Circuits

What are some DIY electronic circuits you can build to listen to light? This page shows you some examples.

Light-to-Audio Circuit

Here is the schematic and solderless breadboard layout for a basic light-to-audio circuit.  Its only components are a phototransistor, resistor, capacitor, potentiometer, output jack and a 9 volt battery snap.  Connect the output to an analog 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 and more. Or you can use it as a pickup for experimenting with optical tonewheels.

Solderless breadboard version of the light-to-audio circuit. The light sensor, a phototransistor, is shown by the arrow.

What are some limitations of this simple circuit?  Its noisier than a good photodiode circuit.  Also, it lacks automatic gain control. So if the light is too bright, the phototransistor will be in a saturated state and the output will go silent.  This circuit also lacks a headphone output.  To make it portable you may want to add an external circuit to amplify the audio so it can drive headphones.  Or just by itself you may connect it to the mic input of a portable audio recorder and use it for 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 good sensitivity and clean audio, using a phototransistor or photodiode is the best solution.

Original Lite2Sound

The first Lite2Sound was powered by a 3V lithium coin cell and featured a photodiode on the tip of a flexible bend-and-stay wire. The circuit had a simple tone control and a 1/4″ audio line output, but lacks a headphone jack or automatic gain control. The schematic is shown below.

Lite2Sound PX

The Lite2Sound PX photodiode amplifier removes some of the problems in the simple light-to-audio circuit. It has a headphone jack and volume control for portable use. It also has automatic gain control (AGC) in its preamplifier. It compensates for variations in the ambient light level.  Otherwise without AGC, the sensor preamp will overload and go silent when the light intensity is too strong.

Lite2Sound BC

In collaboration with Boldport Club, a special DIY kit version of Lite2Sound was manufactured in 2018. Its circuit is more basic than Lite2Sound PX because it lacks automatic gain control, although it’s very portable and capable of capturing interesting sounds. Lite2Sound BC fits in a mint tin styled case with color graphic sticker and a fun PCB, designed by Ben Barwise. (web link)

Here’s the schematic for Lite2Sound BC. It is deliberately minimal. A transimpedance amplifier is AC-coupled to drive an LM386 audio power amplifier which gives its output to the headphones jack.  To listen without headphones, you can make soldered connections to an 8 ohm dynamic speaker instead.

Summary

The circuit schematics shown above illustrate several practical photodiode amplifiers for listening to baseband modulated light. These use a PIN photodiode for light to current conversion, a transimpedance amplifier for current-to-voltage conversion, and an AC-coupled audio gain stage.

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