In monostable operation we have only one state stable and other state unstable. We have a input named **Trigger** to the 555 Timer. When we give no trigger timer stays in the stable state but when we give trigger then timer goes to the other state for a fixed time period and then goes back to the stable state. The stable state for 555 Timer is LOW state while HIGH state is unstable state. Hence 555 Timer has a LOW output voltage initially. When we given trigger then timer output voltage goes from LOW to HIGH and stays HIGH for W time delay and then resets again.

A Multi-vibrator is 2-state circuit which has either a zero or one or two stable states. And as in monostable operation of 555 timer we have one stable state, hence we also call this timer as **Monostable Multi-vibrator. **Functional diagram of monostable multi-vibrator is given on next page. In the diagram, as we have three 5 Kohm resistors in series hence the circuit is called 555 Timer (Triple 5 timer). Due to this arrangement we have 2Vcc/3 voltage at node A and Vcc/3 voltage at node B.

Initially we have output equal to zero i.e. Q’= 0 & Q=1. As Q=1, transistor gets ON and hence capacitor is discharged and hence S becomes ZERO and R is also ZERO as initial value of Trigger is Vcc.

Hence in stable state

**S=0 R=0 output=0 and capacitor C is discharged**

When we give a trigger at the input (i.e. a LOW voltage pulse is given for small time), lower op-amp gives 1 as voltage at –ve terminal becomes less than Vcc/3. Hence R becomes 1 and Q becomes 0 and Q’=1 and output goes HIGH. Now as Q=0, this cuts-off the transistor and hence capacitor is allowed to charge through resistance R. When capacitor voltage becomes greater than 2Vcc/3, output of upper op-amp becomes 1 and hence S=1, R=0 which makes Q=1 and Q’=0. And output is again reset. Hence a trigger at the input makes output as 1 for some time W i.e. a rectangular pulse of width W is obtained.

The value of W is slightly more than the time in which capacitor is charged from 0 to 2Vcc/3. We know that in one time constant RC, capacitor is charged to 63.2% but we need to charge capacitor to 2Vcc/3 = 66.6%. if we solve the equations then we’ll get

**W= 1.1 RC**

**The following waveforms represent the working of monostable:**