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The control voltage is two-thirds of the VCC voltage applied at the supply pin

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The control voltage is two-thirds of the VCC voltage applied at the supply pin. This is so because of the internally built voltage divider. This pin can also be supplied with an external voltage directly through a potentiometer. This external voltage can be varied by varying or adjusting the potentiometer. Increasing or decreasing the applied voltage make lengthens or shortens the capacitors charging and discharging time. Therefore, it can be deduced that the frequency can be varied by successfully varying the applied input voltage (control voltage). This control voltage can be availed to the pin through the potentiometer. Incidentally, it may be availed through a transistor circuit’s output.

The voltage-controlled oscillator uses the 555 timer IC as the core component this IC should be configured first to form an astable multivibrator before it is used as an oscillator.&nbsp.An astable multivibrator can be defined as a timing circuit with an output that oscillates between the logics low and high without stopping, as a result, it creates a full train of pulses.

The main difference between this circuit and the standard 555 timer circuit is that the 555s pin 5 is connected to an external voltage supply.&nbsp. Pin five is the 555s control voltage pin. This pin allows the user to adjust the threshold voltage. This is the voltage that pins two and pins six are compared to by the internally built comparators. The outputs from these comparators normally control the internally built flip flop circuits that toggle 555 timer’s output, then, adjusting the control voltage at pin five varies the frequency at which the 555 timer’s output is toggled at. The increased voltage at pin 5 decreases the output oscillation frequency. Decreasing this voltage increase the oscillation frequency at the output.

Comparing the waveform of Vout with the waveform of Vc for the NOR gate based oscillator.&nbsp.