Remote Volume Control

Sometimes a generally useful gadget comes out of a search for a solution to a specific problem. In this case, we were attempting to use the HauntMaster Terror Tone with a powered subwoofer.

The Terror Tone instructions recommended not leaving it on all the time, and just hitting the tone when you need it. We considered putting the CD on "repeat", and just switching the subwoofer on and off. But each time the power was applied, the subwoofer made a loud "thump" noise.

We needed a silent way to remotely turn the sound on and off at will, and hopefully change the volume.

Concept
Parts
Electrical Construction
Packaging, rev 1
Packaging, rev 2

Concept

The idea here is very simple: if you use a simple cable to plug the CD player's "line out" into the powered speakers, you could set the volume control on the speakers for whatever maximum sound level that you want. But the CD, set on "repeat" would always play at that level. If only you could put a potentiometer or variable resistor in the line, between the CD player and the powered speakers, you could turn the sound down and back up again.

Now all you need to do is find a resistor whose value can be changed remotely, and which is electrically isolated from the sound flowing through it.

The Cadmium Sulphide photoresistive cell is a simple solution to this problem. It functions as a variable resistor. Its value changes as the light on it changes. It provides optical isolation.

[This is not to be confused with a photovoltaic cell that generates electricity when exposed to light, as are used in solar power applications.]

Parts

We decided to build this as an audio patch cable with the volume control built in. We went for RCA connectors, and made it stereo. You can tune this to fit your needs.

Required parts:

male RCA plug, red
male RCA plug, black
female RCA socket, red
female RCA socket, black
clear C7 lamp
C7 lamp holder, with cord and plug
hookup wire; we used "zip cord"
(2) Cadmium Sulphide photo cells

We got the Cadmium Sulphide photo cells from Radio Shack. They sell an experimenter's package of 5 assorted cells ("CDS Photoresistor Multi-Pack", Cat.# 276-1657, $2.49 as of October 2001). Although any of the cells in the package are likely to work, I sorted through them with a multimeter, and selected the two with the lowest resistance when exposed to light.

These small lamps are intended for use inside miniature buildings used for Christmas decorations. They are sometimes called "pinch lights" because they are held in place with a springy metal clamp at the bottom.

I got this one at Dollar Tree for $1 (October 2002).

You will also need some simple electonics tools, like a soldering iron.

The end product will need some sort of housing.

Electrical Construction

Start by making two simple patch cables, each one having a male connector at one end and a female at the other. Connect the center conductor of the male to the center conductor of the female, and shield to shield. This is simply a straight through cable extender. You can even plug it in and test it at this point.

Now, take the cable and find which wire carries the signal for the center pin on the connector. Cut that wire.

Solder a photo cell in series there.

The result is a short extension cable with a photo cell in the middle. Build two.

Now take the C7 lamp assembly, and somehow arrange things so that the photo cells are forced to look into the lamp (see "Packaging").

Now, whenever the lamp is turned on, the photo cell resistance drops and the audio signal goes straight through. Turn off the lamp, and the photo cell resistance goes up, and the audio signal is choked off, or at least significantly decreased. And if you dim the lamp, you can turn the volume up and down in fine variations.

We chose to control the lamp via X-10, which gives us full control over the lamp (and thus sound) via wireless remote control.

Packaging, rev 1

This is another one of those projects that almost never happened. It was Halloween. We had already deployed most of our props. With the "just one more" glint in my eye, I hunched over my soldering iron, as the first TOTs arrived. The electrical assembly was easy.

As a stopgap packaging approach, I used some masking tape to tape the photo cells securely around the C7 lamp.

It was ugly, but it held. The lamp got hot, but didn't seem to get hot enough to pose any kind of danger.

Packaging, rev 2

The right way to finish this thing is to get a small metal "project box", just large enough to hold the C7 lamp, lamp fixture and photo cells.

Thank you for visiting. Your comments are welcome.
. . . . . .