California University of Pennsylvania
Department of Applied Engineering & Technology
Electrical / Computer Engineering Technology
EET 215: Introduction to Instrumentations
Lab No.5
Voltage Level Detectors
Names:
1.
2.
3.
Signature:
Date
Comparators
Objective of the Experiment
The main objective of this experiment is to understand the basic functions the 741 as a voltage
comparator
Learning Outcomes
Students will demonstrate:
-
the ability to understand the inverting and the noninverting zero crossing detectors
the ability to understand voltage level detectors (inverting and noninverting)
CAUTION: It is imperative that students exercise care when handling the ELVIS unit. Make sure all
connections are correctly made. When in doubt, check with your instructor to verify proper circuit
connections.
Introduction
The LM741 is a general purpose operational amplifier. Figure -1 shows the pin-out of the DIP used in
lab.
Figure -1:
LM 741C
Background Information
The 741 op-map is used either as a voltage amplifier – linear operation, or as a voltage comparator.
Preliminary Investigation:
Refer to this link to answer the following questions:
http://www.ti.com/lit/ds/symlink/lm741.pdf
123456789-
What is the maximum rated power supply voltage for the LM741C? ------------------- Volts
What is the maximum power dissipation for the same op-amp? --------------------- mW
What is the maximum input offset voltage (Vio) ? ---------------------- mV.
What is the maximum input offset current (Iio)? ----------------------- nA
What is the typical input impedance?
------------------------MΩ
What is the output short circuit current (Maximum output current)? -------------------- mA
What is the typical common mode rejection ratio?
------------------------dB
What is the typical slew rate? ------------------------------- V/us
In the same datasheet, look at the schematics diagram of the LM741. Of the two transistors at
the output stage, which transistor is a PNP and which is an NPN? ---------(NPN), -------(PNP)
Refer to class notes as a reference as you do this lab.
Components Needed:
LM741C op-amp
10KΩ OR 5KΩ potentiometer
Experiment
A- Zero crossing detector – part -1
1With the power to the board turned OFF (top right corner), connect the circuit shown, follow
the steps below and plot the output waveform. Also, from the output waveform, record answers below.
15V
VCC
7
Vin
3
6
2
4
LM741CN
0
VEE
-15V
2Turn the circuit board’s power ON. From the function generator, apply for Vin (at pin 3) a
triangular waveform at 3Volts (pk) and a frequency of 100Hz.
3-
On the scope, monitor both channels, VIN and Vout (op-amp output is at pin 6)
Hints: - Using the BNC connector, apply the triangular waveform to pin 3 of the op-amp. To monitor
this input, connect the signal (pin 3) to AI0+.
- Of course connect the BNC ground to the common ground on the board. Also, connect
AI0- to this common ground.
- Connect the op-amp output to AI1+ and connect AI1- to the common ground on
board.
See below and notice the setting on the Scope. ENSURE that the COUPLING on the scope is set
to DC.
4-
Place a screen shot of your result below.
5-
From the output waveform, what is the value of +Vsat ? ------------------ Volts
6-
What is the value of –Vsat? ---------------------- Volts.
7-
What is the most appropriate name for this circuit based on the response? (pick one)
a-
Inverting voltage level detector
b-
noninverting zero crossing detector
c-
inverting zero crossing detector.
B- Zero crossing detector- Part -2
1With the power to the board turned off, connect the circuit as shown below. Simply, you will
only need to make two changes (move the input from pin 3 to pin 2 of the op-amp and move the ground
from pin2 to pin 3. Also, make sure the ELVIS input AI1+ is moved to pin 2
15V
VCC
7
0
3
6
Vin 2
4
LM741CN
VEE
-15V
2-
Turn the power on and reapply the same input and monitor the input and output waveforms on
the scope.
3-
Place a screen shot of your result below.
5-
From the output waveform, what is the value of +Vsat ? ------------------ Volts
6-
What is the value of –Vsat? ---------------------- Volts.
7-
What is the most appropriate name for this circuit based on the response? (pick one)
a-
Inverting voltage level detector
b-
noninverting zero crossing detector
c-
inverting zero crossing detector.
Instructor sign-off:
C-
Voltage Level Detector
1-
Turn the power to the circuit board off.
2-
Connect the circuit shown. Set the output of the potentiometer to 1Vdc. (Use the +5V
DC from the board as an input to the potentiometer. This +5Vdc is found at the bottom
of the strip below the +15V, -15V, and Ground.)
3-
Once the potentiometer’s output is set to 1Vdc, apply it to pin 2 of the op-amp.
4-
The input (Vin) is still the same as Vin used previously (3Vpk triangular waveform at 100
HZ.)
5-
The rest of the setup is identical as previously. Monitor Vin and Vout on the scope.
VCC
15V
VCC
Vin
Vout
2
VCC
1
5.0V
VCC
0
5kΩ
Key=A
50 %
VEE
-15.0V
6-
7-
VEE
Place a screen shot of your result below.
What is the appropriate name for this circuit based on the response? (Pick one)
a-
Inverting voltage level detector
b-
noninverting zero crossing detector
c-
inverting voltage level detector.
8Do not disconnect the circuit – you will need it to answer some of the challenge
questions.
Challenges
1What happens to the square wave output if the threshold level is changed from 1V to
1.5V – Be specific ?
2What would happen to the output if the threshold level is moved to pin 3 and the input
is moved to pin 2?
Be Specific and provide screen shots of resulting output waveform for part 2.
You may now disconnect the circuit and turn the power off.
3- What does the following circuit do? Explain its operation (be precise.) Do not construct the
circuit.
Needed information:
1- The PRC is a photo resistive cell. Its resistance is about 500KΩ in total darkness and about
300Ω in bright light.
2- The potentiometer is used to set the threshold (sensitivity adjustment)
Instructor approval of experiment completion: