Introduction: Re-Transmitter

I wanted to see if I could modify one of those cheap car transmitters that people use to send music from an mp3 player to a car stereo and use it to send music from my computer to the kitchen radio.

The cheap transmitter: I picked one up that had both USB and line-in input possibilities.

These are really cheaply made, but they work well enough to transmit signals over short distances, so my initial idea was to open one up and tweak the antenna and antenna driver circuit. The first thing that I noticed when I opened it up, was that it didn’t really have an antenna… or… it had a wire that acted as the antenna… sort of. But it didn’t seem like it was designed for the kind of range that I wanted to get.

Things you'll need:
x1 cheap FM transmitter
x1 BJT transistor (2n2222 worked well)
x1 100 ohm resistor
x2 220 ohm resistors
x1 22pF capacitor
x1 10nF capacitor
x1 radio antenna
x1 good times

Step 1: Disassemble and Remove Voltage Regulator

The other thing that I noticed was that, even though the thing is packaged to work with 12 volts from a car cigarette lighter, the internal circuitry only needs 5 volts to operate. So we removed the internal 5-volt regulator.

Step 2: Extend Wires

After removing the voltage regulator, and the plug for the cigarette lighter, we were left with this, the case with the circuits and display, and 3 wires sticking out the top. The red wire is the positive power lead where we need to connect 5 volts, the black wire is ground, and the white wire is the built-in antenna that doesn’t really work very well.

Step 3: Build Circuit

I used a simple BJT amplifier circuit to boost the modulated audio signal, and drive a bigger antenna that I removed from an old radio.

Step 4: Good Times

Put it all on a breadboard temporarily, plugged in some Daft Punk, and ... Success!! The little amplifier circuit increased the range of the transmitter from about 1 meter to about 10 meters.



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    44 Discussions

    Contagem de Miren este tambien

    Actually we only need a piece cable with proper length attached into proper pad of this gadget to increasing its range.
    I already try myself and measuring the distance in the open Air, with only a piece cable I got round 80 meters, as I wrote the experiment on my blog post:

    It's written in Indonesian, but there were Google Translate button if necessary.

    will give u up to 2 km rang

    Dear all get good idea from circuit


    2 years ago

    I have got one from ebay, externally is the same but it has all in one
    pcb, without usb pcb and with regulator 78M05 on the main pcb. It does not have an antenna wire and i cannot find any antenna terminal.

    1 reply

    yeah. I can't see a clear antenna lead or pad either...

    anyone has got any results in open area ?

    Really a good idea! I got 15 meters with it! I used the same model fm transmitter.


    I took mine apart and found an "ANT" terminal on the USB board, but it had nothing hooked up to it. With a 39 inch piece of wire tacked on to it, it boosted the signal over 20 dB (100 times). The scope shows a short wire receiving the broadcaster 3 feet away, vs. a local station that was of comparable power (but 20 miles away). A few dozen feet away, it's down to a similar level. Further away, back down into the noise.


    what did u use for power source??

    i have 9 volt battery and i dont know much about resistors so can u tell me how much capacity resistor should i use?? or should use anything else instead of resistor???


    1 reply

    A 5-volt power supply from an old phone.

    If you need to turn 9 volts into 5 volts, look into a LM7805 integrated circuit. I don't know how to do it with resistors...

    Unfortunately it didn't seem to change anything. Thanks for the suggestion though! I just discovered that my car's antenna is in the trunk's side, so I'll just use the larger antenna and move the whole transmitter back there.

    Great ideas! I built the circuit from your schematic, however, and I couldn't make it work at all. I used a NTE123 transistor (same as a BC547). Any ideas?

    LOL to your photo!

    Congrats man! Now I know how to amplify my antenna signal! =D