Experiment 6 : ZERO CROSSING DETECTOR, WINDOW DETECTOR AND

SCHMITT TRIGGER

AIM: To design and construct zero crossing detector, window detector and Schmitt trigger and test their working.

COMPONENTS AND EQUIPEMENTS REQUIRED:

Bread board, 741 IC, resistors, capacitors, Function generator, CRO, regulated power supply

THEORY:

Comparator is a circuit that produces two output state +Vsat and -Vsat by comparing the input signal with a reference voltage. If the input signal is applied to the inverting terminal of op-amp then for input voltages less than Vref, the output is equal to +Vsat and for input voltages greater than vref, the output voltage is equal to –Vsat. If the reference voltage is zero, then the comparator can be used as zero crossing detector. If the applied input signal is a sinusoid, then for positive half cycle, the output is –Vsat and for negative half cycle, the output is +Vsat thus producing a square wave. That is the zero- crossing detector can be used to convert a sinusoidal wave form to a square wave.

Fig.6.1. Zero Crossing detector

PROCEDURE:

1. Place 741 IC on the bread board and connect external components.2. Connect dual power supply to op-amp.3. Apply sinusoidal input of 5v p-p and 1khz frequency.4. Observe the output and plot input and output waveforms on graph sheet.

Model graph:

Fig.6.2

WINDOW DETECTOR:

Sometimes one may like to mark the instant at which an unknown input is between two threshold levels. This can be achieved by a circuit called window detector. The circuit diagram for window detector is shown in figure. Since the current through both operational amplifiers is zero, the branch with three 10Ω resistors acts as a voltage divider network. The input voltage is applied to non-inverting terminal and reference voltage is applied to inverting terminal of both op-amps. However the reference voltage applied to inverting terminals are not equal. The reference voltage at inverting terminal of op-amp1 is ((12×20)/(10+10+10))=8V and the reference voltage at inverting terminal of op-amp2 is ((12×10)/(10+10+10))=4V. If the input voltage is between 0 and 4v, the output of both op-amps is equal to –Vsat. As a result LED1 switches on. Remaining two LEDs are in off condition. If the input coltage vi lies between 4 and 8, then the output of opamp 1 is + v sat and opam 2 is -Vsat . This swithes LED 1 to off state and LED 2 to on state. The LED 3 is in off condition. When the input voltage is greater than 8 volts, the out put of both opamps is +vsat.

Therefore LED3 is in on condition. Therefore depending on which of the LEDs are in on, we can find the input voltage levels.

12V

AD8011AN

3

24

7

6

Vi

AD8011AN

3

24

7

6

1

R1 10k

5

R2 10k

R310k

9

0

7

LED2

LED3

LED1

2

R5

4.7k

3

R6

4.7k

8 R7

5.6k

4

R8

5.6k

6

0

0-12V

Opamp2

Opamp1

Fig.6.3. Window Detector

PROCEDURE:

1. Place 741 IC on the bread board and connect external components.2. Connect dual power supply to op-amp.3. Connect a variable input voltage source to non-inverting terminal.4. Vary the input voltage and observe the switching of LEDs.

THEORITICAL PRACTICAL LED1 LED2 LED3 0 < vi < 4 ON OFF OFF 4 < vi < 8 OFF ON OFF 8 < vi < 12 OFF OFF ON

SCHMITT TRIGGER:

The other name for Schmitt trigger is regenerative comparator. In Schmitt trigger positive feedback is used which forces the op-amp to drive in saturation. The voltage divider network connected at the output, provide positive feedback. The input voltage is applied to the inverting terminal. The voltage at non-inverting terminal can be obtained using superposition theorem. Let us assume initially Vi < VN, then the output is equal to Vsat.

The voltage at non-inverting terminal is

----------------------(1)

This voltage is known as VUTP.

If the input voltage is less than VUTP, the output is vsat. When the input crosses beyond VUTP, the output switches to –vsat.

Now the voltage at non-inverting terminal is equal to

--------------------(2)

The above voltage is known as VLTP

When the input voltage is less than VLTP, the output voltage switches from –Vsat to +Vsat. Note that VLTP < VUTP .

The difference between these two voltages is known as Hysteresis and can be written as

Because of hysteresis, the circuit triggers at a higher voltage for increasing signals than for decreasing ones.

DESIGN:

PROCEDURE:

1. Place 741 IC on the bread board and connect external components.2. Connect dual power supply to op-amp.3. Apply sinusoidal input of 5v p-p and 1kHz frequency.4. Observe the output and plot input and output waveforms on graph sheet.5. Find the values of VLTP and VUTP and compare with theoretical values.

Transfer Characteristics

To find the transfer characteristics of Schmitt trigger consider the circuit shown in figure

The UTP and LTP of the circuit is 5.7 volts and 3.3 volts respectively. Connect the power supply at input increase the voltage from 0 to 10 volts and note down the output voltage. Next decrease the input voltage from 10 to 0 volts and note down the readings. Plot a graph between input voltage and out put voltage.

Model graph

Fig. Transfer Characteristics of Schmitt trigger

Result: The circuits for zero crossing detector, window detector and schmitt trigger were designed and analysed.

Viva Questions

1. What are the non-linear applications of op-amp?2. Explain the operation of i) inverting comparator ii) Non-inverting

comparator.3. Explain the applications of comparator.4. What are the applications of op-amp? Explain any one practical

application.5. What is the other name for Schmitt trigger?6. Which feedback is used in Schmitt trigger?7. Define i) UTP ii) LTP8. Define Hysteresis9. With an example, explain how the Schmitt trigger circuit can be used to

design an on-off controller.

0 1 2 3 4 5 6 7 8-15

-10

-5

0

5

10

15

iNPUT Vi

OU

TP

UT

V

o

HTPLTP

Vsat

-Vsat