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One-Shot Mono-stable multi-vibrator

Bi-stable devices remain in either of their two states unless the inputs force the device to switch its state. The device remains in its alternate state unless the inputs are changed again to force the device back to its original state. A mono-stable device only has a single stable state and it remains in its stable state. It temporarily changes to its unstable state when it is triggered. It remains in its unstable state for a predetermined length of time and then it automatically switches back to its stable state. The length of time for which the device remains in the unstable state is determined by the time constant determined by the Resistor and Capacitor connected externally to the mon-stable device. The output of the device is a pulse having a time duration determined by R and C. These mono-stable devices are also known as One-Shots. Figure 25.9. One-Shots are of two types, the nonretriggerable and retriggerable.

Figure 25.9a Circuit diagram of a One-Shot

Figure 25.9b Timing diagram of a One-Shot

The One-Shot is triggered by applying a short pulse at the input of the NOR gate at time interval t1. The One-Shot is in its stable state with output at logic zero at time interval < t1. The logic high triggering pulse at the input of the NOR gate sets its output to logic low. The logic low output of the NOR gate is inverted into logic high by the NOT gate and the One-Shot is in unstable state at the start of interval t1. The logic high output of the NOT gate is connected back to the second input of the NOR gate, which maintains the output of the NOR gate at logic low. When the output of the NOR gate is set to logic low at interval t1, the capacitor C begins charging through the Resistor R. The charging time (in seconds) is determined by the time constant RC. During the charging of the capacitor during interval t1 to t2, the input of the NOT gate remains at logic low, therefore the output of the NOT gate remains in the unstable state at logic high. When the capacitor is fully charged to potential +V (logic high) at time interval t2, the NOT gate input also become logic high, which sets the output of the NOT gate to logic low. With the setting of the NOT gate output to logic low at interval t2, the One-Shot id switched back to its stable state. The interval t1 to t2 during which the One-Shot is in its unstable state is determined by the time constant RC.

1. Nonretriggerable One-Shot

A nonretriggerable One–Shot is triggered to its unstable state.

a.The One-Shot output remains in the unstable state for a fixed period of time on each trigger input.

b.The One-Shot will have to return to its stable state before it can be triggered again. If it is already in its unstable state due to application of a trigger input, a new trigger input will have no effect.

c.The duration of trigger input pulses has no effect on the output pulse duration. The One-Shot is triggered either on the positive or the negative edge. Figure 25.10

2. Retriggerable One-Shot

A retriggerabe One-Shot operation is very similar to that of the Nonretriggerbale One-Shot except that the retriggerable One-Shot will retrigger even if it is in its unstable state. Figure

25.11. The retriggerable and Nonretriggerbale are available in Integrated Circuit form.