OK-VCO 40106

I am going to build an OK (over-kill) Voltage Controlled Oscillator using six oscillator outputs from a single 40106 IC (Schmitt Trigger).

To predict the frequency range output of the IC, we will be using the following formula;

fs = n / RC

fs = 0.81/ RC at 10V supply


fs = 1.32/RC at 5V supply.

In this case, I will be using 12V supply for optimum output signals (typically, the 40106 can handle 18V max).

Given R = 100,000 Ohm (potentiometer), C = 0.47x 10^(-6) Farad

fs at 12V = 12.89 Hz (lower frequency at human minimum f hearing range threshold, 20Hz).

I am going to add a pull-up resistor of 47 Ohm in series with the 100K ohm pot. Turning all the way down to 0 ohm pot will give me 47 Ohm (increasing the frequency).

fs at 12V, 47 Ohm, 0.47 uF = 27,433 Hz (slightly higher frequency of human maximum f hearing range threshold, 20KHz).

From the analog square wave signal output, we can use multiple stage of low pass filter (capacitor to ground) with buffered output signal to shape the waveform from square to saw-tooth to triangle to sine wave signal. The output waveform can be predicted and observed through circuit simulation.

To control the frequency (pitch) output via external controlled voltage (CV) such as CV-MIDI keyboard. We can refer to Rene VCO-CV schematics at https://www.schmitzbits.de/vco4069.html

Input options for modulating this VCO would be as follows;

CV/oct (pitch), FM and Sync. Note that I will build a dedicated LFO with PWM modulation option.