1
$\begingroup$

I'm looking for the cheapest possible way to add digital mode operation to an existing laptop(with TRRS headset jack)/baofeng UV5R combination.

The specific solution I'm looking for help to achieve (although happy to hear comments about cheaper pathways) is to build a passive (as in unpowered) vox circuit on the redundant channel of the TRRS jack and use it to close to ground the baofeng PTT sleeves.

What would the said vox based PTT circuit look like?

I can probably write a driver for hamlib to interact with and create a loud 1000Hz tone for the vox circuit later (and happy to add to the question after I do).

I've looked at the following SE questions but they don't quite scratch this itch:

vox-mic-that-emulates-ptt almost describes it enough but its a broken link.

getting-a-ptt-signal-for-digital-radio-kit-right-channel-ptt describes how one might try. Has any one this work? Looks more complicated than I would have thought it needed to be, and it doesn't appear the solution is an analog passive circuit which would be important, so it was a single cable and not hacky split cables.

you would need need a circuit with something like an envelope detector and a comparator, driving a transistor that connects to the actual "ground to transmit" line on the radio. Ideally with a potentiometer to control the comparator's reference level so you can tweak the sensitivity.

Finally how-to-make-a-ptt-button-for-baofeng-uv-5r shows the wiring at the baofeng end but nothing about the TRRS side or the vox circuit needed.

Improve this question
$\endgroup$
4
  • $\begingroup$ Any reason not to use the in-built VOX on the UV5r? That way you don't need to do anything other than send audio to the HT through the mic jack... I suspect that is the cheapest way to do. $\endgroup$
    – webmarc
    Commented Oct 8 at 19:51
  • $\begingroup$ @webmarc I have read that the uv5r vox tail is too long for many digital modes, whereas I know it works fine with a PTT such as digirig. $\endgroup$
    – Mr Purple
    Commented Oct 8 at 20:45
  • $\begingroup$ Sounds like a good reason to me! $\endgroup$
    – webmarc
    Commented Oct 8 at 21:41
  • 1
    $\begingroup$ @MrPurple I assume we're talking about AX.25 Packet / APRS, not WSPR or PSK31? It's a good idea to use the spare audio channel. Another idea: Consider just making a faster-vox circuit yourself, triggered by the regular audio burst. Then you could change the tail speed as you wish. Two concerns: You may still need to add a sound burst before the packet, if the radio doesn't switch to Tx fast enough (but packet has quite a long starting tone). Also, is there enough power or voltage availailable from the Baofeng microphone bias output, to turn on a transistor/mosfet for PTT? $\endgroup$
    – tomnexus
    Commented Oct 9 at 16:41

1 Answer 1

Reset to default
4
$\begingroup$

Attached below are two examples for a galvanically isolated VOX-PTT that needs no power supply.

The circuit at the top is fast, less then 1ms to close the PTT switch and about 3ms to open it. It is based on a patent (US4438356) but I had to do some mods to the circuit to make it work. The NMOS transistor should be a small super or ultra logic level type. The JFET is keeping turn-off time as small as possible. The impedance ratio of the transformer depends on the available max. audio level.

The second circuit uses a photovoltaic driver instead of a transformer. It needs a certain min. audio level to be able to drive a few mA through the LED and generate enough voltage at the gate of the NMOS. The MOSFET should also be a super/ultra logic level type. With around 4ms/6ms for PTT-ON/OFF it is a tad slower than the first example. The time the radio needs to switch to TX and back again is of course on top of that. Some radios are quite slow in this regard.

enter image description here

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .