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DIGITAL ELECTRONICS

CIRCUITS LAB MANUAL

Submitted To:

Sir Amir

Group members :

Ammara Tanveer 2011-EE-304

Huma Asif Javed 2011-EE-307

Samrah Imtiaz 2011-EE-312

Mahnoor Shafiq 2011-EE-313


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Aim:To design the Schmitt trigger circuit using IC 741 .

Apparatus required:

S no. Equipment/Component

name

Specifications/Value Quantity

1 IC 741 refer 1

2 Cathode Ray Oscilloscope (020MHz) 1

3 Multimeter 1

4 Resistor 10k 3

5 capacitor 0.1 f, 0.01 f 1

6 Regular power supply (0 -30V),1A 1

Theory:

The circuit shows an inverting comparator with positive feedback. This circuit converts arbitrary wave forms to a

square wave or pulse. The circuit is known as the Schmitt trigger (or) squaring circuit. The input voltage Vin

changes the state of the output Vo every time it exceeds certain voltage levels called the upper threshold voltage Vutand lower threshold voltage Vlt. When Vo= - Vsat, the voltage across R1 is referred to as lower threshold voltage,

Vlt. When Vo=+Vsat, the voltage across R1 is referred to as upper threshold voltage Vut. The comparator with

positive feedback is said to exhibit hysteresis, a dead band condition.

The Triggering or Threshold voltages are obtained by the voltage divider R1-R2. Suppose the output voltage is +Vsat.

Now the Voltage across R2is Vut. Which is equal to +VsatR2/(R1+R2). When the input Voltage exceeds the voltage

across resistor R2, output goes to -Vsat. Now the voltage across R2is Vlt. Which is equal to -VsatR2/(R1+R2). When

the input voltage goes lower than this voltage, output goes to +V.

Characteristics of the Schmitt trigger:

Assume that input voltage vinis greater than the + Vsat, and output voltage vOUTis at its negative extreme (point 1).

The voltage across R2in the figure is a negative quantity.As a result, vin must be reduced to this negative voltage level (point 2 on the characteristics) before the outputswitches positively (point 3). If the input voltage is made more negative than the Vsat, the output remains at +

+VOUT(points 3 to 4). For the output to go negative once again, v inmust be increased to the + V satlevel (point 5 on

the characteristics).In figure, the trip points are defined as the two input voltages where the output changes states. The upper trip point

(abbreviated UTP) has a valueUTP = Vsatand the lower trip point has a valueLTP = VsatThe difference between the trip points is the hysteresis H and is given asH = + Vsat(- Vsat) = 2 VsatThe hysteresis is caused due to positive feedback. If there were no positive feedbac k, would equal zero and the

hysteresis would disappear, because the trip points would both equal zero.Hysteresis is desirable in a Schmitt trigger because it prevents noise form causing false triggering. To design a Schmitt trigger, potential divider current I2is once again selected to be very much larger than the op-

amp input bias current. Then the resistor R2is calculated from equation

R2 = UTP/I2and R1is determined fromR1= (VOUTUTP) / I2


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Circuit Diagrams:

Design:Vutp= [R1/(R1+R2 )](+Vsat)

Vltp= [R1/(R1+R2 )](-Vsat)

Vhy= VutpVltp

=[R1/(R1+R2)] [+Vsat(-Vsat)]

Procedure:1. Connect the circuit as shown in fig.

2. Apply an arbitrary waveform (sine/triangular) of peak voltage greater than UTP to the input

of a Schmitt trigger.

3. Observe the output at pin6 of the IC 741 Schmitt trigger circuit by varying the input and

note down the readings as shown in Table 1 and Table 2

4. Find the upper and lower threshold voltages (Vutp, Vltp) from the output wave form.

Reading:Supply Voltage = 12 V

R = 10k

R1 = 10k

Threshold Level = 7.500V

Upper trigger level Lower trigger level

7.500V -7.500V

Precautions:

Check the connections before giving the power supply. Readings should be taken carefully.

Results:

UTP and LTP of the Schmitt trigger are obtained by using IC 741 as shown in Table .

Inferences:Schmitt trigger produces square waveform from a given signal.


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Wave forms:


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Aim:To generate a pulse using monostable multivibrator and to generate Symmetrical square waveforms using IC 555

Timer .

