Here is the schematic for a simple light-to-audio circuit you can build DIY style on a solderless breadboard. It only requires a phototransistor, a resistor, a capacitor, a potentiometer, and a 9 volt battery. Connect the output to an audio input such as a guitar amp, line input on a mixer, or the mic input of a recording device. You’ll be able to hear modulated light from things like your room lighting, IR remote controls, LED displays, optical computer mouse and much more!
What are the main limitations of this very simple light-to-audio circuit? For one, it lacks automatic gain control. Therefore, if the room light is too bright, the phototransistor will be in a saturated state, and the sound suddenly “cuts out” (just when it was getting good :D). This circuit also lacks a headphone output. To make it portable you must add an external circuit to amplify the audio and drive your headphones. But, there’s no reason you couldn’t connect it to the mic input of a portable audio recorder and get started with field recording.
You could also try connecting a CdS variable resistor (LDR) or a silicon solar cell to the input of an audio amplifier. However, for high sensitivity and clear audio, using a phototransistor or photodiode is the best solution.
The Lite2Sound PX photodiode amplifier solves the problems with the simple light-to-audio circuit. It has a headphone jack and volume control so you can conveniently take it out into the field and explore. Lite2Sound PX also has an automatic gain control (AGC) in its preamp. This lets it compensate automatically for changes in the ambient light level. This means it can work outdoors in the bright sun, or in a dark room, without having to fool around with the preamp’s gain settings.
Lite2Sound PX is an easy-to-build DIY kit, with all the parts included. It is available from Rare Waves for USD $49 (kit), or $74 (pre-assembled)