Question

Describe the operation of a monostable multivibrator using transistors.

Answer 1

A monostable multivibrator, also known as a "one-shot" or "monostable circuit," is a type of electronic circuit that has two stable states: a stable state (quiescent state) and an unstable state. When triggered, it temporarily switches to the unstable state and then returns to the stable state after a fixed period of time. This time delay is typically determined by external components like resistors and capacitors.

Here's a basic explanation of the operation of a monostable multivibrator using transistors:

Components needed:

Two transistors (typically NPN bipolar junction transistors)
Resistors
Capacitors

Operation:

Quiescent State (Stable State):
Initially, one of the transistors is in the ON state (saturated) while the other is in the OFF state (cutoff). The transistor in the ON state allows current to flow from the positive power supply (Vcc) through the collector-emitter junction and to ground, while the transistor in the OFF state blocks the current flow.
The capacitor is charged to Vcc through a resistor and is in the fully charged state (almost no voltage drop across the capacitor).
Triggering:
When an external trigger signal (a pulse or a momentary change in voltage) is applied to the circuit, it briefly switches the OFF state transistor to the ON state.
Unstable State:
The ON state transistor starts conducting, allowing current to flow from Vcc to ground through its collector-emitter junction.
The capacitor connected to the collector of the ON state transistor begins to discharge through a resistor and the base-emitter junction of the OFF state transistor.
Time Delay:
The time delay of the monostable circuit is determined by the RC time constant, where R is the resistor value and C is the capacitor value. The time delay (T) is given by T = 1.1 * R * C, approximately.
During this time delay, the OFF state transistor remains in the ON state due to the discharge of the capacitor through the base-emitter junction.
Return to Stable State:
After the time delay, the voltage across the capacitor drops below the threshold level required to keep the OFF state transistor in the ON state. The OFF state transistor returns to the OFF state (cutoff).
The monostable multivibrator reverts to its quiescent state, with one transistor ON and the other OFF, and the capacitor fully charged once again.

In summary, a monostable multivibrator using transistors functions as a triggerable, one-shot circuit that produces a fixed time delay when triggered, then returns to its stable state. The timing of the delay is determined by the values of the resistor and capacitor in the circuit.