Intercom

  
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This intercom box takes input from any 2-way radio (FRS, GMRS, etc), cell phone, and a stereo audio source and amplifies them for listening via a headset in your helmet. The headset is a modified Chatterbox stereo headset. The standard Chatterbox mic is removed and replaced with a noise-cancelling mic with a high density windsock.
Intercom - schematic

This should provide good wind noise rejection and a convenient mount for the mic in a full face helmet. The audio isolation transformer (L1) eliminates echo resulting from the fact that the FRS radio doesn`t share the same ground for both audio and mic. Since the cell phone does share a ground, the transformer is unnecessary. If you wish to power this box and your other devices off of bike power, have a look at Bill Bowden`s Hobby Circuits, particullarly the Variable Voltage Regulator. This curcuit will alow you to transform bike power into a clean voltage source for your FRS, CD, amplifier, etc. Cut the mic off the Chatterbox headset and solder on the WM55 mic. Insert the WM55 into the windsock and zip-tie the opening closed. Now the mic can be velcroed onto the helmet chin bar. Mount the headphone speakers and carefully tuck in all of the wires. You want to leave the plug where it`s not in the way, but easily accessable to attach the cord. Well, take the parts you`ve collected and follow the diagram below. The output of the amp and the input from the mic are connected to the hatterbox headset via a 6-pin mini din connector. Although it is not noted, I speced a 3. 5mm stereo jack for the "To FRS Audio Out" in hopes of getting a radio with the shared ground type headset plug like a cell phone. It may be necessary to use two mono jacks, one for the FRS audio and one for the FRS mic. Either way, we ought to be able to make...



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