Adding Bluetooth Support to an FM Transmitter

19 Jul 2014

A few years ago I wanted to build an FM transmitter from a kit. I settled on the FM25B from Ramsey. It’s delivers really nice stereo quality for the price. I did a few experiments with it for a while and when I got tired of that I started using it as a way to broadcast music around my house. My main stereo system has a radio and I also have a shower radio. It’s really nice to play music or internet radio in the morning and be able to walk around or take a shower and continue to listen.

The way I had set it up before was that I just always had a hanging 1/8th cable coming off my bookshelf. This really felt like a hassle sometimes and it’s unsightly, so this weekend I decided to try to retrofit the transmitter with a bluetooth audio receiver.

Parts

I looked around on the internet and I found what I think would be a good device for this but I didn’t want to wait for shipping so instead I walked over to Best Buy and bought one of these. This thing has a battery (which we won’t need).

Since I wanted to power the device with the same power source as the radio, I knew i would need some kind of voltage regulator. I am assuming this thing runs off 3.3V because it is powered by USB so I stopped by RadioShack to look for a 3.3V regulator but could only find a 5V regulator. I decided to just try and make this work anyway.

Build

First step was to find some good points to leach off the radio. I looked around on the circuit and tested some spots until I found a place located after the filtering caps that was providing a clean power signal. I suspected that the 5V might be fine if the device had an internal regulator. I built this test rig and let it sit for a while to see if anything got hot.

Turns out it handled it just fine. Next thing I did was freeform solder the circuit then stick it into the transmitter. I ran the audio cable out the back and looped it back into the input. I can easily hardwire this later but for now it’s not noticeable.

Now when I am at home, I can just choose my transmitter in my bluetooth options and it will route all the audio to all the speakers in my house.