For those of you who are trying to interface something to a rig
which has a single two-conductor plug for both microphone input
and push-to-talk connections, such as many of the handheld rigs
including the Icom IC-2AT and IC-02AT, here are three circuits
that may be useful to you:

::: Electret condenser microphone to radio:

MIC--------------+--------------------O------------> Mic/PTT on radio
 |               |                    |
 |               |                    |  <-- Shield
 +------/\/\/----+                    |
 |     (20-30K)         /  <-- PTT    |
 |                     /      switch  |
 +--------------------+   +-----------+------------> Ground on radio


::: Dynamic microphone to radio:

MIC  +-----------------------O----------> Mic/PTT on radio
COIL C                       |
     C                       |
     C             /         |  <-- Shield
     |            /          |
     +-----------+   +-------+----------> Ground on radio


::: High-level input (such as from tape recorder, RTTY AFSK generator,
     or Packet TNC output) to radio:


HIGH LEVEL OUTPUT:                                 RADIO INPUT:
                          (0.47 uf)
Audio to transmitter -------||-------+----O------> Mic/PTT on radio
                    (25K)            |    |
Push-to-talk -------/\/\/------------+    | <-- Shield
                                          |
Ground -----------------------------------+------> Ground on radio
