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Draw the diagram of a monostable multivibrator using IC 555. With the help of waveforms at the trigger input, across the charging capacitor and at the output Design the same for a pulse width of 11ms.

Subject: Liner Integrated Circuits

Topic: Special Purpose Integrated Circuits

Difficulty: Medium

Draw the diagram of a monostable multivibrator using IC 555. With the help of waveforms at the trigger input, across the charging capacitor and at the output explain its working. Design the same for a pulse width of 11ms.

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Timer IC555 as Monostable Multivibrator

Also known as one shot multivibrator.

monostable

Operation:

When a negative ( 0V ) pulse is applied to the trigger input (pin 2) of the Monostable configured 555 Timer oscillator, the internal comparator, (comparator No1) detects this input and “sets” the state of the flip-flop, changing the output from a “LOW” state to a “HIGH” state. This action in turn turns “OFF” the discharge transistor connected to pin 7, thereby removing the short circuit across the external timing capacitor, C1.

This action allows the timing capacitor to start to charge up through resistor, R1 until the voltage across the capacitor reaches the threshold (pin 6) voltage of 2/3Vcc set up by the internal voltage divider network. At this point the comparators output goes “HIGH” and “resets” the flip-flop back to its original state which in turn turns “ON” the transistor and discharges the capacitor to ground through pin 7. This causes the output to change its state back to the original stable “LOW” value awaiting another trigger pulse to start the timing process over again. Then as before, the Monostable Multivibrator has only “ONE” stable state.

For an output pulse of 11ms duration, We assume, capacitor C1 = 0.uF and calculate the value of R1 using the formula given below:

t = 1.1 x R1 x C1

for t = 11ms , R1 = 100k

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