Multivibrators

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The one-shot is a monostable multivibrator device. That is, this device only has one stable state and one unstable state. When triggered the device changes from its stable state and remains there for a fixed period of time, known as the pulse width, before returning to its stable state. Often, an external resistor and capacitor will be used to control the pulse width. The logic symbol for the one shot is shown in Figure 2-17 and the corresponding logic circuit is illustrated in Figure 2-18.

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Figure 2-17 One shot logic symbol

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Figure 2-18 One Shot logic circuit

Let us assume that the zero state is the stable state. When a pulse is applied to the trigger input, the output of the NOR gate (A) goes LOW. This HIGH-to-LOW transition is coupled through the capacitor and input of the inverter (B). The apparent Low at the input of B results in a HIGH output which is then feed back to the input of A, Keeping its out put LOW. Up to this point the trigger pulse has caused the output of the one-shot, Q, to go HIGH.

The capacitor immediately begins to charge through R. The rate at which it does so is determined by the time constant RC. What the capacitor charges to a given level, which appears as a HIGH to B, the output goes back low. The width of the pulse is determined by the formula:

t_w= 0.7 RC_EXT

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One shots are used in a variety of applications where one pulse if a specific pulse width is required.