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NE555 oscillator – 555 timer chip can achieve  clock pulse, time delay circuit, or PWM (square wave) using only a few passive components.

The pin diagram of this integrated circuit with a total of 8 pins appears below;


NE555 can work between 4.5V – 18V
Thanks to its wide operating voltage range, it can be used both TTL  and CMOS ICs. 200 mA current can be taken from OUT pin number 3 of the 555 timer.


The 555 timer ic can be operated in two different ways;

  • Monostable
  • Astable

The common feature of the two operating system is that the capacitor C changes as it charges and discharges over the R resistor,  creating a square wave (PWM) from the output pin.

Monostable Multivibrator



The monostable circuit works with 1 resistor (Ra) and 1 capacitor (C)

Briefly, the logic of work is as follows;

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If the voltage applied to the trigger input (Pin 2) of the 555 oscillator is less than 1/3 of the supply voltage (Vcc), a pulse occurs at the output (Pin 3).

The pulse ends when the voltage of the capacitor (C) equals 2/3 of the supply voltage.

  • Duration of the pulse is determined by the formula below according to Ra and C values.

555-timer-circuitAstable Multivibrator

In this mode, trigger input (Pin 2) and threshold (Pin 6) are connected. Thus, unlike monostable circuit, NE555 works as a self-triggering multivibrator. Capacitor C is charged through resistors Ra and Rb while it is discharged through resistor Rb.



  • You can calculate the duration of the signal received from the output (Pin 3) according to the following formulas according to the values ​​of Ra, Rb and C.
High Time : tH=0.693(Ra+Rb)C
Low Time : tL=0.693(Rb)C
Period : 0.693(Ra+2Rb)C
Duty Cycle: (Ra+Rb)/(Ra+2Rb)

555-pulse-generatorIn the sample calculation;

Ra=4K7 = 4700 Ohm
Rb=1K5 = 1500 Ohm
C=22u = 22 micro farad

According to these values, the following oscilloscope image is formed.



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