Thanks very much for the help, Rob and Gil.? It's making more sense now.? I changed the circuit to this:
(from??schematic, page 6)
I thought that the point of the 4N25 opto-isolator was to separate the PTT ground from the circuit ground, but in my radio G and MG are the same.
With measured ~110mVac at the sound card output (my phone with spotify), I get ~500mVac at PTT but the led doesn't light.? When I take the phone away it doesn't change.
I'd be grateful for ideas on how to debug this.? Also I'm not very clear on what the LM158 op-amp is is for, besides increasing the input voltage.
