I own this simple CB Radio: http://www.cbforumas.lt/upload/manuals/alan/alan_42_um.pdf I would like to connect its 2.5mm microphone jack and my computer's 3.5mm audio out jack. I have tried simple adapters and cables, but they fail, because the 2.5mm jack requires a push to talk microphone. I'm not unfamiliar with soldering, but which contact layout do I have to use for the 2.5mm jack and what do I have to connect to what do make this work?


Most connector plugs make contact at 3 points:
1 Tip (pointy bit),
2 Sleeve (opposite the tip), and
3 Ring (between tip and sleeve).
Depending on the rig, two of these will carry audio. One of the audio lines also goes to the Push To Talk switch, with the other side of that switch having the third wire. When you push the switch, you short those lines together.
If you can't get a schematic for your radio, you'll need to figure out which is what.

Buy a 2.5mm plug, a 3.5mm socket, and some wire.
Put the plug in your CB, try shorting pins on it, until you activate the transmit.
Take it out, permanently wire up the short, then add connections to the 3.5mm socket.

Presto! Audio through, PTT remanently on. Everyone gets annoyed at you because you "hog" the airwaves.

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    $\begingroup$ As much as I enjoy electronics I would not recommend the 'try shorting pins on it' method. The corresponding schematic should be obtained and I would also verify correctness of the schematic by properly opening up the unit and inspect visually around the microphone socket. $\endgroup$ – captcha Sep 23 '14 at 3:59
  • $\begingroup$ captcha, I agree: my solution is a quick & dirty one. Personally, I also believe permanently blasting the airwaves is something that should be left to the commercial radio stations. $\endgroup$ – Alan Campbell Sep 23 '14 at 4:12
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    $\begingroup$ Thanks, hogging the airwaves is just what I want to do. Joke aside, I used a small speaker to find out which contacts were transporting the audio signal and aded a switch to toggle my input. $\endgroup$ – Andreas Hartmann Sep 23 '14 at 6:38

If you want to record into the computer, you need to connect the Transceiver 3.5 jack to the 2.5 mm jack on the computer.

If you want to send computer audio to the microphone input, you need to do the inverse, but take into account that very possibly, the microphone input of the transceiver is combined with the PTT switch.

So you should connect a capacitor in series with the microphone line, in the order of 100nF to maybe 1 uF or so. The positive side of the capacitor, in case you use an electrolytic cap, should probably point to the transceiver.

Also, very probably, you'll have to attenuate the audio from the computer in order not to saturate the input.

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    $\begingroup$ Plus add a 600Ohm-600Ohm isolation transformer to prevent ground loops that may audibly distort the signal if you're planning to run the radio off a wall-wart adapter. $\endgroup$ – captcha Sep 23 '14 at 4:03

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