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PTT Switch Box





This PTT switch box was built as an accessory for the BM-800 PC Microphone project. Apart from adding a PTT (Push To Talk) switch, it also allows the mic to be used with both a PC and radio transceiver simultaneously by splitting the audio two ways; one being a fixed high level feed to the PC and the other a variable low level feed to the radio. It can be powered either from the +5 volt rail of a spare USB port or if available, the voltage on the mic socket of a radio (which can be up to +9V but that's fine). Because the PTT is a locking type (I like to waffle), there's an LED to indicate when transmitting. Handy to warn if transmit has been activated unintentionally. Other than that, it's essentially just a central hub for the various cables used to connect everything together, details of which are given later on with their respective pinouts. The cables shown are for the radios I currently own and use, but always consult the user manual of your radio if unsure of the exact wiring of the microphone socket, as they can differ between manufacturers.




PTT switch box schematic (looking at rear of DIN sockets).


The switch box houses three 5 pin DIN sockets. The two labelled 'MIC' and 'PC' (input from the microphone and output to the PC) are interchangable and the one labelled 'RIG' feeds off to a radio transceiver. A 10uF capacitor was added to block the DC voltage that some radios have on their mic input (for powering electret mics) and is not used for the PC output. The sockets are wired so that no damage can occur if the plugs are accidentally inserted in the wrong order. The 10K resistor and 5K variable preset adjust the audio level only to the radio and also prevent the low impedance mic input of the radio from dragging down the audio feed to the PC. This simple arrangement works really well as the radio requires a lot less signal than the PC. For the actual PTT switch I wanted something old school, so I chose a 'Chicken head' knob! The rotary switch used was taken apart to weaken the spring tension so that it requires less force to turn, otherwise if too stiff the whole enclosure would move when operated.




Stripboard layout
Inside the switch box enclosure there's a little board that hosts a group of PCB headers. They are all JST XH types except for the one labelled 'J1' which is a Molex KK type that is used as a shorting link (or jumper). The reason for all this 'socketry' is because I like to play around with this kind of thing and so wanted a modular design that can be easily unplugged, modified and plugged back in without disturbing anything else. It can of course all be hard wired if preferred. All three DIN sockets carry power on pin 3, but pin 3 of the RIG socket can be isolated from the rest by removing J1. This is so that when powering the mic from a PC, the USB 5V doesn't clash with the voltage on the mic socket of the radio. If powering from the radio, then J1 should be left in place, but the USB plug must be disconnected from the PC, again to stop the voltages clashing. The LED is a 3mm flat top cylindrical type which being flush with the front panel makes the unit look less home made, but I guess that Chicken head knob has already given the game away!







Cable pinouts for PTT switch box to microphone.




Cable pinouts for PTT switch box to PC line input / USB 5V


The cables shown below are for a couple of transceivers that I own and use but yours may differ, so always consult the user manual if unsure.





Cable pinouts for PTT switch box to AnyTone AT-778UV radio






Cable pinouts for PTT switch box to Baofeng / Quansheng type radios. See warning here





For maximum RF screening it's always best to use good quality shielded cable and connectors. I recommend the reasonably priced Van Damme Tour Grade Classic XKE microphone cable, Rean NYS322 5 pin DIN plugs and for the microphone a Neutrik NC3FXX 3 pin XLR line socket. A metal enclosure is also recommended. The one I used was purchased from Amazon for less than £10.





DISCLAIMER: PLEASE BE AWARE THAT THE INFORMATION SHOWN HERE IS FOR GUIDANCE ONLY AND AS SUCH, I DO NOT ACCEPT LEGAL LIABILITY FOR ANY LOSS, DAMAGE OR INJURY CAUSED BY ITS USE. REMEMBER, EVERYTHING IS AT YOUR OWN RISK!