Rarely does an effects company look back to the turn of the 19th century for its technological innovations, but Zvex Effects’ “Mad Scientist,” [Zachary Vex], did just that when he created the Candela Vibrophase.

At the heart of the Candela is the lowly tea light. Available for next to nothing in bags of a hundred at your local Scandinavian furniture store, the tea light powers the Zvex pedal in three ways: First, the light from the candle powers the circuit by way of solar cells, second, the heat from the candle powers a Stirling engine, a heat engine which powers a rotating disk. This disc has a pattern on it which, when rotated, modifies the amount of light that reaches the third part of the engine – photoelectric cells. These modulate the input signal to create the effects that give the pedal its name, vibrato and phase.

https://hackaday.com/2018/11/19/a-candle-powered-guitar-pedal/