Timing Circuits


As we have generally slow changing DATA signals i.e. DATA signals have slow rising and falling edges. And use of these signals creates problems in the working of Digital circuits. Hence to avoid these problems we use SCHMITT TRIGGER to sharpen up the edges of DATA. The transfer characteristics of this trigger are as follow:

We have only 2 output voltage levels Vo- & Vo+ and there are 2 input voltage thresholds V1 & V2. the SCHMITT TRIGGER follows the above characteristics i.e. when ever input voltage is increased from 0, we have the output voltage as Vo- and output remains the same till input voltage is less than V2. Hence when ever there is an increase in the input voltage, transition would occur only when input voltage becomes greater than V2.

When-ever input voltage is decreased from the high value (>V2), we have the output as Vo+ and the output remains the same until input voltage becomes less than V1.

Hence behavior of SCHMITT TRIGGER depends on whether input is increasing or decreasing and trigger is said to give hysteresis of V2 – V1. This is used to eliminate the effect of noise on the signal. We illustrate this fact using the following example.

Suppose we have the following signal with superimposed noise and we take two cases: with small hysteresis V2 – V1and with large hysteresis V2 – V1—and notice the effect of SCHMITT TRIGGER

Small hysteresis V2 – V1+As voltage is less than V1+, hence initial output is Vo-. When the voltage increases to greater than V2, output jumps suddenly from Vo- to Vo+. Now output voltage remains same till input voltage drops below V1+. When voltage drops below V1+, we get the output as Vo-. After some time input voltage again increases beyond Vand hence output again jumps to Vo+.

Large hysteresis V2 – V1–: As voltage is less than V1- , hence initial output is Vo-. When the voltage increases to greater than V2, output jumps suddenly from Vo- to Vo+. Now output voltage remains same till input voltage drops below V1+. When voltage drops below V1+, we get the output as Vo-.

We represent the outputs of SCHMITT TRIGGER below:

And we can see that we get a clean square wave for large hysteresis while for small hysteresis we get many transitions in the output but it is still better than the output without using SCHMITT TRIGGER.

Timing Circuits

555 Timer: Astable Operation

In Astable operation, we have no stable states. Hence we say that timer doesn’t stay in any of the two states indefinitely i.e. vibrates between the two states. Hence we don’t need trigger in this case. This is also called Astable Multi-vibrator. This is also called free-running multi-vibrator. When-ever we give power to the timer, we get the rectangular oscillating output signal.

The following diagram would explain the working of the Astable Multi-vibrator:

Working of this circuit is similar to Monostable multi-vibrator. In this circuit voltage of the capacitance oscillates between Vcc/3 and 2 Vcc/3.

Suppose initially we have Q=0 & Q’=1. As Q is 0, transistor is turned OFF and hence capacitor starts charging through R1 + R2. When the voltage of capacitor goes greater than 2 Vcc/3, output of upper Op-amp gets 1 and hence S=1 & R=0 and due to this Q becomes HIGH and Q’ goes LOW.

Now as we have Q=1 & Q’=0. As Q is 1, transistor is turned ON and hence capacitor starts discharging through R2. When the voltage of capacitor becomes less than Vcc/3, output of lower Op-amp gets 1 and hence S=0 & R=1 and due to this Q becomes LOW and Q’ goes HIGH.

Now again we have Q=0 & Q’=1 and whole procedure is repeated. Hence we get the oscillating output as illustrated follow:

In the figure above W is equal to the time in which capacitor is charged to 2 Vcc/3 from Vcc/3 and P is equal to the time in which capacitor is discharged from 2Vcc/3 to Vcc/3. Hence

W= 0.693 (R1+R2) C

P= 0.693 R2 C

So the time period of output is T= 0.693 (R1+2 R2) C

We can vary the duty cycle of output pulse by changing the value of R1 & R2 and duty cycle is defined as

D= W/T = 0.693 (R1+R2) / 0.693 (R1+2 R2) C = (R1+R2)/ (R1+2 R2)

And frequency of the timer is F= 1/T = 1.44/ (R1+2 R2) C

Timing Circuits

555 Timer: Monostable operation

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:
Timing Circuits

Timing Circuits: Introduction

The timing circuits are the special purpose circuits which are generally used in digital circuits. We have the following important types of timing circuits:

1. 555 Timers are used in timing circuits very often as they are more reliable and lost cost. We have the two modes of operation of 555 Timer:                

  • Monostable operation
  • Astable operation

2. Schmitt Trigger: This is used to sharpen up falling and rising edges of DATA signal.