Here's a quick and fun miniproject that I did for the heck of it. My school recently abolished its entire telephone system, prompting everyone to throw away their old room phones. With a giant mountain of bakelite beginning to pile up in recycling, I 'recycled' one and brought it back to my room.

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Wondering what I could do with it, I got a call on my cell phone. I was playing around with the old phone with both hands and trying to talk at the same time, but I hate bluetooth headsets. After hanging up, I tried putting the old phones handset between my ear and shoulder. Fits wonderfully, and lets me use both hands while I talk. Rummaging through my parts bins, I quickly found my old bluetooth headset, and got to taking it apart. Sorry for taking these pictures after it was done, but there was absolutely nothing here a person cannot do without a multimeter and a bit of can-do spirit. closed
This project is little more than taking existing functionality and supersizing it. I needed three buttons in addition to the microphone and speaker already present in the handset. After a jount through my school's parts bins, I started laying out how everything would be wired up. closed
Probing around with the multimeter showed that there were nice big test points for all the buttons, the speaker and microphone. Soldering 30 gauge wire for the buttons, and using the wire from the handset for the speaker and microphone, I quickly wired up the board. Popping out the handset's old volume control made room for the three buttons (volume up, down, and call buttons). As you can see, all these buttons simply bypass the original buttons, with the speaker and microphone from the handset wired up to where the original mic and speaker were. closed
After pairing the bluetooth device to my cell phone, I tried a quick call. I was AMAZED by the call quality this handset delivers! It sounds clearer than my phone's stock speaker, and the listener could hear me just as clearly through the handset mic as through the original cell phones mic. Definitely a worthwhile project, especially because it's free (if you have all the parts) and it only takes about 3 hours, with most of that time being cleaning the handset, probing out the wire pads, etc. closed