555 Timer as Monostable Multivibrator

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.

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 : 

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.

       +2/3VCC. = VCC. (1 – e-t/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.


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