Needing a simple mono earpeice for the car inspired me to modify an existing Nokia 2.5mm earpiece that I happend to have spare (I actually had a couple of them.)

1) First job was to get the pinouts for both connectors, this was the easy bit (above)

2) Next I opened the Nokia microphone section, I thought it may be glued but it was just held together by clips, if you use a small enough screwdriver it is quite easy and doesn't mark the case.

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3) Now I removed the little board that had the mic and switch on it and has all the wires soldered onto it, it is quite tricky, be careful not to strain the wires.

4) Following the nokia diagram I unsoldered the wire to (3) and then cut it off, this is only used in a stereo headset.

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5) Now I de-solder the wire to (2), and then soldered onto the now empty (3)

6) Finally I soldered a little piece of wire between (1) and (2), I actually soldered to a little pad connected to (1) Reassemle.

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You are now left with a earpiece wired like the SPV diagram. Please Note: The headset I used was the original Nokia one that was supplied with the 8210 phones, I don't know how much headsets differ but I would think most are simmilar, if you have a different one you may have to trace the wire connection back to the plug