NOTE: This is a first cut page only. I've already made changes to the schematic, and I haven't even started breadboarding it up yet. Eventually I'll upload a revised schematic and better instructions for etching you own PC-board and everything.



Building a DIY Intercom Mixer Box for your Motorcycle

Keywords: motorcycle, Chatterbox, AutoCom, FRS, GMRS, cell phone, audio, headset

Original Design by Paul Rothenberg

Parts List

Overview

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. 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.

Assembly

Headset Modification

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.

Box Assembly

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 conversion plugs...

Circuit Notes...

Helmet mic, FRS mic, and PTT

After careful examination of the circuit around the condenser mic and consulting the WM55D datasheet. The mic can be driven by up to 10v and therefore can probably share the regulated voltage source used by the stereo amp. I was initially confused by the arangement around the FRS mic, PTT switch, and the helmet mic. A standard consdenser mic hookup looks like this. You just drive the mic with 1.5v-10v and pull the output off the positive lead with a capacitor. Now, the PTT switch is solely for the FRS radio and just throws the 10K resistor between the FRS mic and ground, putting a load on the mic circuit and sending a PTT signal to the radio. The helmet mic is always active and the cell phone is always listening, but the FRS only transmits when PTT is pressed.

Cell, FRS, and audio hookups

I hate asymmetry. Let's get that out in the open right now. Look at Paul's original diagram...  Do you see the symmetry? Neither do I! Ok, the audio source, cell, and FRS all use an easy to get 1/8" stereo audio plug. Paul designed his orginal box for FRS/GMRS radios which take the two pin (1/8" mono - 3/32" mono) earphone/mic plugs. I'm redesigining it to use radio and cell phones with the 3/32" single pin headset standard. These share a ground between the mic and speaker so the FRS input plug wiring should look just like the cell input. With the mic grounds and the speaker grounds being joined. If this becomes problematic, a separate mic and speaker ground can be achieved by linking the cell and FRS grounds to the mic ground via capacitors (0.1uf).

Stereo Amp...

Paul may have had bunches of computer speakers lying around to canabalize, but I plan to build this low power stereo amp using a chip amplifier (National Semiconductor LM386-1). I also found a 12mm stereo volume control audio potentiometer (Panasonic EVJ-C20F02A14) which I can use in the place of the two pots needed for the amp linked to above. This low part count, low power, bass boosted amp should solve my amp problems for less than $5/unit.

New Parts List by Module


Circuit Diagram
6 pin mini-din headset plug
 Motorola standard cell headset plug