74121: Monostable Multivibrator (Single, Not Retriggerable)

This is a monostable multlvibrator 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 10 and 11 determines the pulse width in combination with a resistor between pin 11 and pin 14. The resistor can range from 2 to 40K and the capacitor from 10 pF upward.

There are several ways to trigger the monostable multivibrator, determined by what you do to the A1, A2, and B inputs.

If A1 and A2 are grounded, bringing B from ground to a positive level produces triggering. This is a level-sensitive input with Schmitt trigger snap action or hysteresis.

If A1 is high and B is high, bringing A2 from high to low triggers. If A2 is high and B is high, bringing A1 from high to low triggers.

Most other combinations of A1, A2, and B inhibit the operation.

Be sure to properly terminate all three Trigger inputs, The circuit cannot be retriggered during the on time and a duty cycle of less than 75% is recommended. 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 are needed, the 555 is a better choice.

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