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: