Here is the schematic,
I used the CTS line on the RS-232 (yes my new PC still has a hard-wire RS-232, you can still get them on Lenovo) I haven’t tried a USB to 232 converter but it should work in windows, wine in Linux doesn’t support them. The only thing I wish was a way to force RTS high via echolink, this would eliminate the need for a battery.
PIC of the setup
5 V from USB port, I tried this too much noise, tons of noise over mic
V+ from Mic port, voltage to low only 2.5V could not trip CTS pin all the time.
Yeasu Mic (MH-45), for some reason they use a transistor output on PTT line ???