Wednesday, 4 July 2012


Monostable multivibrator also called monoshot, has only one stable state as its name implies. It has also one quasi-stable state. Monostable multltivibrator are used for generating a pulse of desired time period when a trigger voltage is applied. An external trigger forces this circuit to go to quasi-stable state from its stable state and remain in that state for an amount of time determined by the discharging time of the capacitor. R and C are the timing elements and C1 is the speed up capacitor.
This circuit shows a monostable multivibrator using single bias supply. As soon as the power supply is switche ON, transistor Q1 goes to cut off state ane Q2 goes to saturation state due to regeneraive action. The stable state voltages are Vc1=Vcc, Vc2=Vce(sat)+Vre, Vb2=Vbe(sat)+Vre and Vb1<Vbe(cutin)+Vre . 
The moment a negative trigger is applied at the collector of the transistor Q1, transistor Q2 goes to the cut off state resulting Vc2 jumps to Vcc. This sudden rising of the voltgae gets coupled to the base of the transistor Q1 through the commutating capacitor C1 because capacitor acts as a short circuit for instantaneous change of voltage. Therefore, the transistor Q1 goes to ON state. The collector voltage of the transistor Q1 suddenly drops by an amount Ic1Rc1, where Ic1 is the current though the resistance Rc1.
The base voltage of Q2 suddenly drops by an equal amount. Now the  polarity of the voltage across the capacitor C is such a wax that the negative is at its rigit side. Now it charges from this negative potential to +Vcc through R and Q1. When the potential at the right side of the capacitor reaches the cut in voltages, Q2 turns ON. Due to the surge of base current, a momentary overshoot of voltgae occurs, but soon it settles at satuaration voltage+Vbe(sat). Thus the circuit comes back to the stable state. It will continue till the next trigger comes at the collector of Q1. The time duration of the quasi-stable state is given by the expression T=0.69RC.
Components for wiring this circuit
Q1=Q2= BC107
R1 = 47K
R2 = 56K
Rd = 6.8K
Rc1 = 4.7K
Rc2 = 4.7K
R = 100K
Re = 1K
Cd = 0.01μF
C1 = 4.7pF
D = 1N4001
Vcc = 12V
trigger - squre waves
Vc1, Vc2 are outputs