A Monostable Multivibrator, also known as a one-shot multivibrator, is an electronic circuit that generates a single output pulse in response to an external trigger. It has one stable state and one unstable state, which is temporarily occupied when a trigger pulse is applied. The circuit stays in its unstable state for a predetermined time before returning to the stable state.
Monostable multivibrators are commonly used in timing applications, pulse width modulation, signal shaping, and generating time delays. Their simplicity and versatility make them ideal for tasks such as generating control signals and synchronizing digital systems.
Monostable multivibrator using 555 timer IC is as shown in the following Figure. This 555 timer is called monostable multivibrator because it has only one stable state. Resistor RA and capacitor C are components connected externally to the IC-555. Threshold voltage (6) and discharge (7) pins are connected to each other.
- Initially the SR-FF is set (Q = 1); transistor T is driven into saturation, and the capacitor is by passed by the transistor. Therefore, the capacitor voltage VC = 0, and also the output voltage Vout= 0 (as Q = 1)
- When the negative going trigger pulse (which should be more than 1/3 VCC) is applied at trigger input of lower comparator, the comparator output goes high; and SR-FF is reset (Q = 0), forcing 𝑄 to go high and transistor T turns off.
- As T is off, capacitor starts charging through RA. Now, the output will remain high (from t1 to t2,in waveform shown).
- At time t2; the voltage across the capacitor VCC becomes more than 2/3 VCC and upper comparator output goes high. This will set the SR-FF (Q = 1).
- Since, SR-FF output Q = 1; transistor T is ON, and hence, capacitor discharges, and also output goes low. The output remains low till the next trigger pulse is applied.
From the waveform of monostable multivibrator, it is clear that, the ON time TON of the output voltage is same as charging time of the capacitor.
Therefore, TON → is the time taken by capacitor to charge from 0 to 2/3 VCC.
The voltage across capacitor increases exponentially and is given by;
Vc =Vcc[1–et/RC]
As capacitor charges through RA; let us replace R by RA: Hence,
Vc=Vcc[1–et/RAC]
At t = t2 (TON), the capacitor voltage (VC) reaches 2/3 VCC:
Therefore,
VC=2/3VCC = VCC[1–eTON/RAC] or eTON/RAC =1–2/3 = 1/3Therefore,TON = 1.1RAC
Applications:
A monostable multivibrator can be used in many applications, few important applications are
1. Frequency divider
2. Missing pulse detector
3. Pulse width modulator
4. Pulse position modulator etc
