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'Car FM transmitter to spy bug' Answered

Hello, i am looking for some tips on how to transform an FM mp3 transmitter to a spy bug.. i have that transmitter you plug into the lighter in your car, put an aux cable or an usb in it, and it then trasmits to a set frequency so you can tune in with your radio.. i was wondering would it be possible to just get rid of the jack, and solder a microphone instead? If not, how would I go around that?


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1 year ago

I know it's Bluetooth it is from some Headphones what all does it do and where is it from

Jack A Lopez
Jack A Lopez

2 years ago

I think the Wikipedia article for, "Electret microphone", had some hints for how to bias those things.


Well, it has a circuit diagram. Specifically this one:


Typical value for the resistor in series with the supply (V+) is about 10 Kohm, and a typical value for the DC blocking capacitor is about 10 uF.

I am not sure if just that circuit alone will give you a signal with sufficient amplitude; i.e. close to line level amplitude, around 1 volt RMS,


like would be produced by a typical music player with a headphone jack.

If not, maybe you need a tiny audio amplifier, to go in between the microphone circuit and your transmitter gizmo.

Another possible problem you might encounter, is those transmitters for use with a car radio, are extremely weak. The reason for that is because, in your car, everything is about 1 meter distance from everything else.

So you might want to test that, to see how much range you can get, before you get too serious about whatever hardcore espionage you have planned for this thing.

Also, in the event you really want listen in, to a room that is only a few meters away, use headphones, or record it silently and listen to it later, or something quiet enough the bug cannot hear its own audio output, because if it does, then you, and likely the people you're listening to, will get to hear a screeching feedback whine, and that can be kind of embarrassing give-away that you're trying to do some listening.

I mean, that's something to think about if you're trying to be stealthy.

Final note: Do I have to mention that most people do not like being listened to surreptitiously? Do I have to mention that there are laws about transmitting too much RF power, in parts of the spectrum you do not have permission to use? I mention that one in case you might be contemplating a more powerful transmitter.

No? Well, someone will probably comment here, for to tell you what you're doing is super-duper, unethical and illegal, and you know, it is really important to nod your head and pretend you're listening, to people who like say things like that.


2 years ago

No and no again, sorry.
A mic only works if you add the required electronics to get the signal to line level - to use the audio intput of your transmitter.
The bug will then still be useless as they are designed for very, very short range only.
Even with a proper antenna they struggle to get more than 10-15m - and for that distance I just use a sensitive microphone.

Check for wirless microphones instead.
The cheap once usually operate in the FM band and can reach up to 50m, with a few modifications up 100.
The more professional onse can go much further but use dedicated hardware, meaning you need a suitable receiver as well.
Another option is to use a microcontroller, WiFi and a microphone to get the range of what WiFi has to offer on the cheap.
Last but not least there is Google.
Not hard to find to find circuits for FM transmitters from simple to pro level.
Not hard to upscale those with more powerful transistors or a dedicated amplifier either....