Here it is demonstrated how one can build VCO(Voltage Controlled Oscillator) using 555 Timer an explains how it works. 555 Timer is a popular timer IC that can be used to build many electronics circuit application such as FSK(Frequency Shift Keying) modulator, oscillator, pulse generator, schmitt trigger etc. The 555 timer mainly operates as astable and monostable mode. The VCO is an application example of 555 timer operating in astable mode. Voltage Controlled Oscillator(VCO) is an electronics circuit which produces varying frequency due to varying amplitude. In other words when varying voltage is applied to VCO it produces signal with varying frequency. We can say that VCO is amplitude to frequency converter.
Before explaining the circuit and working principle, watch the following video which demonstrates how the NE555 timer VCO works.
The NE555 VCO on breadboard is shown below.
In the above 555 timer VCO circuit schematic, the resistors R1 and R2 together with the capacitor C1 sets the base operating frequency. The value of these passive component was selected to output a square wave with frequency of 151.66KHz with the help of online 555 Timer calculator. The input voltage at CV pin at this 151.66KHz frequency is about 2.5V which is the middle of the potentiometer.
The RV1 is a 10KOhm potentiometer which has voltage range from 0V to 5V. The potentiometer is connected to the control voltage pin 5. By varying the potentiometer we can change the output signal frequency. So when the potentiometer is roughly at the middle the input voltage is about 2.5V and the calculated frequency value is around 151.66KHz. When the potentiometer is varied such that the input voltage is 5V then the frequency is around 37KHz. And at the other extreme when the potentiometer is varies such that the input voltage to the CV pin is 0.1V to 0.3V the frequency is around 250KHz. So this 555 Timer has input voltage range of 0V to 5V and corresponding frequency range of 37KHz to 250KHz. These are simulation result. The following video shows simulation of 555 timer VCO in electronics design software. It demonstrates how the changing voltage changes the frequency of the output signal.
On building the above NE555 timer VCO circuit on breadboad, the following were the measured result. At 30mV input via the potentiometer, the frequency was around 20KHz, when the pot was at 2.55V the frequency was 118.7KHz and when the potentiometer was turned to 4.84V the frequency was around 32KHz. Higher than 4.84V or lower than 30mV input voltage the signal wave vanished from the oscilloscope.
VCO(Voltage Controlled Oscillator) are used in radio transmitter and receiver such as FM transmitter, AM modulator. It is alternative to ordinary oscillator such as crystal oscillator, Hartley Oscillator or Colpitts Oscillator.
