Pass the line level audio signal through a CdS cell

This could be wired up on a generic pcb board. The LED and Cell can be placed inside a 3/16" id, 1.3" long black piece of surgical tubing. The tube flexibility allows better control over shading.

This is soft so your finger can pinch off the light passing through for natural volume quieting. You want to use a CdS Cell with a high off resistance, 1M ohm is typical. This could mimic an actual volume loop sound perfectly. 

This image below shows a small circuit board "the manual finger volume" placed next to the Popper Button. In practice either method/finger can cutoff the theremin sound with much greater attack than a volume loop. This is due to the little distance traveled by a body movement to achieve the results

Possibly the LED & CdS mounted in a support with a wrap around elastic wrist band would be effective for finger pressing.

This 3.5mm TRS breakout cable to 1/4" is used for audio In and Out.

Designing this as a belt/waist mounted setup seems interesting.

With a Pitch Only theremin using the complete Pitch Popper to the left, the middle finger holds the sound quiet so the index finger can pop-on the notes using the red button. This could become as natural as tapping out a tune on your kitchen table top. The white LED uses 13 ma @ 6 volts with four AA cells, they should last a year or two, if they start getting weak so will the sound.

< Image is 2x actual size

The main red Popper Button has the advantage of "Popping On" the sound and it can be triggered by several other electronic methods, other than a big red button. 

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