Powering FM Transmitter via USB / Other Sound Options


Does your car stereo deck have an AUX input? If so, then you’re final step is simple: connect the computer audio output to the deck input.

The stock stereo in my ’04 Civic doesn’t have an aux input, so I’ll need to follow another route.


An auxiliary input adapter is a great option if you want to spend around $80-$100, but I want to do this on-the-cheap. Otherwise I could have just bought a Zune or iPod (albeit a much less interesting venture).


I’m using a Belkin FM transmitter to broadcast the computer music over 88.7 FM (if you’re really keen, get a bumper sticker that says “ON AIR” to advertise your new station!). There’s one major problem with using this particular FM transmitter:

  • It only runs with AAA batteries



The Solution: USB Power

AAA batteries produce 1.5 volts each, so we need to create a 3 V power source using the 5 V USB port. There are four wires that make up a USB cable; the standard is that the red and black cable make up the 5V / ground, and the other two are used for data communication.


Using a simple voltage divider we can drop the 5 volts down to about 3 volts.



We can superimpose our new “battery” into the FM transmitter. I could wire this all permanently, but I prefer to be able to remove it if necessary. I trimmed a K’NEX rod to the size of a AAA battery and covered the ends in aluminium foil to act as terminals. I shorted other battery terminal using a nail. Shorting isn’t bad in this case; all it’s doing is eliminating the need for a second battery. One 3 volt battery is effectively the same as two 1.5 volt batteries.


The Result:


I also wrapped the FM transmitter in a loop of wire to improve the range.




One Final Note: Maintenance

Adding in new songs is not a problem. I can bring the computer back inside and remotely drag over songs/edit playlists by plugging it into the network. Another option is to use a crossover cable or wireless USB stick and directly connect to the computer with my laptop.




And, with that, the project is done! It works great, and I can forever forget about burning music CDs.



1. Introduction

2. Computer Software & Setup

3. Wiring & Hardware Installation

4. Powering FM Transmitter via USB / Other Sound Options

  1. Peter
    September 2, 2009 at 9:20 pm

    This is very interesting. Your project sounds like a lot of fun. Thanks for the detailed write up and the excellent collection of photographs.

  2. Amrit
    September 2, 2009 at 6:36 pm

    This project looks great.
    Good job and nice car, have fun!

  3. Graeme
    September 10, 2009 at 9:45 am

    Nice write-up, Michael!

    I noticed that in your voltage divider circuit diagram you show 50 and 100 Ohm resistors. In the photo I see that you actually did use a 100 Ohm resister. It would be more efficient using larger resistor values when you make a voltage divider. The resistors are just dividing the voltage and smaller resistors draw more current (V=IR). This is waste current that is going straight to ground and thus your circuit is drawing more power than it needs.

  4. November 4, 2010 at 4:41 am

    most cheap USB Cables are not very durable and it can cause errors in data transfers too ::

    • November 4, 2010 at 6:10 am

      In this case, the USB cable solely provides power (the two data wires are not used).

      The FM transmitter is broadcasting the audio from the sound output jack on the computer.

  5. Joe
    February 20, 2012 at 8:45 pm

    That specific FM transmitter is a Belkin transmitter. In fact, it is the only version of the belkin fm transmitter that provides the best reception on the selectable fm stations and also properly transmits the bass and treble. I’ve tested several other fm transmitters and found that they either offer poor reception or poor audio quality. I’ve been working on modding mine as well. I’ve hardwired a line inside on the battery terminals and replaced the audio input plug (it wore out to the point that it frayed). I’m going to use a car adapter to usb port device and then drop the voltage from 5v to 3v. I find that this specific transmitter responds well to a variable voltage (3v +/- .5) If the sound comes out of your speakers with a extra resonance in the bass bump, then the voltage is a bit too high. On the other hand if the voltage is too low it will only affect the transmitters radius.

    • February 21, 2012 at 7:16 am

      Very interesting about the voltage level versus sound quality – I’ll have to experiment with that myself. The dedicated car USB adapter is a good idea as well, and certainly more efficient than the linear regulator / voltage divider that I’m using. Thanks for sharing!

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