October 7, 2025

Introduction

After working with several projects that used phototransistors, I became curious if they could be used to replace a bipolar transisor in an audio circuit.

An IR emitter is used to "transmit" the sound over the air into the phototransistor. Pulling the IR emitter further away from the photo transistor has the effect of making the signal "quieter"... maybe lowering the gain? I also tried replacing the IR emitter with a laser which sounded terrible. It's possible with some different circuitry the laser could sound okay, but for what I did it was pretty bad.

The sound of this circuit with the IR emitter was also.. not my favorite. It wasn't the IR emitter at fault, even with only the two bipolar transistors it sounded cheap, but it really needs some tuning to get a nicer sound. Either way, the IR emitter is probably kind of pointless anyway. Well, not totally pointless since if the neck of my guitar got struck by lightning while I was playing, my guitar amp would be protected from damage due to optoisolation.

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Video

YouTube: https://youtu.be/fvUKbY8ovUE

Explanation

I started by looking at one of Joe Davisson's guitar pedal designs called the Vulcan Overdrive. I wasn't really looking into making something that sounded great, so I basically just copied the transisor parts of this, leaving out the diodes and sometimes some of the capacitors. I started with 2 transistor stages that had 0.1uF caps on the base of 2N3904 transistors, a 1.5M ohm pullup on the base (I didn't have 4.7M ohm), 1K ohm on the emitter to ground, and 5k ohm resistor on the collector pulled up to 3v. It sounds pretty awful when it clips, but that's okay.

I dropped an IR emitter on the collector of the second transistor. On the other side of the IR emitter is a phototransistor running on a separate 3v battery source, 10k ohm resistor on the collector, emitter to ground, and collector separated from the guitar output jack by a 0.1uF capacitor.

Above is the main setup for this project. A Mini-Marshall battery powered guitar amp is used for the sound output. The breadboards have some extra things on them that aren't connected... but the main circuit here can be seen: two 2N3904 transistors and the IR circuit. An oscilloscope probe is connected to the phototransistor.

This is a closeup of the circuit.

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