The Tremulus Lune by Commonsound is one of the coolest, simplest, most easily modifiable optical tremolo circuits around, which was precisely the reason why I attacked this project early on in my DIY pedal-building game. It’s easy to build, easy to modify, has a low parts count, and has a very easy-to-debug schematic full of all kinds of features. I am really the kind of musician that likes having many features and a wide range of options to choose from, enabling me to modify my sound for one style or instrument, and using the same piece of hardware for something completely different. So the Tremulus was perfect for me.
At its core, the Tremulus Lune is basically two modules interacting to produce a single effect. The first module is a simple input and output buffer centered around the TL072 dual op-amp, with a light-dependent resistor (LDR) stuck in between them. The second module is a low-frequency oscillator (LFO) that controls how light is emitted from an LED. The LED lights up at the speed, depth, and waveform governed by the oscillating voltage of the LFO, which interacts with the LDR in the buffer module, thereby proportionally controlling the amount of signal being let through or resisted by the LDR from the input buffer to the output buffer. Thus, the relationship between the oscillating LED and the LDR used to modulate the volume envelope of the input signal is what makes this an optical tremolo.
Tremolo parameters are really fun to tweak, and in my opinion deserve the freedom that knobs allow for continuous modification. Speed and Depth are an obvious must for any tremolo. Symmetry and Smoothness I believe are also necessary, because they allow you to shape the tremolo volume envelope to anything from a sine, square, triangle, and sawtooth waveforms. Spacing gives you extra control over the pulsewidth, which if that’s your thing, gives you an extra dimension of musicality from the circuit itself (letting you cross over into a sort of analog granular synthesis territory). Finally, a Gain control is crucial, as tremolo circuits by nature typically create the perception of outputting signal volume that is lower than unity—while this is not entirely true, a little extra gain helps compensate for this perception and is particularly useful when tweaking all of the other parameters.
This version of the Tremulus Lune was based on Commonsounds revision v2.4.1 of their schematic. For more history and information about the original Tremulus Lune, check out the Commonsound website. They also include information about mods and tweaks that you can use to modify this circuit.
Note that this is the price only for the circuit board. You will need to source your own parts.