VOX circuit for radio transceivers

VOX (voice activated transmission) activates the station’s transmission (press PTT) when it detects sound (modulation signal) at the microphone or (if the transceiver has one) at the audio input.
This circuit is useful when, say, you drive (car or motorcycle) and you can’t use a hand to press PTT to talk. You speak directly and the VOX automatically activates the broadcast.
For radio amateurs it is useful in digital modes, when you connect the radio to the sound card of the computer, but not the PTT (you cannot, do not want, the software does not have). Newer transceivers have such audio interfaces (including vox).

My YAESU FT-2400 (VHF-FM-50W) has no VOX or audio interface, so I made one earlier, presented here.
But that VOX circuit didn’t work so reliably. Either it kicked in or out in the middle of the transmission, or it never disconnected the transmitter, I found the station once on Tx after a quarter of an hour, that you could boil an egg on it.

In short, here is the new work:

Just a few details:
The final drawing that gave exceptional results

The way I did the components board.

The article that was the inspiration of the project, with sufficient explanations

and let’s hear y’all in digital.

Circuit VOX pentru stațiile radio
VOX (voice activated transmission) pune stația în emisie (apasă PTT-ul) când detectează sunet (semnal de modulație) la microfon sau la intrarea audio (dacă transceiverul are).
Acest circuit e util când, să zicem, conduci (mașina sau motocicleta) și nu poți să folosești o mână să apeși PTT ca să vorbești. Vorbești direct și vox-ul activează automat emisia.
Pentru radioamatori se folosește în modurile digitale, când conectezi stația la placa de sunet a calculatorului, dar nu și PTT-ul (nu poți, nu vrei, nu are softul).  Stațiile mai noi au o asemenea interfață audio (inclusiv vox).

Stația mea YAESU FT-2400 (VHF-FM-25W) nu are vox, nici interfață audio, așa că am făcut eu una mai demult, prezentată aici.
Dar circuitul acela de vox nu a funcționat prea stabil. Ba nu cupla, ba decupla la mijlocul transmisiei, ba nu mai decupla niciodată, am găsit stația în emisie după un sfert de oră, că fierbeai un ou pe ea.

Pe scurt, iată noua lucrare:

Doar câteva precizări:
Schema finală care a dat rezultate excepționale.
Cablajul la care m-am gândit eu.
Articolul care a stat la baza proiectului, cu explicații suficiente

și să ne auzim cu bine în digital.


  1. Hi Richto,
    just build your circuit but relay , when i put tone, turns On and imediately Off.
    After some tests i removed R8 of 10K and works normal. What’s your opinion? Lack of signal or should i change R9 by a potenciometer?
    Would appreciate help. Thank you.

    1. Hello Jose,
      R8 is not very important. It makes the circuit a bit more stable. There is R9 (sensitivity) which helps. Put a potentiometer and try to adjust to your needs. Perhaps your signal is small.
      Be careful, if you make it very sensitive, the transmitter might stay ON for a very long time.
      Let me know if it works.
      Cheers 🍻

    1. Hey Richto,
      I only now saw the error. It’s 0.2uF.
      This value is not critical, you can put whatever you have at hand, only bigger, not smaller.

  2. Can i use ne5532 instead of 358?
    I think they have identical pinout and booth dual op amp in a package.
    Thank you.

    1. Hey Ratskie,
      I did a bit of research and it looks like the specifications are even better for ne5532. It might work, but I can’t guarantee for sure.
      The design is very robust and it’s worth trying. Let me know how it goes.

  3. Buna ziua!

    On the circuit diagram when it says “to PTT” does that go to a 5v relay to mechanically complete the circuit between the microphone and the radio?


    1. Hello Joseph.
      The answer is no, this goes to the PTT pin on the transceiver. So you need to identify the PTT pin on the microphone connector (this is how I did it), an connect it to this relay contact. The PTT is activated when switched to ground, and this is the whole purpose of this circuit.
      If you need more help, please email me at yo3rak-at-gmail.com

Leave a Reply

Your email address will not be published. Required fields are marked *