Pulse-R Solar Panel Microphones

A few years ago I came across the work of UK-based artist Frazer Merrick, in particular his use of small solar panels and bicycle lights in experimental music performance. Inspired, I set out to make a few such solar panel microphones of my own.

When photons strike a photovoltaic solar panel, the panel converts that electromagnetic radiation into DC electric current. If you were to connect the output of a solar panel to a speaker and put it out in the sun, no audible result would be produced (though you might see the cone of the speaker move momentarily), because the voltage output of the cell would be relatively static (as the radiation from the sun is rather consistent on a clear day). However, if you replace that sunlight with a light source that oscillates at a fast enough rate, you would begin to hear the resulting voltage oscillation as an audible tone. LEDs are one such oscillating light source: even though they appear to us as a consistent light source, all LED’s flash on and off at very high rates. The output of these LEDs resembled that of a square wave generator, and in fact when shone on a solar panel, the sound output is very close to a regular square wave. Fluctuations in brightness, such as those found in the pulsing modes on some LED bicycle lights, are achieved by pulse-width modulation of the LED output, which in turn translates to a PWM effect in the sound output as well. The frequency of these lights is not consistent: they appear to be set by manufacturers to a wide range of values for reasons that are beyond my comprehension, but conveniently most tend to oscillate around 200-1000 Hz, easily within the range of human hearing when converted to sound.

The Mechanics

Construction

The construction of these devices is exceedingly simple. While Frazer’s design incorporates a simple preamp into a custom circuit board, I have found simply connecting the positive and negative leads of any cheap 1-2V solar panel directly to an audio jack to be perfectly effective, with the output level of the device not dissimilar to that of a hot guitar pickup. I designed a simple 3D-printed housing for mine, and made a few to use in stereo and as modulation sources for my Eurorack system.

Performance

These devices make for an unusually expressive performance device. While the timbre produced by most LED lights is not particularly unique, the range of musical results possible by using different lights together with a looper, exploiting flashing modes for phasing rhythms, and varying the distance of the light source to panel to control volume is really staggering. As a performance interface, it is very easy for the audience to understand the relationship between the flashing lights in one’s hand and the sounds they hear (though I have found that the sound generation mechanism itself requires a bit of explanation prior to performance or in program notes), and as a performer I find them to be very expressive. The lack of pitch control and the non-equal-tempered nature of the pitches found in most LED lights make them a sort of a microtonal found-sound instrument. This can make them challenging to perform with other pitch-quantized instruments, but with a little bit of curation and experimentation I have found it possible to do so.