get that valuable 2 Volts during a call and nothing while the phone is idle. Furthermore, make sure the SIM card is mounted correctly, as demonstrated on the photo above.

Step 5: Utilizing the Signal

After we make sure we get 2 Volts during calls, we can design our circuit according to the intended use. I will demonstrate the most simple case, where this signal is used in order to open or close a external circuit, as an on/off switch would.

The 2 Volts cable coming from the device, is connected to the base of the transistor and the emitter to the common ground of the GSM bug battery AND that of your external power source. For illustration purposes, I use a 9 Volt battery.The last pin of the transistor, the collector, gets connected to the circuit you want to control, as does the positive pole of the battery. In my case, I connected the positive pole of the battery and the collector pin of the transistor to a terminal, in order to make the connection to the external circuit easier.

If everything has been properly connected, when a 2 Volt signal arrives to the base of the transistor, the emitter pin gets grounded thus the external circuit (the one we want to control) closes. On the opposite case, when no signal is present at the base of the transistor, the external circuit is open. Depending on the external circuit you want to drive, you might have to use a relay, but I will not elaborate more on that in this tutorial.

Step 6: Future Potential and Possible Use Cases

Using this inexpensive GSM bug to control other circuits or sending signals opens up a large field of opportunities. It can be used in home automation, surveillance and remote control applications, instead of a more expensive and complicated GSM module. I have composed an indicative Use Case diagram that illustrates numerous occasions where this simple "hack" could be leveraged. I would be glad to know your ideas as well, so don't hesitate to contact me if you have any remarks or questions.

Interested in the real life applications of the hack? Then check this 10$ trackable keychain I made based on the above!