# 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

• Roger Inanout

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

• ravi

Do you have a cct for digital clock using ic 4026

• febri

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

• Tara chand

I want operate SSR through potntional meter 0-220vac

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

• D.Mohankumar

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

• dknaoremcha

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

I think you can adapt this circuit
electroschematics.com/6950/555-duty-cycle-control/

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

• Jim Keith

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.

• Jim Keith

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.

• dknaoremcha

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

• dknaoremcha

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