74123: Monostable Multivibrator (Dual, Retriggerable)

This is a dual monostable multivibrator or pulse generator. Each half of the circuit must be triggered. Each half of the circuit may be used separately.

In response to a trigger, the Q output goes high and the Q' output goes low, staying there for a predetermined time and then returning to the initial state.

A capacitor connected between oscillator pins and a resistor connected from oscillator pin B on either side to +5V determine the pulse width. The resistor can range from 5K to 25K and the capacitor from 10 pF upward.

There are two ways to trigger the monostable multivibrator. If input A is held low, bringing B from low to high triggers. If input B is held high, bringing input A from high to low triggers.

The Clear input should remain high. If grounded, it inhibits triggering and returns the circuit to the state with Q low and Q' high.

The circuit may be retriggered at any time. Be sure to properly terminate all Trigger and Clear inpufs. Certain forms of clip-on digital testers can upset the operation of this stage, particularly on the Resistor and Capacitor inputs.

Unless very short times or complementary outputs or retriggerability is needed, the 555 is a better choice of monostable multivibrator.

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