Re: Circuit for second PTT that will mute Mic but allow line-in...

Rob French (KC4UPR)


Thanks for the schematic!  That all makes sense to me, and it matches the math in your previous post, so I appreciate putting a picture to the words.

I know in the past I have connected my sound card output directly to my mic input without a trim pot any sort.  But I also haven't done any of the TX preamp adjustments to my uBITX that I've seen, so perhaps my audio, even if higher than the mic, was still fine getting into the TX stages.

As far as (potentially... haven't made my mind up) removing R1/R2 and replacing D2 with a trimpot (essentially a blend control, is what I think you are proposing?), I'd actually considered using an op amp mixer to blend the two inputs.  Your version would certainly be simpler.

If I wanted to ensure I had headroom for higher amplitude audio signals, could I replace the +5V with +12V?  Here's my thought.  Down where you have the Mic PTT, of course switching that to ground is how the PTT would work.  But, I was thinking I could have a diode connected between the Raduino and the switch.  The cathode end would be connected to the top of the switch, as would the "Mic PTT" input in the schematic you created.  Voltage present when the switch is open (+12V I think) wouldn't exceed the reverse voltage of the diode.  When the switch is closed, that would ground out both the Raduino PTT pin (via the diode) as well as the Mic PTT line in the circuit above.  Forward voltage drop across the diode would still land me in logical-zero land for 5V logic.   I'm sure I'm missing something here, but it makes sense in my head...

Thanks again for putting this together!


Join to automatically receive all group messages.