Apparatus required:

Sr no. Equipment/Componentname Specifications/Value Quantity

1 IC 555 See datasheet 1

2 Resistors 100k 1

3 Resistors 100k 1

4 Capacitors 100f, 0.01f 1

5 Diode 1N4148 1

6 Regular power supply (030V),1A 1

7 Cathode ray oscilloscope (020MHz) 1

Theory:

Monostable operation:

A Monostable Multivibrator, often called a one-shot Multivibrator, is a pulse-generating circuit in which theduration of the pulse is determined by the RC network connected externally to the 555 timer. In a stable or standby

mode the output of the circuit is approximately Zero or at logic-low level. When an external trigger pulse is

obtained, the output is forced to go high . The time the output remains high is determined by the external RC

network connected to the timer. At the end of the timing interval, the output automatically reverts back to its logic-

low stable state. The output stays low until the trigger pulse is again applied. Then the cycle repeats. The

Monostable circuit has only one stable state (output low), hence the name monostable. Normally the output of the

Monostable Multivibrator is low. When the power supply VCC is connected, the external timing capacitor C

charges towards VCC with a time constant (RA+RB) C. During this time, pin 3 is high (VCC) as Reset R=0, Set

S=1 and this combination makes Q=0 which has unclamped the timing capacitor C.

Astable operation:

When the capacitor voltage equals 2/3 VCC, the UC triggers the control flip flop on that Q=1. It makes Q1 ON and

capacitor C starts discharging towards ground through RB and transistor Q1 with a time constant RBC. Current

also flows into Q1 through RA. Resistors RA and RB must be large enough to limit this current and prevent damage

to the discharge transistor Q1. The minimum value of RA is approximately equal to VCC/0.2 where 0.2A is the

maximum current through the ON transistor Q1.

During the discharge of the timing capacitor C, as it reaches VCC/3, the LC is triggered and at this stage S=1, R=0

which turns Q=0. Now Q=0 unclamps the external timing capacitor C. The capacitor C is thus periodically charged

and discharged between 2/3 VCC and 1/3 VCC respectively. The length of time that the output remains HIGH is the

time for the capacitor to charge from 1/3 VCC to 2/3 VCC. The capacitor voltage for a low pass RC circuit

subjected to a step input of VCC volts is given by

VC = VCC (1- e-t/RC)

Total time period T = 0.69 (RA + 2 RB) C

f= 1/T = 1.44/ (RA + 2RB) C


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Circuit Diagrams:


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Design:

Monostable operation:Consider VCC = 5V, for given tp Output pulse width tp = 1.1 *RA*C

Assume C in the order of microfarads & Find RA .

Astable operation:Formula: f= 1/T = 1.44/ (RA+2RB) C

Duty cycle, D= tc/T = RA + RB/(RA+2RB)

Model calculations:

Monostable operation:If C=100 f , RA = 10k then tp = 1.1 sec

Astable operation:Given f=50 KHz and c=0.1F, D=67%

50 KHz = 1.44/ (RA+2RB) x 0.1x10-6 and 0.67 = RA+RB/ RA+2RBSolving both the above equations, we obtain RA & RB as

RA = 98.12k

RB = 95.24k

Given f=10 KHz and c=470 pF, D=67%

10 KHz = 1.44/ (RA+2RB) x 470x10-12 and 0.67 = RA+RB/ RA+2RB

Solving both the above equations, we obtain RA & RB as

RA = 104.39 K

RB = 101.32 K

Procedure:


1. Connect the circuit as shown in the circuit diagram as shown in Fig 1.2. Apply Negative triggering pulses at pin 2 of frequency 1 KHz as shown in Fig 3(a).

3. Observe the output waveform and capacitor voltage as shown in Fig 3 (b)and (c) and

measure the pulse duration

4. Theoretically calculate the pulse duration as Thigh=1.1.*Ra*C

5. Compare it with experimental values.

Astable operation:

Symmetrical square waveform generator:1. Connect the diode OA79 as shown in fig (ii) to get D=0.5 or 50%.

2. Choose Ra=Rb = 10K_ and C=0.1F

3. Observe the o/p waveform as shown in Fig 4(c), measure frequency of oscillations and the

duty cycle and then sketch the o/p waveform.


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Waveforms:


Result:The input and output waveforms of 555 timer monostable multivibrator are observed as shown in Fig 3(a), (b), (c).

Inferences:Output pulse width depends only on external components RA and C connected to IC555.

Unsymmetrical square wave of required duty cycle and symmetrical square waveform can be generated.

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