Question

Mention and explain the working principle of two types of one-shot!

Answer 1

1] Monostable Multivibrators

the Monostable Multivibrator circuit has only ONE stable state making it a “one-shot” pulse generator. When triggered by a short external trigger pulse either positive or negative.]

WORKING PRINCIPLE

Consider the MOSFET circuit on the left. The resistor R and capacitor C form an RC timing circuit. The N-channel enhancement mode MOSFET is switched “ON” due to the voltage across the capacitor with the drain connected LED also “ON”.

When the switch is closed the capacitor is short circuited and therefore discharges while at the same time the gate of the MOSFET is shorted to ground. The MOSFET and therefore the LED are both switched “OFF”. While the switch is closed the circuit will always be “OFF” and in its “unstable state”.

When the switch is opened, the fully discharged capacitor starts to charge up through the resistor, R at a rate determined by the RC time constant of the resistor-capacitor network. Once the capacitors charging voltage reaches the lower threshold voltage level of the MOSFETs gate, the MOSFET switches “ON” and illuminates the LED returning the circuit back to its stable state.

Then the application of the switch causes the circuit to enter its unstable state, while the time constant of the RC network returns it back to its stable state after a preset timing period thereby producing a very simple “one-shot” or Monostable Multivibrator MOSFET circuit.

Monostable Multivibrators can produce a very short pulse or a much longer rectangular shaped waveform whose leading edge rises in time with the externally applied trigger pulse and whose trailing edge is dependent upon the RC time constant of the feedback components used. This RC time constant may be varied with time to produce a series of pulses which have a controlled fixed time delay in relation to the original trigger pulse .

2] Bistable Multivibrator

The Bistable Multivibrator is another type of two state device similar to the Monostable Multivibrator but the difference this time is that BOTH states are stable.

Bistable Multivibrators have TWO stable states (hence the name: “Bi” meaning two) and maintain a given output state indefinitely unless an external trigger is applied forcing it to change state.

The bistable multivibrator can be switched over from one stable state to the other by the application of an external trigger pulse thus, it requires two external trigger pulses before it returns back to its original state. As bistable multivibrators have two stable states they are more commonly known as Latches and Flip-flops for use in sequential type circuits.

The Bistable Multivibrator circuit above is stable in both states, either with one transistor “OFF” and the other “ON” or with the first transistor “ON” and the second “OFF”. Lets suppose that the switch is in the left position, position “A”. The base of transistor TR₁ will be grounded and in its cut-off region producing an output at Q. That would mean that transistor TR₂ is “ON” as its base is connected to Vcc through the series combination of resistors R1 and R2. As transistor TR₂ is “ON” there will be zero output at Q, the opposite or inverse of Q.

If the switch is now move to the right, position “B”, transistor TR₂ will switch “OFF” and transistor TR₁ will switch “ON” through the combination of resistors R3 and R4 resulting in an output at Q and zero output at Q the reverse of above. Then we can say that one stable state exists when transistor TR₁ is “ON” and TR₂ is “OFF”, switch position “A”, and another stable state exists when transistor TR₁ is “OFF” and TR₂ is “ON”, switch position “B”.