As varying input voltage changes output frequency of the VCO, it is also known as “voltage-to-frequency converter”.

As shown in the following figure, the Pin 5 terminal of a VCO is the voltage control pin. This pin regulates the trigger to threshold the applied signal levels. Usually, the applied VCC voltage at the supply pin is more as the control voltage will be two-third of the supply pin voltage. The in-built voltage divider causes this division. The pin can also have direct voltage from an external source through a voltage regulating potentiometer. The time for capacitor charging and discharging shall increase with corresponding voltage increase, while the reverse is true in case of decreasing the applied voltage. Hence, the frequency variation is directly reciprocal to the applied voltage, also known as a control voltage. A potentiometer or a transistor circuit output can supply such input control voltage.

A chip called “555 timer IC’ is the core component of a voltage-controlled oscillator. Accordingly, its use as an oscillator requires its configuration, before using it, for forming a stable multi-vibrator.&nbsp.A stable multi-vibrator is a timing circuit that has an output of constant oscillation, between the given logic of high and low, without any stopping in between. This produces a complete chain of pulses.

While the circuit with only standard “555 timers” cannot be connected to any external voltage supply, the “555tmer IC” circuit has the capability of being connected to such voltage supply source through pin 5. The user can regulate the threshold voltage through this pin, which is the control voltage pin. The threshold voltage at pin 5 is compared to voltages prevailing at pin 2 and pin 6 through the in-built voltage comparators. The in-built flip-flop circuits are usually controlled by the outputs received from these comparators, as these circuits toggle the output of the 555 timers. Adjustment of a control voltage applied to pin five results in the 555 timer output frequency variation.