74122: Monostable Multivibrator (Single, Retriggerable)

This is a monostable multivibrator or pulse generator. The circuit must be triggered.

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 pins 11 and 13 determines the pulse width in combination with a resistor between pin 13 and pin 14. The resistor can range from 5K to 25K and the capacitor from 10 pF upward.

There are several ways to trigger the monostable multivibrator, determined by what you do to the Al, A2, B1, B2, and Clear inputs.

If A1, A2, and B2 are high, a low-to-high transition on B1 triggers. If A1, B1, and B2 are high, a high-to-low transition on A2 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 four trigger inputs. 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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