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pulse generator

555 Pulse Generator Circuit

This is a pulse generator with adjustable duty cycle made with the 555 timer IC. The circuit is an astable multivibrator with a 50% pulse duty cycle. The difference from the standard design of a 555 timer is the resistance between pins 6 and 7 of the IC composed of P1, P2, R2, D1 and D2.

The diodes D1 and D2 set a definite charging time for C1 which produces a 50% duty cycle in a normal case. The duty cycle (n) is dependent on P1 and P2 in the following manner:

n = 1 + P2/P1

If P2 = 0 (n = 100%) then the frequency can be approximately calculated with the following formula:

f = 0.69/((2*P1 + P2 + 4.7kΩ)*C1)

Pulse generator circuit diagram

pulse generator

Oscilloscope Captures

As you can see in the captures the duty cycle is not between 0% and 100% but it is within reasonable range. I’ve used a 20K for P1, 100K for P2 and 10nF for C1.

Printed circuit layout of the pulse generator

pulse generator pcb

Components List

C2 = 10µF
C3 = 0.1µF
R1 = R2 = 4.7K
D1 = D2 = 1N4148
IC = 555
C1, P1 and P2 must be calculated

48 Comments

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  • i1673687mvrht-net

    I have mounted this circuit on a breadboard with 20K for P1, 100K for P2 and 10nF for C1. Deos not work. Do not lose your time and change schematics.

  • neha-hule11gmail-com

    can you tell me a circuit for which we can keep the pulse width constant and vary the rate of that pulse.

  • itsmesiva88gmail-com

    what is the maximum range of frequency can we generate using this circuit?
    Is it possible to generate 4.3MHZ?

  • omkar

    hello guys i m designing a pulse generator using fpga and i have designed one.anyone know how to aadjust the on time by default we get 50% i want to reduce it to 25%.please reply with you idea thank you and i am using digital circuits mostly.

  • lavender

    hello there, I have tested this circuit and it is working , however I would like to know how to calculate its pulse width? when I use to calculate it with astable formula I am not getting the same answer with the simulation.

  • Ra'ed

    thanks for this good way to use 555

    I would like to ask small question .

    If I use
    1n4001
    Instead of
    1n4148

    can the circuit work in the same operation ?

  • Steve

    I made this one and it works fine, I checked it on my scope:

    http://www.rmcybernetics.com/projects/DIY_Devices/homemade_signal_generator2.htm

    Here is parts list: http://www.digikey.com/short/w7nnn

    OR… these work okay: http://www.ebay.com/itm/181096913621

  • oldbeaver

    Can some guru of the Forum help me please?

    The circuit looks ok, but I need something slightly different, and I am electronic lover, not professional.

    Help may be just tell me where they sell a proper kit for what I need, or, tell me tech proper name of what I am looking for.
    This is the desired PMW circuit:

    +B Voltage: 12-14 VDC
    Input signal 1: an almost square signal which varies from 500Hz to 5000Hz app.
    Input control voltage: 1 to 6 VDC coming from a control pot.

    Needed output: square signal with the same frequency as input (variable) width proportional to control voltage (variable duty cycle, following same input frequency).

    Thank you very much in advance.

    Oldbeaver.

  • Azman

    Dear Electro schematic,
    Hi….I have construct and build the circuit. But not working, i dont know why? I have troubleshoot the connection, i check every level voltage, the problem voltage drop at R2 is zero and at R1 is 7.5 V. The power supply is +9 V DC, rite? I dont have +9V DC, i supply 7.5 V…is it working? can I supply 12 V DC? Please help me, Thanks.

  • Kristian

    Which frequency range can it provide ? I mena from 1 Hz to ..? Without calculation just changing the values of the POT ?

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