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Two or three electronic components are used.

CdS Photo Cell Demonstration:
clara_test8.mp3 (180 kb) 04.04.07
No Distortion, Clara's Voice in, Clara's Voice Out

Updated 10.30.07

Illumination
 Volume Control
This has advantages over a hoop antenna



CdS cell is glued to bottom of glass sphere.
 Halogen lamp can be on Music Stand.

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*Do not use the R2 10k resistor if the J1 input is from my RS Ultimate 2 audio output.

This works with most theremin models lacking a volume antenna.

As your hand lowers to cover or shadow the photocell, the volume quiets. The resistance will increase to several mega-ohms. Greater than a  6" window of control from Full Volume to Total Cut Off is possible. This transition creates a clean low volume range. The crystal ball creates the best focus of the incoming light from above that I have found. This setup can work better than a the standard RCA volume hoop method. 

Note:  The lower half of the crystal ball could be painted black to block bottom light.
Features:    This is the simplest and natural volume control design!

This CdS volume control method outputs a clean & undistorted signal (sound).
Standard volume hoops can use a FET transistor or similar which can add unwanted tweak to the signal. 

This illumination setup works excellent in normal room lighting. 
Standard volume hoops require finicky tuning like the pitch antenna.

Wide "fade out range" with control and complete on or off.  
Standard volume hoops are finicky and on/off with little fade.

The Audio Input and Audio Out of this volume control works best at a Line level.

Use shielded audio cable on the out side of the CdS to your main amplifier to avoid 50/60 hz hum.
Home Depot has a package of two for $5    

R1 - CDS Photocell - remove it from any bathroom plug in nightlight, they worked the best. 

I have tried the Radio Shack 5 pack of CdS cells 276-1657 they are worthless. They did not have as effective dark to light response resistance.
R2 - Resistor is 10K 1/4 watt

To mount R1 & R2 with their input & output cords use some imagination. Start by coming up with your own method of a volume control apparatus or stand to hold everything in a position that can be easily accessible while playing.

The photocell should be mounted on its own enclosure which can allow for better volume response. (see photo above)

Having the theremin pitch control separate from the volume control allows for better placement of the two controls. Sometimes you may need a left handed volume or a further out reach to the volume for low note playing.
Note: Indirect halogen lamp light works better than incandescent bulbs to minimize any 50/60 hz hum.
         Battery powered LED lamps are the best.