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Audio Over Ride Box

Here's a little project I've been working on for a while and it seems to be working pretty well so I figured it was time to share it around. The goal of this project was to create a little box that would allow me to run several audio inputs into a small box and run one output to my motorcycle helmet. I wanted the audio inputs to be prioritized such that if a higher priority input went active, it would override the current input I was listening to. I had the following inputs in mind:
Stereo music input from high or low power car stereo. Lowest Priority. This input would be heard in both left and right helmet speakers.
Passenger intercom. Medium Priority. This input would be of medium priority and override the stereo on the left helmet speaker.
CB Radio. Medium Priority. This input is also of medium priority and would override the stereo on the right helmet speaker.
Radar detector. Highest Priority. This is the highest priority and would override both the CB and the stereo on the right helmet speaker.

The LED's on the box are (from left to right) Radar Dector, CB Radio, Intercom, Stereo Rt, Stereo Lt (yeah, I know this is backwards; just turn the box around). With no inputs, the stereo LED's (green) are always on since these are the low priority circuits. The other LED's are yellow. As the other inputs become active, the corresponding yellow LED will light and the overridden circuit's LED will turn off.

A nice feature of this box is that if I'm not using the intercom, I could run the CB radio into both of the medium priority inputs and thus be able to listen to the CB through both the left and right speakers.
Also, these inputs could be any audio input and not limited to the ones I've listed. There is one exception. If the input signal ground is not the power ground, the signal input should only be used on the stereo input. The stereo input has been designed to handle high power stereos which have 2 wires for each channel as opposed to low power car stereos which can just run a common ground for both left and right channels resulting in a three wire setup, like a Walkman.

Below are a few pictures of the box and a schematic for the circuit as well as a parts list. Thanks are in order for Tommy Melnik who is a member of the ST1100 email list. He took my rough hand drawn schematic and turned it into the nice .PDF file below. Thanks Tommy! I also need to thank Bill Kumpf who gave me some good advice along the way and an ear to bounce my ideas off. I'm a software guy with some EE background (enough to get me into this project)  and Bill is a real EE who actually helped me work out some the details. Thanks Bill!

The box itself measures   4" x 3" x 1.5"

This is the guts of the box. Relays are used to switch between audio inputs. I've experienced no trouble with relay bounce or chatter.  Since these pictures were taken, I've added 2 small isolation transformers on top the relays (only space I had for the upgrade w/o ripping things up) so that high power stereos will work with the unit. The circuit board has solder pads on the underside for each hole. The componets are wired together using wire wrap wire and solder. There is a plan to layout the artwork for a printed circuit board so that assembling these babys would be alot easier. There's no date for the completion of that project.
As you can tell, the inputs are RCA jacks. The output connector is a standard 1/8" stereo plug so you can plug in your walkman headphones, ect.  I plan to upgrade this output plug to a 5 pin DIN plug compatable with most helmet headsets (J&M ect...) and add another input for a microphone. The mic input would not be switched but just serve as a means to connect your CB's mic so that it can get run into the 5 pin DIN headset connector.

Here are the links to the Schematic and Parts List PDF files (Thanks again Tommy).
NOTE: The LM339 parts have the incorrect pin #'s for power and ground on the schematic. The correct pin #'s are : Power: Pin 3, Ground: Pin 12

Schematic PDF File

Parts List

The parts list should have two more components added to it they are as follows:

2 - Audio Output Transformers - Radio Shack P/N 273-1380   $1.99 ea.

Circuit Adjustment Instructions

If you should choose to build this circuit you will need to adjust the potentiometers to set the trigger levels for the overriding inputs.  Each input circuit has 3 potentiometers. They are as follows:

Radar In :      R8,   R6,  R1
CB In           R16,  R14, R9
Intercom In : R27,  R25, R21

Adjust all pots until only the green LED's are lit. You will only need to do this first adjustment for input circuits that have their yellow LED's lit. When only the green LED's are lit, no input circuit is tripping an override condition and the stereo input is switched to the headset.

Next, adjust each pot for a particular input, in the order they're listed above. Adjust the first pot until the the input's yellow LED lights. Then turn it the other direction until it extinguishes and the green LED relights. Then turn it a smidge more in the same direction. Do this slowly. This extra smidge of a turn is to make sure the trip level is not right on the edge. If it is on the edge, the circuit may drift a bit and you could find yourself out on a rally somewhere with the CB circuit constantly tripped with no way to get back to listening to your stereo (How do I know??). To correct a situation like that, readjust the input circuits pots, this time making the extra "smidge" of a turn a little bigger.  Take the Radar input circuit, for example: Adjust R8, then R6 and then R1.  Repeat this for the CB and Intercom circuits.  When completed, the green LED's should be lit. If you apply a signal to any of the inputs, the corresponding yellow LED should light and the input should be switched to the speaker output. Once these adjustments are done, your circuit should be tuned properly. If you wanted to fiddle a little more just to verify your settings are correct, you can adjust any one of the three pots in a given input circuit, a smidge in the "trip" direction, and the yellow LED should light.

Good Luck!.


Article By: Jeff McCracken

August 2001

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