A simple VCO (voltage controlled oscillator) for audio

It is possible to generate square waves suitable for audio output using a battery, Schmitt Inverter, capacitor, small speaker, and resistor. A highly recommended option is to use an OPAMP to boost the signal.

Here’s a video demonstration:

I’m actually quite pleased with the result. Throughout the rest of the discussion I will assume the use of an OPAMP. A schematic in its pure form looks like this:

The Schmitt inverter is the triangle on the left. The one on the right is an OPAMP, which has been configured as a buffer.

A good choice for R1 is a 100K potentiometer, and for C1 a 0.01uF capacitor. I was able to generate signals in the range 140Hz to 17kHz (approx).

Schmitt inverters are typically sold as “Schmitt Hex Inverters”, meaning that there are six of them on a single chip, rather than just one. A 74HC14 is a suitable choice.

An equivalent circuit to the above, but this time showing chip pinout connections, is as follow:

The relevant internals of the chips are also shown.

You can construct an LFO (Low Frequency Oscillator) using an R1 of 20K ohms (variable, of course) and C1 of 100uF. I was able to generate frequencies in the the range 0.7Hz – 280Hz with these values.

Why would you want to generate these signals at such comparatively low frequencies? Well, you could use it for blinky lights. The frequencies are low enough to be visible. But what I really had in mind was to use it as a control signal to a “stepper” module. The LFO can be used to control the “beats” of a synthesiser, i.e. how fast our synth will play.

More of that in a future post.


About mcturra2000

Computer programmer living in Scotland.
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