I was unable to find an answer to this question, but feel free to link any additional threads I may have missed.

In my car, I have an aftermarket amp, and multiple HAM and public service radios. I would like design a circuit that would allow me to listen to music(source 1) through the stereo as loud as I want, but when audio is detected from one of the radios (Source 2), the music is muted and audio from the radios can be heard. Source 2 will likely need some sort of audio squelch circuit because the some amount of signal may come through the radio speakers even when there is no audio to listen to. My understanding of circuits is very basic and I would like to learn more about this particular subject. I need help understanding how to trigger music muting based on audio from the radios, and the switching the audio.

Ideally, all radios would be connected to some sort of audio mixer through low level headphone or RCA input to 1 output to the audio switching device as source 2. The OEM head unit output (Source 1) to the amp is high level speaker wire. Input to the amp is currently high level speaker wire. The amp can accept low level RCA inputs, but the head unit requires high level output.

Could you please recommend a path to solving this problem? I lack the knowledge at this point of the correct research terms, but I would like to learn enough to design my own circuit.

Thank you very much! Solving this problem would be extremely helpful to me.


  • \$\begingroup\$ Hi @LevisJeanz, welcome. FYI this forum has pretty tight rules about the types of questions that are allowed, and solicitations for hardware recommendations are not allowed. You can make the format of the question more specific by phrasing it in a different way. E.g. "can I use device of type X to do Y", where X and Y are both well defined. Something of the form "can someone recommend X for the sake of Y" is considered off limits by the folks who pay the bills. You can change your question with the 'edit' button. Cheers \$\endgroup\$
    – Pete W
    Feb 19, 2021 at 20:55
  • 1
    \$\begingroup\$ Thanks for that input Pete. My intention is to build my own circuit, I have rephrased my question accordingly. \$\endgroup\$
    – LevisJeanz
    Feb 19, 2021 at 20:58
  • \$\begingroup\$ These are called VOX circuits or Voice activated switches. You can find plenty of circuits online for that. \$\endgroup\$
    – Passerby
    Nov 29, 2021 at 0:21

1 Answer 1


If I understand correctly, you have multiple sources for your car sound amplifier, and you want certain communications radio outputs to automatically override the main music source (the "head unit") in feeding that amplifier.

One way to start is to make all those sources reasonably compatible. You can find "speaker to line level output" converters inexpensively. This will convert your head unit speaker outputs to line levels, to roughly match the comm radio outputs.

Now you need to design a circuit that accepts multiple line level inputs, senses audio on some of them, and automatically switches that input to the output. The sensing circuit will need a diode and capacitor to convert the average audio level to a DC signal. You'll want control over the sensitivity (creating a squelch threshold), and time constants for turn-on and turn-off.

You might be able to do this all in the analog domain but it would be more educational for you to use a microcontroller like an Arduino at the heart of it (search Arduino and find tutorials on microprocessors). You will also need to learn about voltage regulators and filtering to power the microprocessor in a 12V auto system.

The microprocessor analog inputs can do the sensing. For switching the audio, research "CMOS Analog Switch". Create an initial design and bring it back here for review. It would be a very educational project.


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