Monostable 555 Multivibrator Circuit

Monostable 555 timer multivibrator circuit (one shot monostable multivibrator) is a retriggerable mono shot pulse generator. The name ‘Mono stable’ indicates that it has only one stable state. The unstable state is called ‘Quasi stable state’. The duration of stable state or the pulse width is determined by the charging time constant of the RC network. We can transfer the multivibrator from stable state to quasi stable state by using  push button trigger switch. The animated (simulated) working of the mono stable 555 multivibrator is shown in this article. You can use our monostable 555 timer calculator tool to find the duration of pulse(width) easily.

Components required

• IC 555
• Resistors (1MΩ, 100Ω)
• Capacitors (1µF, 0.001µF)
• LED Push button switch

Working of monostable multivibrator

• Output of monostable 555 multivibrator remains in its stable state until it gets a trigger.

• Primarily the transistor and capacitor are shorted to ground, this state is considered as the stable state of monostable 555 multivibrator.

• As we know, when the voltage at the second pin of 555 IC goes below 1/3 Vcc, the output becomes high. This high state is known as ‘Quasi stable state’. (Refer Astable multivibrator using NE 555)

• The trigger causes the transition from stable state to quasi stable state.

• So when we press the button (Trigger), the voltage at 2nd pin become less than 1/3 Vcc (disconnected from Vcc) and hence the output becomes high.

• Then the discharge transistor is cut off and capacitor starts charging towards Vcc (Refer the internal circuit below). Charging of capacitor is through the resistor R1 with a time constant R1C1. 

• As the capacitor voltage increases and finally exceeds 2/3 Vcc, it will reset the internal control flip flop, there by turning off the 555 timer IC (more than 2/3 voltage at the threshold pin (6th pin) causes IC to reset).

• Thus the output goes back to its stable state from Quasi stable state.