A modular synth is composed of various oscillators and filters that can be chained together in different ways to produce a variety of sounds. This synth is composed of a voltage controlled oscillator(VCO), a voltage controlled low pass filter(VCF), a low frequency voltage controlled oscillator(LFO), and a small amplifier and speaker. The default configuration uses the VCO to generate an audio waveform that is then passed through the VCF before being sent to the speaker. The strength of the VCF’s filtering is modulated by the LFO.
Build the VCO circuit. The LM358 op amps are single supply so they take +VCC and Ground for power. The TL072 op amps are dual supply so they take +VCC and -VCC for power. It should output a square wave centered around 0. Adjusting the potentiometer will change the frequency of the wave.
Build the speaker amplifier circuit. Connect the output of the VCO to the input of the amplifier. You should be able to hear your the pitch change as you adjust the frequency potentiometer on the VCO.
In order to build the VCF, you need two vactrols. These are made up of an led and a photoresistor heatshrinked together such that the led shines on the photoresistor. Here’s a link to a tutorial on how to make them: https://www.youtube.com/watch?v=grP3pgTkaDs. Note: the tutorial suggests using flat topped leds. I had no issues making vactrols with standard rounded top leds.
Build the LED driving circuit for the VCF. The potentiometer should control the brightness of the leds. When adjusting the potentiometer, it is best not to leave the leds at a very high brightness for long periods to decrease the likelihood that they will burn out. At this step, you should test the resistance of your vactrols. With the LEDs off, you should measure a resistance on the order of a few hundred kilohms. With the LEDS fully on, the resistance should drop to a few hundred ohms.
Build the filtering section of the VCF. Connect the VCF in between the speaker amplifier. The output of the VCO should be connection to the input of the VCF and the output of the VCF should be connected to the speaker amplifier.
When the LEDs are fully on, the square wave from the VCO should be unaffected. When the LEDS are on the fully off, no signal should make it through the filter. When the leds are partially on, the signal coming out of the VCF should look like a distorted square wave on the oscilloscope. The 100k potentiometer controls the resonance of the circuit.
Build the LFO circuit. It should output a triangle wave centered on zero. The potentiometer controls the frequency of the oscillations. Connect the output of the LFO to the input of the VCF through a 1M ohm resistor. We’re now driving the filter with a oscillating signal. This should add a woom woom effect to the sound. The potentiometer on the VCF now controls the DC offset of the signal we are driving the leds with. Adjusting this varies how discrete each woom woom sounds.
Optional Step. Build the linear to exponential converter circuit. The two npn transistors must be hot glued together. This ensures that they remain in thermal contact. Insert the converter after the VCO’s potentiometer as shown in the large circuit diagram. This will make changing pitch on the synth sound more “musical” since the frequency of notes is exponential. The input and output adjust trimpots can be adjusted to change what linear voltages correspond to which exponential voltages. I’ve adjusted mine so that 11V input gives 11V output.