A Monostable Multivibrator, often called a one-shot Multivibrator, is a pulse-generating circuit in which the duration of the pulse is determined by the RC network connected externally to the 555 timer. In a stable or stand by mode the output of the circuit is approximately Zero or at logic-low level.
When an external trigger pulse is obtained,( when the Switch is closed ) the output is forced to go high (VCC). The time for which the output remains high is determined by the external Resistor and capacitor C network connected to the timer. At the end of the timing interval, the output automatically reverts back to its logic-low stable state.
The output stays low until the trigger pulse is again applied. Then the cycle repeats. The Monostable circuit has only one stable state (output low), hence the name monostable. Normally the output of the Monostable Multivibrator is low.
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Consider VCC = 5V, for given t
Output pulse width t = 1.1 R C
C in the order of microfarads & Find R
C =0.1 µF , R = 10k then t = 1.1 mSec
Trigger Voltage =4 V
Connect the circuit as shown in the circuit diagram.
Apply Negative triggering pulses at pin 2 of frequency 1 KHz (or) as connected switch.
Observe the output waveform and measure the pulse duration.
Theoretically calculate the pulse duration as T=1.1. RC
Question & Answer
1. Is the triggering given is edge type or level type? If it is edge type, trailing or raising edge?
Edge type and it is trailing edge
2. What is the effect of amplitude and frequency of trigger on the output?
Output varies proportionally.
3.How to achieve variation of output pulse width over fine and course ranges?
One can achieve variation of output pulse width over fine and course ranges by varying capacitor and resistor values respectively
4. What is the effect of Vcc on output?
The amplitude of the output signal is directly proportional to Vcc
5. What are the ideal charging and discharging time constants (in terms of R and C) of capacitor voltage?
Charging time constant T=1.1 RC
6. What are the applications of monostable Multivibrator?
Missing Pulse Detector, Frequency Divider, PWM, Linear Ramp Generator.