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PTT Switch Box
 After
finishing the BM-800 PC Microphone
project, I wanted to be able to use
it with my handheld transceiver as well, and so some kind of
interface would be required that incorporates a PTT switch. I also
decided that it should have an internal battery to power the microphone
rather than using 5 volts from a USB port, to make it a completely
'stand alone' unit. As the BM-800 is designed to work on 5
volts, I used a 4 x AA battery box mounted on the inside of the front
panel supplying 6 volts. The preamp in the microphone only consumes
about 1.5mA so the battery should last several months with intermittent
use. Mostly though, this project is more about the different
connection cables and their pinouts.
 PTT switch box schematic (looking at rear of DIN sockets).
The switch box
houses two 5 pin DIN sockets and a 3.5mm mono jack socket. One DIN (B)
is the input from the microphone and the
other (A) is an output to a radio transceiver, with the jack being an
output that feeds the line input of a PC. There are also two
latching push switches,
one is the PTT of course and the other is an on-off switch to
disconnect the battery when not in use, coloured red and green
respectively.
A 10uF capacitor was added to block any DC voltage on the mic input of
the radio (there to power electret mic capsules) and is not used for
the PC output. The pinouts for the
DIN sockets are such that if the plugs are accidentally reversed no
damage will occur. The two resistors form a potential divider to
attenuate the drive to the rig and also act as a kind of isolator so
that
the radio's low impedance mic input doesn't 'drag down' the audio feed
to the PC, which works well as the radio requires less signal than the
PC.
 Pinouts for PTT switch box to microphone.
 Pinouts for PTT switch box to PC line input.
 Pinouts for PTT switch box to Baofeng or similar type radio.
 The PTT switch box is housed in an extruded aluminium enclosure that matches the BM-800 microphone.
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