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    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

    555 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.

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
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    25 Responses to "1 Hz Pulse Frequency Generator with 555"

    1. Thanks for circuit but how can improve the circuit.
      Which give better stability in frequency?

    2. If we use IC SCL 1444A circuit with a 32.768 kHz crystal better stability can be obtained

      • dknaoremcha says: on November 4, 2012 at 8:40 am

        Sir, I have a 700mA LED driver which has a pin to supply less than 500Hz from a PWM if I want to dimming the led in 1:100%. Can you please provide me a PWM circuit.

        With regard,
        Debenkumar Naorem

      • Jim Keith says: on November 4, 2012 at 12:26 pm

        I think you can adapt this circuit

        Adjust frequency to 500hZ by selecting C2 and eliminate C1.

      • dknaoremcha says: on November 4, 2012 at 4:24 pm

        Thanks Sir, but isn’t there something easier. This vaiable capacitor is hard to find in my state. and also the pot of 2M.

        Can u please tell me about SG3524 IC?

      • Jim Keith says: on November 4, 2012 at 5:17 pm

        No air variable is required –that is only for variable frequency –a lower value pot may also be selected and C2 may be scaled up to provide the correct frequency.

        Quite easy in my opinion –the SG3524 is no easier.

      • dknaoremcha says: on November 4, 2012 at 11:44 pm

        Many many thanks for your valuable reply Sir, I will try the circuit. Unfortunately 555 is also not available but I will find from Delhi. Here is the link of the datasheet of my driver-www.maxictech.com/en/UploadFiles/MT7201_rev1.1_EN.pdf
        I have changed the Rs to 2×0.33Ohms in parallel and lighting 2x3watt Philips rebel (3vF/700mA) and its giving a good brihtness. Its quite good for my daughter’s reading but in the late night I want to decrease the brightness. I am not an expert in electronics but I love to experiment with electronics. Please read the datasheet and tell me what else I can do if you have some more idea to adjust the brightness.

        Thanks again

        With best regard,
        Debenkumar Naorem

      • Jim Keith says: on November 5, 2012 at 3:57 am

        No PWM or duty cycle generator is required. It clearly states that a DC voltage applied to pin 3 will control brightness. That is what I would do. Connect pot across battery –if greater than 3V, add series resistor to top of pot to bring voltage down.

    3. what should i do if i dont want to use variable resistor????
      can i use a 100k ohm resistor instead of it???

    4. I want operate SSR through potntional meter 0-220vac

    5. bagaimana merancang dan membuat sendiri osilator dari op amp TL084 dengan keluaran sinyal persegi?

    6. Do you have a cct for digital clock using ic 4026

    7. I love Annie too; the name I mean.The article was also very informative.Thank you.

    8. 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,

    9. 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?

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



      • 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.

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

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

    13. 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.

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

    15. 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?


    16. Now, here is the schematics

    17. nickn2011yahoo-com says: on February 21, 2016 at 5:56 am

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

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