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# 1 Hz Pulse Frequency Generator with 555

Here is a 1Hz pulse/frequency generator using the popular timer IC 555 which is wired as an Astable Multivibrator. The output pulses can be indicated visually by the LED. An Astable Multivibrator, often called a free-running Multivibrator, is a rectangular-wave generating circuit. Unlike the Monostable Multivibrator, this circuit does not require any external trigger to change the state of the output, hence the name free-running. This circuit can be used in applications that require clock pulses.

## Schematic of the 1Hz Pulse Generator Circuit

An Astable Multivibrator can be produced by adding resistors and a capacitor to the basic timer IC 555.The timing during which the output is either high or low is determined by the externally connected two resistors and a capacitor.

Clock: A clock is simply a square wave i.e. alternate high & low states. Each alternate high-low forms a clock cycle with a specific frequency & duty cycle. Frequency is the number of cycles completed in 1 sec & duty cycle is the ratio of the time period of high state to the time period of the low state.

We can set the 555 to work at the desired frequency by selecting the right combination of resistances & capacitance.

Frequency = 1.44 / {(R1 + 2R2) * C1}

Also, 555 can produce waves with duty cycle else than the 50 % cycle.

Duty Cycle = (R1 + R2) * 100/ (R1 + 2R2)
where duty cycle = Ratio of time period when the output is 1 to the time period when the output is 0.

Circuit Working

Capacitor C1 begins charging toward VCC through resistances R1 and R2 (VR). Because of this, the charging time constant is (R1 + R2( VR)) C. Eventually, the threshold voltage exceeds +2/3 VCC, the comparator 1 has a high output and triggers the flip-flop so that its Q is high and the timer output is low. With Q high, the discharge transistor saturates and pin 7 grounds so that the capacitor C1 discharges through resistance R2 (VR) with a discharging time constant R2 C.

With the discharging of capacitor, trigger voltage at inverting input of comparator 2 decreases. When it drops below 1/3VCC, the output of comparator 2 goes high and this reset the flip-flop so that the timer output is high. This proves the auto-transition in output from low to high and then to low. Thus the cycle repeats.

##### Related Tutorials

• nickn2011yahoo-com

How would you generate a 2 KHz tone soundwave with a 555 timer

• Michel

Now, here is the schematics

• Michel

Hello,

I get some kits “gene 1KHz sinus” (see Schematics).
I would like to change the frequency down to 440 Hz.
I think it needs to change C3, C4, C5, C6 value, may be also R7, R4, R5, R6 but I d’ont kow how to calculate the new valors.
Any body can help me?

Cordialement,
Michel

• Guest

How you guys find the r1 and r2 in the frequency of 555 timer?

• Derek

Great circuit! I having troulble with one of my homework promblems that involes setting up a 555 timer in astable mode and using a potentiometer to get certain frequency ranges. The problem is I don’t know where to start. If you can help please email me.

• anon

How would the two circuits differ if SR latch is implemented using NAND gates or NOR gates?

• Kristian

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

• Rob

Sir, what must I change to make this a variable output from 0…1kHz? The duty cycle must be 50%, no variable PWM needed.

Thanks,

Rob

• Jim Keith

As most know, the 555 is generally considered a poor choice for a linear VCO, however…

Will post a circuit that does this shortly. Unfortunately, it is busy with an LM358 op amp current source and an output D-Latch to provide 50% duty cycle. It uses the “Inverted 555” trick that I previously posted. However, it should have good linearity over the range of 1HZ to 1kHZ –single power supply.

I also have on the back burner an XR4151 VCO that provides excellent linearity to above 10kHZ, but it requires ± power supplies.

Beyond this, I have an entirely new VCO concept that has not yet left my cranial simulator –it uses an LM339 and single power supply.

• pushpa

i would be very thankful if someone suggests me how to generate 1Hz pulse with 20% duty cycle .Is it possible to generate a pulse less than 50% duty cycle with 555Timer?

• Tomek

You need to use a diode placed in parallel to VR. The values of resistors are: R1=29 kOhm, VR=115 kOhm

• tzaii

I enjoy the simplicity of the layout. I am curious of the amount of voltage at the output. I am looking for a way to create 1 to 4 hz and a voltage output of about 100 to 200 mVolts where the amps don’t exceed 3 mAmps. I could throw together this schematic which I probably will do when I get to the coast of California in a day or two but in the meantime, if anyone as this assembled perhaps you could throw a multimeter on it and let me know.
All my best,
tzaii

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