Monostable Multivibrators

A monostable multivibrator, also known as monoshot, is the one in which one of the states is stable and the other is quasi-stable. The circuit is initially in the stable state. It goes to the quasi-stable state when appropriately triggered. It stays in the quasi-stable state for a certain time period, after which it comes back to the stable state.

Figure below shows the basic monostable multivibrator circuit.

Monostable multivibrator

The circuit functions as follows. • Initially, transistor Q2 is in saturation as it gets its base bias from +VCC through R. Coupling from Q2-collector to Q1-base ensures that Q1 is in cut-off. • When a positive trigger pulse of short duration and sufficient magnitude is applied to the base of transistor Q1, Q1 starts conducting. • As the voltage across the capacitor C cannot change instantly, negative voltage is applied to the base of transistor Q2 and it turns OFF. Therefore, the output goes to the HIGH state. • Since there is no direct coupling from Q1-collector to Q2-base, which is necessary for a regenerative process to set in, Q1 is not necessarily in saturation. However, it conducts some current. • The Q1-collector voltage falls by Ic1Rc1 and Q2-base voltage falls by the same amount as voltage across a capacitor (C in this case) can not change instantaneously. • In nutshell, the moment trigger is applied, Q2 went to cut-off and Q1 started conducting. But now there is a path for capacitor C to charge from VCC through R and the conducting transistor. The polarity of voltage across C is such that Q2-base potential rises. The moment Q2-base voltage exceeds the cut-in voltage, it turns Q2 ON, which due to coupling through R1 turns Q1 OFF. And we are back to the original state, the stable state. • Whenever, we trigger the circuit into the other state, it does not stay there permanently and returns back after a time period that depends upon R and C. This state is called the quasi-stable state.

Figure below shows the timing waveforms of a monostable multivibrator.

Timing waveforms of monostable multivibrator

In a conventional monostable multivibrator, once the output is triggered to the quasi-stable state by applying a suitable trigger pulse, the circuit does not respond to subsequent trigger pulses as long as the output is in quasi-stable stable. After the output returns to its original state, it is ready to respond to the next trigger pulse. Retriggerable monostable multivibrators respond to trigger pulses even when the output is in quasi-stable state.

Figure below shows output pulse width in the case of a retriggerable monostable multivibrator for repetitive trigger pulses. The output pulse width, that is, the time period of the quasi-stable state equals (n - 1)Tt + T.

Output pulse width for retriggerable monostable multivibrator for repetitive trigger pulses

Some of the commonly used digital ICs that can be used as monostable multivibrators include the following. TTL family • 74121 (single monostable multivibrator) • 74221 (dual monostable multivibrator) • 74122 (single retriggerable monostable multivibrator) • 74123 (dual retriggerable monostable multivibrator) CMOS family • 4098B (dual retriggerable monostable multivibrator)

Figure below shows the circuit using IC 74121 as a monostable multivibrator for HIGH-to-LOW edge triggering.

HIGH-to-LOW edge triggering of 74121 configured as monostable multivibrator

The output pulse width can be computed from T = 0.7RC. Recommended ranges of values for R and C, respectively, are 4 K to 40 K and 10 pF to 1000 F. IC provides complementary outputs. That is, we have stable LOW or HIGH state and corresponding quasi-stable HIGH or LOW state available on Q and outputs.

Figure below shows the use of 74123 as a retriggerable monostable multivibrator for HIGH-to-LOW edge triggering.

HIGH-to-LOW edge triggering of 74123 configured as retriggerable monostable multivibratorHIGH-to-LOW edge triggering of 74123 configured as retriggerable monostable multivibrator