Easy, Simple Voltage Controlled Oscillator (VCO) or Frequency Modulator (FM)

So as an extension to my post on easy amplitude modulator circuits, we can use the same method in order to make a very simple voltage controlled oscillator. There are a few oscillator circuits that could be used but for now I will use a simple schmitt trigger relaxation oscillator.

See my 'Circuit Help Index' page for further help on these circuits.

As can be seen in the circuit diagram, a variable light source such as a bulb or LED can be used to control the LDR (Light Dependent Resistor) and ergo control the RC timing constant of the timer thus altering its frequency.

The output of this circuit is a voltage controlled square wave or triangle wave depending on which side of the resistor you take the voltage from, just remember not to offset the frequency by loading the circuit.

There are other circuits you could use this for as long as a resistor is used to control frequency. Examples include 555 astables and transistor based astables.