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IC 555 Pinouts, Astable, Monostable, Bistable Modes Explored

IC 555 Pinouts, Astable, Monostable, Bistable Modes Explored

The post explains the basic pinout details of the timer IC 555, and how to configure the IC in its standard or popular astable, bistable, and monostable circuit modes. The post also details the various formulas for calculating the IC 555 parameters.

Our hobby world would be less interesting without IC555. It would be one of our first IC to use in electronics. In this article we are going to look back at history of IC555, their 3 operating modes and some of their specifications.

IC555 was introduced in 1971 by a company called “Signetics”; it was designed by Hans R. Camenzind. It was estimated that about 1 billion units is manufactured every year.

That’s one IC555 for every 7 people in the world. The Signetics Company is owned by Philips Semiconductor. If we look at the internal block diagram of IC555 we can see three 5K ohm resistors connected in series, so we got name as 555 timer. But some hypothesis claims that it was arbitrary.

How IC 555 Works

IC 555 internal circuitry

A standard IC555 consist of 25 transistors, 15 resistors and 2 diodes integrated on a silicon die. There are two versions of the IC available namely military and civilian grade 555 timer.

The NE555 is a civilian grade IC and has operating temperature range of 0 to +70 degree Celsius. The SE555 is military grade IC and has operating temperature range of -55 to +125 degree Celsius.

You will also find the CMOS version of timer known as 7555 and TLC555; these consume less power compared to standard 555 and operate less than 5V.

CMOS version timers consist of MOSFETs rather than bipolar transistor, which is efficient and consume less power.




· Pin 1: Ground or 0V.

· Pin 2: Trigger or input.

· Pin 3: Output.

· Pin 4: Reset.

· Pin 5: Control.

· Pin 6: Threshold.

· Pin 7: Discharge.

· Pin 8: Vcc.

3 Modes of timer:

· Bistable or Schmitt trigger

· Monostable or one shot

· Astable

Bistable Mode:

When the IC555 is configured in bistable mode it works as a basic flip-flop. In other words when the input trigger is given, it toggles the output stateON or OFF.

Normally #pin2 and #pin4 are connected to pull-up resistors in this mode of operation.

When the #pin2 is grounded for short duration, the output at #pin3 goes high; to reset the output, #pin4 is momentarily shorted to ground, and then the output goes low.

There is no need for a timing capacitor here, but connecting a capacitor (0.01uF to 0.1uF) across #pin5 and ground is recommended. #pin7 and #pin6 can be left unconnected in this configuration.

Here is a simple bistable circuit:

Simple Bistable Circuit Using IC 555

When the set button is depressed the output goes high and when reset button is depressed the output goes to low state.R1 and R2 may be 10k ohm, the capacitor may be anywhere between the specified value.

Monostable Mode:

Monostable or one-shot pulse generators are widely used in many electronic applications, where a circuit needs to be switched ON for pre-determined time after a trigger. The output pulse width at #pin3 can be determined by using this simple formula:



T is the time in Seconds

R is resistance in ohm

C is capacitance in farads

The output pulse falls when the voltage across the capacitor equals to 2/3 of the Vcc. The input trigger between two pulses must be greater than RC time constant.

Here is a simple Monostable circuit:


Simple Monostable Circuit Using IC 555

Astable Mode:

This is the most commonly used multivibrator or AMV design, and this would be one of our first circuit implemented for IC555 as a hobbyist (remember alternate blinker LED?).

When IC555 configured as astable multivibrator, it gives out continuous rectangular shaped pulses at #pin3.

The frequency and pulse width can be regulated by R1, R2 and C1.The R1 is connected between Vcc and discharge #pin7, R2 is connected between #pin7 and #pin2 and also #pin6. The #pin6 and #pin2 are shorted.

The capacitor is connected between #pin2 and ground.

The frequency for Astable multivibrator can be Calculated by using this formula:

F = 1.44/((R1+R2*2)*C1)


F is the frequency in Hertz

R1 and R2 is resistors in ohms

C1 is capacitor in farads.

The high time for each pulse given by:

High= 0.693(R1+R2)*C

Low time is given by:

Low= 0.693*R2*C

All ‘R’ is in ohms and ‘C’ is in ohms.

Here is a basic astable multivibrator circuit:


Simple Astable Circuit Using IC 555

For 555 IC timers with bipolar transistors, R1 with low value must be avoided so that the output stays saturated near ground voltage during discharge process, else the ‘low time’ could be unreliable and we may see greater values for low time practically than calculated value.

PWM Circuit

If you want the output less than 50% duty cycle i.e. shorter high time and longer low time, a diode can be connected across R2 with cathode on the capacitor side. It's also called the PWM mode for the 555 IC timer.

You can also design a 555 PWM circuit with variable duty cycle two diodes as shown in the above figure.



About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

8 thoughts on “IC 555 Pinouts, Astable, Monostable, Bistable Modes Explored”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Hi Swagatam,
    I have a circuit that uses a ne555p astable timer that is on for 240 seconds(4-min) and is off for 1 second, then on for 240 sec, etc. My problem is the timing is off (about 1.5 times) for the first cycle when I power up the circuit. Is there a way to make the power up timing cycle be the same as the other cycles? Thanks!

    • Hi Norman,

      you can correct it by configuring the reset pin appropriately, so that each time power is switched ON the IC begins from the start.

      for this you may connect the pin#4 of the IC with the positive line through a 10K resistor, and a add a 10uF capacitor across pin#4 and the negative supply line.

      also make sure to add a 10k resistor across the positive and the negative supply lines.
      hope this corrects the issue…

      • Hi Swagatam, I recently ask you about a problem of my timing circuit using 555 timer was not correct at initial start up. You suggested I add a 10k resistor to ground and a 10k resistor to voltage and a 10uF cap to ground from pin 4 of the 555 timer. I did that with no effect. The initial cycle time is about 5 min. 45 sec. and all other cycles are 4 min as I designed. I have attached a partial circuit showing the timing part of the circuit. Any other suggestions would be appreciated. Thanks!
        You system will not accept my schematic.

  3. If you search my blog you may find a few of them , however if you have specific questions , I can clarify them for you separately….

  4. Hi, current sensing transformer would be unnecessarily complex to calculate and build, instead an opamp or resistive network could be applied for the same results with perfect accuracy.

    for sensing current a simple series resistor could be used

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