The headset is as simple and as generic as possible and includes the option for a helmet mounted PTT switch with all connections available on a 5 pin DIN plug. The 1nF capacitor is included to provide some RF bypass for air band usage.
I recommend the use of genuine Icom radio cables, which are sold as spares for their radio speaker microphones. Air band cables can be used for the UHF, but not the reverse as the UHF speaker mikes don't expose all the connections required for air band despite the pins looking the same. You may be able to get 3rd party cables off ebay, but make sure they support exposing all 3 connections on the small 2.5mm plug.
The switch box includes a preamplifier which is switched in when the air band radio is transmitting. This boosts the electret mike output to the level required for the air band radio.
NOTES ON RADIO PTT CONNECTIONS:
The 2.5mm plug is used on each radio as follows:
ICOM AIR BAND: Base=Common Ground, Ring=Mike+, Tip=PTT (Ground to transmit)
ICOM UHF: Base=Common Ground, Ring=5V, Tip=Mike + and PTT (Connect mike to transmit)
The 3.5mm plug on both radios is: Base=Ground, Ring=Serial, Tip=Speaker Out
Two preamp circuits are included. Both appear to work OK.
A strip board layout for the standard preamp is included.
The preamplifiers include a tank capacitor and load resistor to allow correct biasing of the electret mike in the headset.
Use genuine Icom plugs from spare parts for appropriate speaker mikes, or build your own plugs if you have access to a 3D printer using the instructions available at:
"If I get it wrong can I damage my radio?"
Answer: Well, that's unlikely. But you should make sure that the Timeout Timer on your Air Band radio is set to stop transmission after a minute of continuous PTT activation, in case you have a fault in the PTT circuit.
Air band circuits are prone to squealing transmissions due to RF feedback into the microphone leads. test your setup carefully to avoid this.