Monostable Multivibrator

The monostable multivibrator (sometimes called a one-shot multivibrator) is a square- or rectangular-wave generator with just one stable state. The operation of the monostable multivibrator is relatively simple. The input is triggered with a pulse of voltage. The output changes from one voltage level (stable state) to a different voltage level (unstable state). The output remains at this new voltage level for a definite period of time. Then the circuit automatically reverts to its stable state and remains that way until another trigger pulse is applied to the input. The monostable multivibrator actually takes this series of input triggers and converts them to uniform pulses, as shown in the figure below. All of the output pulses are of the same amplitude and time duration.

Monostable multivibrator block diagram.

A standard transistor monostable multivibrator is shown in the figure below. When the circuit is energized, current through small resistor R₁ and uncharged capacitor C₁ causes Q₂ to saturate. Q₂ has practically no voltage drop across it, so its collector is essentially at 0 volts. This voltage is coupled through R₃ to the base of Q₁. Therefore, Q₁ is cut off. C₁ is charged to nearly V_CC through the output circuit (resistor R₁) of Q₁. The circuit is in its stable state.

Monostable multivibrator with transistors.

Assume that a positive pulse is applied at the input terminal. C₂ and D₁ couples this voltage change to the base of Q₁ and starts Q₁ conducting. Q₁ quickly saturates, and its collector voltage drops to ground potential. This sharp voltage decrease is coupled through C₁ to the base of Q₂, causing Q₂ to cut off. The collector voltage of Q₂ quickly rises to V_CC. This rise in voltage at the collector of Q₂ is coupled through R₃ to the base of Q₁ to keep it conducting.

The monostable multivibrator has now been turned on by applying a pulse at the input. It will turn itself off after a period of time. To see how it does this, look at the figure above again. Q₁ is held in saturation by the positive voltage applied through R₃ to its base, so the circuit cannot be turned off here. Notice that the base of Q₂ is connected to C₁. The charge on C₁ acts like a battery with its positive terminal on the collector of Q₁, and its negative terminal connected to the base of Q₂. This voltage across C₁ keeps Q₂ cutoff. Now, C₁ starts to charge to reverse voltage through R₂ and Q₁. When the voltage at the base of Q₂ is such that it is slightly forward biased, collector current will start to flow. The resulting drop in voltage at the collector of Q₂ is coupled to Q₁ by R₃ which causes Q₁ to turn off.

74121 Monostable Multivibrator

This integrated circuit can generate pulses with a width from about 30 ns to 28 s. The pulse width may be varied by external timing components (R, C) and is approximately defined by the relationship t = 0.7RC. The IC can be triggered by positive or negative input edges.