I want to build a simple circuit, preferably only a few transistors which simply activates when it hears a low level audio, and deactivates when the audio signal does not exist anymore.

No pre-delay and no after delay is needed or wanted. I just want a circuit to activate simply when it hears the packet from my sound card.


I like G4ILO's circuit, based on something by Skip Teller, KH6TY and described in more detail in the VOX Circuit section here.


All circuits will have some sort of latency. Not to mention the QRN and maybe QRM that could trigger your circuit unnecessarily.

So instead of designing the elusive "zero-latency with complete noise immunity" circuit here is an alternative:

Do it in software. Keep the audio path open all the time. With the right program it would be less likely to miss any part of a real preamble.

These kinds of input circuits are rarely connected to speakers so it wouldn't bother you by the white noise in between.

Doing it this way could ensure you didn't miss the preamble because of latency in an external circuit.

Of course, be sure to do your due diligence and post this same question directly into Google and see if/whom has done this before you.

  • $\begingroup$ I think the asker wants something that activates transmit when the computer outputs a packet to the sound card. In either case, I agree it's probably better to do it in software, but modern computers without serial ports don't have a convenient interface anymore, sadly. Can you suggest a simple circuit that lets the software activate the PTT via a USB port? $\endgroup$ – MoTLD Apr 15 '17 at 23:16

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