INSTRUMENTATION & CONTROL

To design a monostable multivibrator using timer IC-555 and verify its functionality.

IC NE/SE 555 timer
CRO probes
Resistors ( 1MΩ, 1/2W carbon type)
Capacitor ( 2.2 µF, 6.3 volts and 0.01 µF)
Bread board and Hook up wires
Function generator
CRO
Variable power supply ( DC )

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 state the output of the circuit is approximately zero or at logic-low level when an external trigger pulse is applied, the output is forced to go high (≈Vcc). The time for which output remains high is determined by the external RC , network connected to the timer. At the end of time interval the output automatically reverts back to its logic-low stable state.
The output stays low until the trigger pulse again applied. Then the cycle repeats. The monostable circuit has only one stable state, hence the name monostable. Normally, the output of the monostable multivibrator is low.

MONOSTABLE OPERATION
According to figure 2, initially when the output is low that is the circuit is in stable state, transistor Q1 is on and capacitor C is shorted out to ground. However, upon application of a negative trigger pulse to pin 2 transistor Q1 is turned off , which releases the short circuit across the external capacitor C and device the output high. The capacitor C now starts charging up towards Vcc through RA. However when the voltage across the capacitor equals 2/3 Vcc, comparator is 1’s output switches from low to high, which in turn drives the output of the flip-flop turns transistors Q1 on, and hence capacitor C rapidly discharges through the transistor. The output of the monostable remains on until a trigger pulse is again applied. Then the cycle repeats.
The figure shows the trigger input, output voltage and capacitor voltage waveforms. The pulse width of the trigger input must be smaller than the expected pulse width of the output waveform. Also the trigger pulse must be a negative going input signal with an amplitude larger than 1/3 Vcc. The time during which the output remains high is given by
Tp = 1.1 RAC (seconds)
Where, RA is in ohms and C is in farads.
Once triggered the circuits output will remain in the high state until the set time tp elapses. The output will not change its even if an input trigger is applied again during this interval tp.

1. Firstly, choose appropriate value of resistor RA and capacitor C.
2. Connect the circuit according to given figure(1). Also connect function generator to pin no 2 and CRO between pin no 3 and ground.
3. Switch “ON” the power supply and apply trigger pulse (negative) with an amplitude larger than 1/3 Vcc from function generator.
4. Observe the output pulse width on CRO and calculate time period tp by tp = 11 RAC.