555 Timer as Monostable Multivibrator

jojo November 2, 2017 28 Comments

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A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network,connected externally to the 555 timer. In such a vibrator, one state of output is stable while the other is quasi-stable (unstable). For auto-triggering of output from quasi-stable state to stable state energy is stored by an externally connected capaci­tor C to a reference level. The time taken in storage determines the pulse width. The transition of output from stable state to quasi-stable state is accom­plished by external triggering. The schematic of a 555 timer in monostable mode of operation is shown in figure.

555-timer-monostable-multivibrator

555-timer-monostable-multivibrator

Monostable Multivibrator Circuit details

Pin 1 is grounded. Trigger input is applied to pin 2. In quiescent condition of output this input is kept at + VCC. To obtain transition of output from stable state to quasi-stable state, a negative-going pulse of narrow width (a width smaller than expected pulse width of output waveform)  and  amplitude of greater than + 2/3 VCC is applied to pin 2. Output is taken from pin 3. Pin 4 is usually connected to + VCC to avoid accidental reset. Pin 5 is grounded through a 0.01 u F capacitor to avoid noise problem. Pin 6 (threshold) is shorted to pin 7. A resistor RA is connected between pins 6 and 8. At pins 7 a discharge capacitor is connected while pin 8 is connected to supply VCC.

555 IC Monostable Multivibrator Operation.

 

 

555 monostable-multivibrator-operation

555 monostable-multivibrator-operation

 

For explain­ing the operation of timer 555 as a monostable multivibrator, necessary in­ternal circuitry with external connections are shown in figure.

The operation of the circuit is ex­plained below:

Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input falls below +1/3 VCC, the output of comparator goes high which resets the flip-flop and consequently the transistor turns off and the output at pin 3 goes high. This is the transition of the output from stable to quasi-stable state, as shown in figure. As the discharge transistor is cut­off, the capacitor C begins charging toward +VCC through resistance RA with a time constant equal to RAC. When the increasing capacitor voltage becomes slightly greater than +2/3 VCC, the output of comparator 1 goes high, which sets the flip-flop. The transistor goes to saturation, thereby discharging the capacitor C and the output of the timer goes low, as illustrated in figure.

Thus the output returns back to stable state from quasi-stable state.

The output of the Monostable Multivibrator remains low until a trigger pulse is again applied. Then the cycle repeats. Trigger input, output voltage and capacitor voltage waveforms are shown in figure.

Monostable Multivibrator Design Using 555 timer IC

The capacitor C has to charge through resistance RA. The larger the time constant RAC, the longer it takes for the capacitor voltage to reach +2/3VCC.

In other words, the RC time constant controls the width of the output pulse. The time during which the timer output remains high is given as

tp = 1.0986 RAC
where RA is in ohms and C is in farads. The above relation is derived as below. Voltage across the capacitor at any instant during charging period is given as

vc = VCC (1- e-t/RAC)

Substituting vc = 2/3 VCC in above equation we get the time taken by the capacitor to charge from 0 to +2/3VCC.

So +2/3VCC. = VCC. (1 – et/RAC)   or   t – RAC loge 3 = 1.0986 RAC

So pulse width, tP = 1.0986 RAC s 1.1 RAC

The pulse width of the circuit may range from micro-seconds to many seconds. This circuit is widely used in industry for many different timing applications.

Comments
  • priyanka
    September 16, 2017

    nice…………

  • Isha
    January 21, 2015

    how we can use monostable multivibrator with couter to measure transient response of an event with resolution upto 1 microsecond.
    plz…share if u have any idea related to this.

  • Chetan Vashisht
    November 15, 2014

    Will this circuit work for extremely low frequencies, say 0.25Hz?

  • monika
    September 21, 2014

    The wave shaping circuit is used to prevent mistriggering by positive pulse edge.
    The width of the triggering pulse must be smaller than the expected pulse width of output wave form

    hat do you mean by this statement.

    • September 24, 2014

      if you want constant width pulse the input pulse should be wider than the required output pulse width. if it is going to be smaller it will be carried out. hence you will not get a constant pulse width output.

  • Omey Rajebhosale
    August 31, 2014

    you said tp=RAC but whats the unit of tp? seconds or micro-sec or mili-sec?

    i mean after calculating RAC do we get tp in seconds or what?

    • Suresh Dhamsania
      April 16, 2015

      In Seconds

  • Rufen
    July 18, 2014

    what are the application of multivibrator? Where we use it?

  • Rufen
    July 18, 2014

    we give triggering input as square wave then what is the purpose of getting the output as square wave

  • June 11, 2014

    In case of moni stable what is charging & dischaging time. Overall time ; and frequency

  • poonam sharma
    June 7, 2014

    i m glad that,todays technology helps students like us to increase our knowledge without doing much efforts and now people are able to share their treasure of knowledge

  • Himanshu Dhiman
    April 24, 2013

    i would also suggest you to add why its called 555 .
    because it have three 5k resistors

  • Himanshu Dhiman
    April 24, 2013

    very precise and clearly explaned…
    thanks i really like your explanation

  • March 16, 2013

    10Q so much

  • March 12, 2013

    hello mr.sedhuraman,I need 5 to 15 seconds adjustable delayed off circuit
    Pls help me, i am a biginer.awaiting your reply .tks

  • raju kanwar
    December 13, 2012

    thanxx.,…..

  • Ashokan.G
    February 20, 2012

    very super explanasion bu i have so many advantage anda disadvantage of this cricket

  • bapuni
    December 22, 2011

    so nice

  • Deepanshu Rai
    December 15, 2011

    by getting this i became very happy bcz my al the problems were solved by reading this…

  • rony69
    November 28, 2011

    nice

  • nagesh
    November 27, 2011

    very nice web site.. thank you..

  • October 14, 2011

    awesome!!

  • vamsi
    August 2, 2011

    gud exolanation

  • Vinay
    July 14, 2011

    Very Nice…

  • Gurman
    June 13, 2011

    very well explained….

  • kalyanghosh
    January 27, 2011

    in case of monostable what is duty cicle

    • nagarjuna
      October 26, 2013

      it is greater than 50% but it is depends on timing values of resister and capacitor

    • Seetharaman
      October 28, 2013

      Duty cycle is one. with the input going high output will change over for a predetermined and revert back to its original state even though the in put is still high.

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