Types of monostable multivibrator using IC555

A monostable mutivibrator (MMV) is a circuit which has only one stable state. After the input trigger pulse is given, the output goes high. It remains high as long as the voltage across capacitor is below 2/3 * VCC , as the capacitor charges and voltage equals 3.3V ( for 5V VCC ), the output goes low and remains low, unless a voltage of lower than 1/3 * VCC is given to the trigger (pin 2) of timer IC 555. Note that the trigger input is “level triggered” and not “edge triggered”. The time for which the output remains high is given by T = 1.11RC. Below given is the circuit diagram along with the output (green) and input (blue) waveform.

Fig 1 - Monostable multivibrator circuit

In the above circuit diagram R = 1Kohm and C = 1uf, Therefore – T = 1.11RC = 1.11ms

On simulator it’s 1.09ms. See the green output waveform.

Two types of Monostable multivibrators -

1. Non retriggerable

2. Retriggerable

Non retriggerable monostable multivibrator

In Non re-triggerable, After the initial trigger pulse is given, the output goes high. It will remain high for time “T” (T=1.11RC). If another trigger pulse comes when output is high then that trigger is neglected. This means that the output is going to remain high only for “T” seconds irrespective of the number of trigger pulses that come after initial trigger. Refer the below given image, look at the waveforms.

Fig 2 - Non retriggerable example

The initial trigger pulse (blue) set the output to high (green). After that four trigger pulses were generated but that didn’t affected the output pulse time duration “T”. It went low after 1.08ms (simulation) close to 1.11ms (theoritical).

Lets see waveform when trigger pulses are given continuously.

Fig 3 - Non retriggerable input/output waveforms

This shows us that the output will go low after 1.11ms and will remain low for the time the trigger input is high. As soon as the trigger input goes low the output turns high again.

Conclusion-

1. When output is low, only the initial trigger sets the output high.

2. The output will go low after time “T” (T=1.11RC).

Circuit simulation link - https://everycircuit.com/circuit/5784033229012992

Retriggerable monostable multivibrator

In retriggerable mode the output will stay high as long as the circuit is triggered frequently within the time duration “T” (T=1.11RC). This means that each time we trigger the circuit when the output is high, then the time “T” is added to the output. Refer the below given circuit and waveform.

Fig 4 - Retriggerable monostable multivibrator circuit diagram

The retriggerable circuit is different from the Non retriggerable, as you can see here there is a transistor (NPN) used to discharge the capacitor. The trigger is provided by giving high pulse to the base of the transistor. If the trigger pulse are fed frequently then the capacitor never reaches 2/3VCC (which is threshold vltg level) and the output remains high. The output will remain high for time “T” (T=1.11RC) if circuit is not triggered after the initial trigger.

Lets see waveform when trigger pulses are given continuously.

Fig 5 - Retriggerable monostable multivibrator input/output waveforms

The above image shows us that the output will stay high if trigger pulses are given continuously.

Conclusion -

1. When output is low, giving a trigger pulse will set output to high.

2. Giving trigger pulses frequently within T=1.11RC, will keep the output high.

Circuit simulation link - https://everycircuit.com/circuit/5115530159325184

Both the 555IC circuits, retriggerable and non-retriggerable are very useful when designing electronic circuits. The applications are vast. I will use these blocks in my future electronic projects. Watch the below given Youtube video (practical working):- 

https://youtube.com/shorts/KWHORnaqYSM?feature=share

If you found this blog useful and want to support me, then you can donate me. The money you give will be used to make more advance electronics projects and setting up better lab.