How to convert a 2.5mm 4 pole to a 2.5mm 3 pole

[Scruffy]

Is it possile to make/buy a conector to convert a 4 pole Nokia, to use a 3 pole headset. If I plug the 3 pole in there is no Mike.

[Tom]

There is a way, but you'll need to solder up a converter:

On a standard 3 pole 3.5mm jack, tip (1) is left audio, (2) is right audio and (3) is ground - that's the longer ring. On your 4 pole you'll have (1) is left audio, (2) is right audio, and (3&4) are mic and ground, though I'm unsure which is which. The problem is that you're connecting mic directly to ground with the longer ring touching both contacts. I'll assume that (3) is mic and (4) is ground, though you'll have to check that :-\.

If you get a 4 pole plug and a 3 pole socket, both 'in-line' type, and a short length of 3 core signal wire, you can wire 1 - 1, 2 - 2, 4 - 3, and leave pin 3 on the jack plug unconnected. That should allow the mic on the phone to still be used normally, but give you L & R audio through the headphones/stereo/whatever you choose to plug it into.  :)