# 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

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

Components List

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

##### Related Tutorials

• Felipe

Hello folks.

The circuit works perfect!.

Here the evidence

• Mike

I used the folling parts and got minim. 4 pules per second up to 30 pules per second with 30-70% duty cyle : C1 = 2,2uF / Pot1 = Pot2 = 50K

• Steve

Here is the parts list, I made a pulse generator with these parts and it worked.

http://www.digikey.com/short/w7nnn

This is the schematic:

• ishan

what is the purpose of two diodes?

• Steve

Finally found a pulse generator that actually works:

• Al

I allso try, not liked me, so I leave it. Better use TL494.

• steve

Hi, I built this pulse generator with the components listed. I used 10µF capacitor for C1. So far I only get around 1 pulse per second and barely any change when turning pot. The duty cycle seems to have better resolution throughout pot range. I swapped 100k and 20k pot with each other and got pulses up to 3 per second. What I need is a frequency range of around 5 to 200 pulses per second. The duty cycle is not that important for my project. I redid this eight times and never got the frequency to go any faster… Is this timer capable of a faster pulse rate? What am I missing here? Thanks..

• Al

R1 and R2 can be 1 kolomh or 550 omh. P1 and P2 can be 200-50 kiloomhs, then work must be best.

• steve

I would like to make this pulse generator. When I shop for the 555 timer I see an endless number of different part numbers. Are all these 555 timers the same?

• R. Smith

Would you be willing to help me with a project involving a pulse generator? I am trying to replicate a circuit that I did not create. “The pulse generator is functionally comprised of an RC-controlled, free-run oscillator, which drives a buffer amplifier stage. The free run oscillator can be adjusted in the range of 2 MHz to 5 MHz via an adjustable resistor. The buffered amplifier stage drives a clamping inverter which in turn directly gates the 5.8 GHz oscillator section of the RCR.” The RCR is a device that has an internal circuit that is bypassed by the new pulse generator.

• Jim Keith

There are subtle differences to keep in mind:
1. Package style DIP or SMD
2. Bipolar or CMOS –projects generally use bipolar unless CMOS is indicated
3. Manufacturer –generally does not matter
4. Temperature range –generally does not matter

In most applications, either bipolar or CMOS devices work equally well.

• kim

hi! i have completed a project to make a pulse generator with less than 50% duty cycle using a 555 timer and 2 potentiometers. but later on, my professor wants me to compute for the time on and time off.
so here’s the problem, the only formula i’ve got is this,
Ton=.7(Ra+Rb)C and
Toff=.7RbC,
i’ve used this formula before for astable multivibrators. But unfortunately the answers i’ve got don’t match the output from my actual pcb circuit.
My other professor told me that i have to use different formula. And again, unfortunalely, i don’t know any. So for those who knew, please answer. and thanks in advance! 🙂

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