For a starting point I went to eBay and found a cheap pair of Bluetooth headphones for about $15
Opening up my old cassette adapter, this is what I found. The gear drive will need to be kept intact, otherwise the tape deck will think that the tape is at its end. Unfortunately this dramatically cut down the amount of space that I had to work with. Fitting everything into the cassette form factor was the biggest challenge of the project.
This is what the electronics of the headphones looked like after teardown. The electronics are pretty straight forward. I'll need to keep the PCB and the battery. To get the adapter to play through the tape deck I took the leads from the speakers and soldered them to the tape write head from the cassette adapter.
It looks like I got lucky. The Bluetooth PCB will fit along the side of the cassette without having to cut it down or move components. I cut out one corner of the cassette to make space for the mini USB port that is used to charge the device.
Here is how I arranged the electronics in the cassette. There was just barely enough space, but everything fits, and the gear drive still works.
All that was left was tricky task of reassembling the cassette. Here is the final product. the button on the top is used to turn the system on and off.
The cassette adapter works great! The gear drive is more noisy than it used to be but if you are playing music you don't notice it. I've been driving around with my new adapter for a few days now. It seems like I get about 6-7 hrs of music time between charges. I had originally hoped to be able to charge while playing but the design of the tape deck in my truck interferes with the cable. Anyway, I'm pretty happy with the results.
~Hovisbread
