Most of the plug in PTT switches have two separate normally open
switch contacts. One contact switches the audio through and the other
contact grounds the transmit line.

If you want to pass the audio through for an intercom then you will
have to jumper across the contact that switches the audio so it is
always active.

On Wed, 24 Sep 2003 20:33:45 -0400, Michael Horowitz
wrote:

I have an A-21 and headphone/mic adapter.
The adapter has the male jacks at one end for the A-21,and female
plugs for the headset and microphone at the other end. There is also a
female 3mm plug which I believe would connect to a PTT switch.
Playing around with a 3mm jack, I have found that I can make the A-21
transmit a carrier by grounding one of the jack connections, but I can
not get audio to pass to the transmitter, so I'm thinking there is
more to the PTT circuitry than I originally thought.

Can someone please describe the circuitry of a PTT switch? - Mike