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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Monostable and Bistable Multivibrators

Triggering Arrangements

In many switching applications the input pulse to a stage is a rectangular wave as shown in Fig 11. If this input is applied to a short CR differentiating circuit the familiar positive and negative pips are produced. If the differentiated waveform is then fed through a diode connected as shown in Fig 11, only the positive-going spikes appear at the output. This acts as the trigger pulse. By reversing the diode, as in Fig 12, a negative-going trigger pulse is obtained.

If we now connect the trigger circuit of Fig 11 to an Eccles-Jordan bistable circuit, as shown in Fig 13, we can cause it to switch between stable states. If TR1 is on and a switching pulse is applied to the terminal marked 'S', the resulting positive spike at the base of TR1 initiates the change-over to the state where TR1 is off and TR2 on. Once TR1 is off, further trigger pulses to 'S' have no effect; they merely tend to drive TR1 further off. To revert to the initial stable state we must apply a pulse to the terminal marked 'R'. The resulting positive spike at the base of TR2 initiates the change-over to the first stable state where TR1 is on and TR2 off. The labels 'S' and 'R' stand for 'set' and 'reset' respectively. We can therefore set the bistable by a pulse to 'S' and reset it by a similar pulse to 'R'. Either of the two states may be selected at will. This has many applications in computers and counting circuits.

In some circuits we have to apply the trigger pulses from a single source to both stages simultaneously. This may be done with the arrangement shown in Fig 14. A series of pulses, all of the same kind, will then cause the circuit to switch between one state and the other. When a pulse arrives it finds one transistor off and the other on. The positive trigger pulses have no effect on the 'off' transistor but they do effect the other transistor, initiating a change-over. The next pulse finds the positions reversed and switches the circuit back to its first state.

The same reasoning applies to valve circuits except that the trigger pulses there are usually negative-going.


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