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I have a project I've been scratching my head over for a few days.

I work a lot with 2-way radios and like to listen to music while I'm doing it. I wear an earpiece for the 2 way radio.

I want to build a circuit box I can plug the audio for the 2-way radio into along with the audio from an MP3 player and have them coming out of the same earpiece but the 2 way radio to cut out the MP3 signal when someone talks over the radio to me so I can hear it.

I thought a resistor on the radio wire but that doesn't seem to help with the problem that I have I need the music to cut out when the radio signal comes through and come back once the radio signal has stopped.

Ideally would also like to add vol controls (variable resistors) for both on the box

Anyone got any ideas or probably something horrendously simple I haven't thought of/overlooked would be greatly appreciated.

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  • \$\begingroup\$ You might want to look up audio ducking and see if that helps. \$\endgroup\$
    – JYelton
    Jun 23, 2020 at 14:56

3 Answers 3

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I know that they sell analog mux ICs that can take in two signals and output one or the other. The fun part is actually knowing when to turn one or the other on right?

The music channel is the "main" channel, as in it is usually the one on. You could use a peak detector on the radio signal and feed that into a comparator to know when to toggle the mux channel. This way would allow you to tweak how long it takes for the radio to switch on (to avoid random noise and such), time to switch off after noise has stopped (to avoid switching to music mid-sentence), and the level at which you toggle. This is a two edged sword though, as you have to tweak it to make it work.

If you can, try recording some waveforms from the radio as it goes into your earpiece. You can import that into a spice program and try it out for yourself.

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I'm the exact same situation, 3m peltor 3m box 2341, and my goals are the same. My plan is:

  • Put diodes on cb headset wire, then route it directly to the headset;
  • Tap on these wire after the diodes, then plug it into an operational amplifier;
  • Take the output of the op-amp, rectify it to DC, add a capacitor of desired value to make a little buffer, then drive a transistor with this DC to drive a normally-closed relay which will open the music circuit.

If you are using a working security helmet like I am, you could maybe add an 18650 inside it for the circuit.

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It's called a ducking circuit, used by DJs and voiceover announcers on radio.

It's essentially a 2 into 1 mixer with level detection on the voice input reducing gain on the music input.

I'd like to draw this as a block diagram because it's important to understand it on that level before dealing with conponents. The music input and the voice input are mixed together, but the music input goes through some kind of gain control stage (a VCA). The voice input also goes to a level detector, the output of which pulls back the gain on the VCA.

In some ways the trickiest part is level detection. You need a fairly short attack time and slow release to make it work well. If you can effectively turn the voltage out into a logic level, the VCA part could even be as simple as a relay which reduces gain by a potential divider. But there are chips available, like the SSM2020, which have both VCA and detector circuit (that one is even stereo) on one chip.

You typically need a bit of trial and error to get the detection part working well.

You probably also want some kind of fixed level control (volume pots) for each signal path.

If you search on ducking circuit you will find quite some circuit ideas.

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