Voltage Controlled Triangle, Square & Combination Wave LFO

A voltage controlled triangle wave, square wave & combination wave Low Frequency Oscillator (LFO) based around a simple 2 op-amp schematic from Ray Wilson’s/MFOS book Make: Analog Synthesizers

NOTE: ALL OF MY DESIGNS ARE DESIGNED FOR SINGLE RAIL (ie: 0–12V) AND USE 4.7K RESISTORS TO CREATE A “VIRTUAL” GROUND POINT. THIS IS USED AS THE BIAS POINT FOR OP-AMPS AND THE GROUND CONNECTION ON ALL JACKS.

FEATURES:
-Triangle wave, square wave & tri + squ. combo wave outputs with indication LEDs
-Adjustable rate from ~10 seconds to ~0.1 seconds with vactrol based voltage control (Including CV IN level adjustment)

WHY: LFO’s provide a slow moving voltage control output for modulation purposes such as vibrato on VCO’s, opening and closing a filters cutoff or tremolo effects with a VCA — the possibilities are quite endless.

HOW:

AN INTEGRATOR & A COMPARATOR:
To paraphrase Ray’s words:

The first op-amp is used as an integrator (Due to the 10uF capacitor in its feedback path) and the second is used as a comparator. The combination of these two configurations with a few extra components allows us to create a pretty nifty LFO circuit.

When power is applied to the circuit, the comparator will be saturated either HIGH or LOW due to the positive feedback path with the 100k resistor.

If we imagine that the comparator is saturated LOW on power up, then the 10uF feedback cap in the integrator will be discharged and the output will be at ground (or Virtual Ground at 6V in our case).
As the comparator is saturated LOW, this will PULL current away from the inverting input of the integrator through the 4.7k resistor and 1M pot in series with each other.
In turn, this causes the output of the integrator to begin ramping up linearly in the positive direction, as it pushes current through the 10uF cap into the inverting input, to balance the current being pulled away via the series resistor/pot combination.
The integrator output voltage will continue to ramp up until it pushes enough current through the 22k resistor, to surpass the current being drawn away by the comparators output via the positive feedback resistor.

When the voltage on the comparators non-inverting input goes slightly above the voltage on its inverting input (Virtual Ground at 6V in this case), then it will suddenly shoot into HIGH saturation.
Now that the comparators output is HIGH, current is pushed towards the inverting input of the integrator via the series resistor/pot combination, causing the integrators ramping output to change into the negative direction/ramp downwards, pulling current through the 10uF cap in order to keep the op-amps inputs balanced.
When the output of the integrator ramps down low enough, to pull enough current through the 22k resistor and surpass the current being sourced from the comparators output via the 100k positive feedback resistor, then the output of the comparator will shoot to LOW saturation again and the cycle repeats indefinitely until power is removed.

The 1M potentiometer in the path between the two op-amps allows us to adjust the rate of the oscillator. When it’s adjusted for less resistance, more current is able to reach the integrators inverting input and the output of the comparator has to rise/fall faster to push enough current through it's positive feedback resistor.
When the rate pot is adjusted for more resistance, less current reaches the integrators inverting input and the comparators output rises more slowly as it’s compensating for less current.

WAVE AMPLITUDES & COMBINATION WAVE:
The triangle waves output is sent to a non-inverting amplifier to raise the output voltage from around 4.2V to around 10.5V peak ie — ~x2 gain.

The square wave has an amplitude of around 10.5V peak due to the fact it’s made up of a comparator.

The output of both the triangle & square waves are sent to a non-inverting summing amplifier, to create a blend between the two — also with an amplitude of around 10.5V peak.

Each of these outputs then also has a transistor buffer being used for the indication LEDs.

VACTROL BASED VOLTAGE CONTROL:
An LED/LDR combination is placed inside black light-tight heatshrink and is then wired in parallel with the 1M rate pot.
A 100k pot is used as a level control for the CV IN signal, being used to light the LED in the vactrol.
Another transistor buffer is used for an indication LED for the CV IN signal.