How to make a Monostable Multi Vibrator using 741 IC ?
As already explained , a monostable multivibrator (MMV) has one stable state and one quasi-stable state. The circuit remains in its stable state till an external triggering pulse causes a transition to the quasi-stable state. The circuit comes back to its stable state after a time period T. Thus it generates a single output pulse in response to an input pulse and is referred to as a one-shot or single shot.
Monostable multivibrator circuit illustrated in figure Â is obtained by modifying the astable multivibrator circuit Â by connecting a diode D1 across capacitor C so as to clamp vc at vd during positive excursion.
Under steady-state condition, this circuit will remain in its stable state with the output VOUT = + VOUT or + Vz and the capacitor C is clamped at the voltage VD (on-voltage of diode VD = 0.7 V). The voltage VD must be less than Î² VOUT for vin < 0. The circuit can be switched to the other state by applying a negative pulse with amplitude greater than Î² VOUT – VD to the non-inverting (+) input terminal.
When a trigger pulse with amplitude greater than Î² VOUT – VD is applied, vin goes positive causing a transition in the state of the circuit to -Vout. The capacitor C now charges exponentially with a time constant Ï„ = RfC toward â€” VOUT (diode Dl being reverse-biased). When capacitor voltage vc becomes more negative than – Î² VOUT, vin becomes negative and, therefore, output swings back to + VOUT (steady- state output). The capacitor now charges towards + VOUT till vc attain VD and capacitor C becomes clamped at VD. The trigger pulse, capacitor voltage waveform and output voltage waveform are shown in figures Â respectively.
The width of the trigger pulse T must be much smaller than the duration of the output pulse generated i.e. TP Â« T. For reliable operation the circuit should not be triggered again before T.
During the quasi-stable state, the capacitor voltage is given as
vc = – VOUT + (VOUT + VD)e-t/Ï„
At instant t = T,Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â vc = – Î² VOUT
So – Î² VOUT =- VOUT + (VOUT + vD) e-T/Ï„ or
T = RfC loge (1 + VD/VOUT)/ 1- Î²
Usually VD << VOUT and if R2 = R3 so that if Î² = R3/(R2+R3) = Â½ then,
T = RfC loge 2 = 0.693 Rf C