December 27, 2025
The original idea was to modulate a variable notch filter via an optocoupled LED, creating a phaser-like effect with a distinct lack phase shift mixing. After many months and almost as many design iterations, a champion emerges: Squidin' Around!
The filter topology resembles a Wien-bridge equalizer, with the ganged potentiometer replaced with two photoresistors. This topology is inherently inverting, so preamplification was also made inverting to maintain audio polarity. I added a non-inverting buffer at the input to keep impedance high. The modulating signal is created with a 555 timer with a controllable frequency of 1-10 Hz. The signal is fed through an op-amp integrator before driving the LED, creating a smoother notch sweep across the spectrum. Everything is powered by a single 9V supply with a Zener reference voltage biasing the audio signal.
See the link below for a demo.
Filter topology.
Breadboard prototype.
PCB population in progress.
Assembled in the housing.
Demo
The original idea was to modulate a variable notch filter via an optocoupled LED, creating a phaser-like effect with a distinct lack phase shift mixing. After many months and almost as many design iterations, a champion emerges: Squidin' Around!
The filter topology resembles a Wien-bridge equalizer, with the ganged potentiometer replaced with two photoresistors. This topology is inherently inverting, so preamplification was also made inverting to maintain audio polarity. I added a non-inverting buffer at the input to keep impedance high. The modulating signal is created with a 555 timer with a controllable frequency of 1-10 Hz. The signal is fed through an op-amp integrator before driving the LED, creating a smoother notch sweep across the spectrum. Everything is powered by a single 9V supply with a Zener reference voltage biasing the audio signal.
See the link below for a demo.
Filter topology.
Breadboard prototype.
PCB population in progress.
Assembled in the housing.
Demo