74HC123 High-Speed CMOS Logic Dual Retriggerable Monostable Multivibrator IC

The  74HC123 designed as a dual monostable multivibrator with resets. A negative to positive reset pulse triggers these ICs, which can be retriggerable. To adjust the time and precision of the circuit, an external resistor (RX) and an external capacitor (CX) are supplied. The modification of Rx and CX allows the Q and Q terminals to provide a wide range of output pulse lengths. The pulse triggering on the A and B inputs, on the other hand, is independent of the rise and fall timings of the trigger pulses and occurs at a certain voltage level.

The output pulse width is extended by retriggering inputs A and B once they have been triggered. After that, a LOW voltage on the Reset (R) pin ends the output pulse. The inputs for trailing edge triggering (A) and leading edge triggering (B) allowed for triggering on either side of the input pulse. Each input on the unused device (A, B, and R) must be terminated high or low if either Mono is not used. External resistance, Rx, is usually set at a minimum of 5k. External capacitance, CX, has a minimum value of 0pF. The pulse width is calculated as tW = 0.45 RXCX at VCC = 5V.

A monostable multivibrator (MMV), also referred as a one-shot multivibrator. This is pulse generator circuit in which the duration of the pulse is determined by the R-C network, connected externally to the 555 timers. In such a vibrator, one state of output remains stable while the other is in quasi-stable (unstable). For auto-triggering of output from a quasi-stable state to a stable state, energy stored by an externally connected capaci­tor to a reference level. The time taken in storage determines the pulse width.