The following Theremin circuit use different method than detecting hand proximity using capacitive coupling method. The oscillator of tone generator are controlled using LDRs. Here is the schematic :
LDR1 assure the frequency of this Theremin, while LDR2 control the volume level. We can lay the LDR in two boxes where we can use our hand to control the aperture of the box, admitting smooth control of light amount that display the LDR. This light is waited to come from ambient light, coming in the box through the hand controlled aperture. This optical Theremin bear a stable ambient light to bring out smooth control. The output will be discovered on a small loudspeaker, but will be in very low volume. You are able to just expand this output with a standard audio power amplifier to get improve loudness.