Monostable

Basic BJT monostable multivibrator.

A monostable multivibrator is an electronic circuit that generates an output pulse. When triggered, a pulse of pre-defined duration is produced. The circuit then returns to its quiescent state and produces no more output until triggered again.

Monostables may be considered as a biased form of multivibrator where it is stable in one state until triggered, then unstable and will return of its own accord.

If repeated application of the input pulse maintains the circuit in the unstable state, it is called a retriggerable monostable. If further trigger pulses do not affect the period, the circuit is a non-retriggerable monostable.

Circuit

In the monostable multivibrator, the one resistive-capacitive network (C₂-R₃ in figure 1) is replaced by a resistive network (just a resistor). The circuit can be thought as a 1/2 astable multivibrator. Q2 collector voltage is the output of the circuit (in contrast to the astable circuit, it has a perfect square waveform since the output is not loaded by the capacitor).

Basic animated interactive BJT bistable multivibrator circuit (suggested values: R1, R2 = 1 kΩ R3, R4 = 10 kΩ).

When triggered by an input pulse, a monostable multivibrator will switch to its unstable position for a period of time, and then return to its stable state. The time period monostable multivibrator remains in unstable state is given by t = ln(2)R₂C₁.

For the circuit shown, in the stable state Q1 is turned off and Q2 is turned on. It is triggered by zero or negative input signal applied to Q2 base (with the same success it can be triggered by applying a positive input signal through a resistor to Q1 base). As a result, the circuit goes in State 1 described above. After elapsing the time, it returns to its stable initial state.

See also

• bistable multivibrator
• astable multivibrator.