Article :  Andy Collinson
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Description
An experimental circuit that converts light to sound. The circuit uses a solar cell as the input transducer, blocks the DC produced by the cell, but amplifies the light received by the cell. Sunlight, moonlight, torchlight all produce sound which is amplified by this circuit.
Circuit Notes
At the input a solar cell converts light to electricity. Our Sun produces a continuous stream of light, which is emitted and absorbed in tiny "packets" called photons, exhibits properties of both waves and particles. This property is referred to as the wave-particle duality.

Sunlight contains a very wide frequency spectrum of electromagnetic radiation. Although not strictly white light, the light produces a hiss, very similar to white noise and analysing the signal received shows that the power spectral density is very wide. The amplifier rolls off around 10Hz and the computer sound card rolls off noise above 20KHz. A spectrogram is shown of the noise received by the circuit.

Solar Cell
A solar cell was chosen for this project has it has a larger surface area and collects more light than a photocell. The one used in my circuit was from Bowood Electronics and delivered about 1.3V DC. The actual voltage is academic as it never reaches the amplifier and is blocked by C1. Because only an ac signal is amplified its not important which way around the solar cell is connected.

The amplifier is a two stage direct coupled amplifier, first stage operates at low collector current for low noise, the second stage is an emitter follower to drive an amplifier or headphones.

In daylight conditions in my room, output was about 40mV RMS but when using a 40 Watt incandescent lamp the output increased to 1V RMS. Care should therefore be taken when connecting the output to any amplifier or headphones. If using low impedance headphones, I recommend using a series resistor of 100 ohms or making R3 a preset, the centre of the preset connects to C2 for a gain control.

Spectrogram

The display above was a signal captured from cloudy daylight received by the solar cell. The spectrum shot was captured by the software Visual Analyser a free program available for Windows and linux available here.

Visual Analyser

The image above is Visual Analyser running on PCLinux with KDE4 desktop. The visual analyser program runs fine under linux, just make sure that wine is installed to run windows executables.

On both Windows and Linux make sure that the input level to the sound card is kept to a minimum before starting or use a resistive attenuator like the one on below:

Sound Sample
A sample of sound about 7 seconds is available here as in MP3 format.