Astable and Monostable Multivibrators

Multivibrators is two state devices which is used extensively in digital electronics. The bistable multivibrators or called filp-flop are the basic memory devices used in sequential logic. Beside bistable multivibrator, there are also astable multivibrator which serves as an oscillator and the monostable multivibrator (on shot multivibrator) which can serve as a pulse source. The NAND gate inverters configuration shown below is one of many ways to create an astable multivibrator : a two state device which is not stable in either state.

A time characteristic of the time constant RC will reach the threshold for switching A low and B high if we start with point A high, then that high voltage charges the capacitor. Until the transition back to the original state occurs , the charging process will reverse. We can build this circuit using half of an IC7400 quad NAND gate.

If the upper part depicted an astable multivibrator circuit, the lower part is a monostable one. The monostable multivibrator or one-shot will produce a single output pulse of length t determined by the value of the resistor and capacitor in response to a positive trigger pulse. The non-retriggerable one-shot will not respond to further trigger pulses which happen during its pulse length. [Circuit’s schematic diagram source: C.R. Nave, Hyper Physics]