# 555 Variable Frequency Square Wave Generator # 555 Variable Frequency Square Wave Generator

This simple 555 Variable Frequency Square Wave Generator produces a variable frequency output of 2800Hz to 120KHz with this values. PARTS LIST R1 2.2kΩ R2 4.7kΩ VR1 250kΩ C1 0.001µF (102)  mylar capacitor C2 0.01µF (103)  mylar capacitor IC1 NE555

The 555 is connected for astable operation. Here the timing resistor is now split two sections, R1 and R2+VR1, with the discharge pin 7 connected to the junction of R1 and R2+VR1. When the power supply connected, the timing capacitor C1 charge towards 2/3 Vcc through R1 and R2+VR1. When the capacitor voltage reaches 2/3 Vcc, the upper comparator triggers the flip-flop and the capacitor start to discharge towards ground R2+VR1. When the discharge reaches 1/3 Vcc, the lower comparator is triggered and a new cycle is started.

The capacitor is then periodically charged and discharged between 2/3 Vcc and 1/3 Vcc respectively. The output state is High during the charging cycle for a time period t1, so that

t1=0.693(R1+R2+VR1)C1

The output state is low during the discharge cycle for a time period t2, given by

t2=0.693(R2+VR1)C1

Thus, the total period charge and discharge is

T=t1+t2

=0.693[R1+2(R2+VR1)]C1 (seconds)

so that the output frequency is given as

f=1/T

=1.443/{[R1+2(R2+VR1)]C1} (You can get frequency in KHz, when R1, R2, VR1 in KΩ and C in µF)

Example:

R1=2.2KΩ

R2+VR1=100KΩ

C1=0.001µF

F=1.443/{[2.2+2(100)]×0.001}

=7.136 KHz

Frequency is adjusted with potentiometer VR1.

To get accurate frequency, you can use regulated power supply.

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