This instructable will show how to make a standard bluetooth headset act as a wireless mono earpiece, using sound from any 1/8" (3.5mm) stereo headphone jack. The microphone can also be used for skype or online gaming for consoles or PC. This instructable may also be useful for those who listen to streaming audio, podcasted radio shows, or listen to audiobooks from their mp3 players. The applications for this are pretty broad, and the price is less than $20.00 if you already own a bluetooth headset.
Step 1: External Bluetooth Adapter
Back in the old days (late '90's-early 2000's), not all phones came with Bluetooth built in. This prompted companies such as Cardo to build external bluetooth adapters such as the BTA II. There are others available, but I got one of these for $.99 (used) on eBay. These can be repurposed with a little soldering to send an audio signal to any bluetooth headset, and deliver an audio signal back from the microphone if desired.
First, obtain your BTA II. Got it? Cool. Now use your thumbnail to pop the cover off. You'll see a battery, a circuit board, and the coiled wire that connects to your cell phone. Three wires connect the wire to the circuit board, on mine they were colored white, red and black. The white is +microphone, the red is +speaker, and the black is the common ground. De-solder these wires after carefully noting their orientation.
Step 2: Find an Old Set of Headphones
You'll need an old set of headphones (make sure the plug is in good shape first) or a 3-conductor lead with a 1/8" headphone plug at the end. Cut the headphone cable to your desired length and strip the outer sheathing, then the inner 3 wires. The colors of the inner wires should be something like, white red and black, or white red and uninsulated. To save yourself some headache in the next step, you may want to twist and tin the exposed wire.
Step 3: Get to Soldering!
Strip the white and red wires and twist them together tightly. By twisting these two together, we're combining the left and right audio signals into one. (As far as I know this wont cause any impedance issues, and worked for me.) The red and white twisted wire pair will be soldered to the circuit board where the speaker + was previously. The ground will be soldered to the circuit board where the ground was previously. Once your solder job is complete, place the circuit board and cable back in the housing and test it out by plugging it into any audio source. If you've paired the headset to the adapter properly and your soldering skills are good, you'll hear audio!
If you want to use the microphone as well, you'll need two monaural 1/8" headphone plugs and leads. (My recommendation is to buy this and cut the cable in half.) You will need to first label the ends so you know which is which when you're done. Strip the ends of both leads and twist the negative (usually black) together. Solder the twisted-together wire to where the black was previously. You may need to be clever here, the hole in the circuit board where these two wires are going is very small. On the lead labeled microphone, solder to where the white wire was previously and on the lead maked speaker, solder to where the red wire was previously.
Step 4: Done!
It should be noted that I haven't actually tried using this as a microphone/speaker, I just use it to roam about my office while listening to streaming audio. The fact that the BTA II has a 7-hour battery makes it ideal for portable use as well. Beside being a really cheap way to get bluetooth audio, this looks a lot more professional than having those white ipod ear buds in while at work!