Wheatstone Bridge and Voltage Comparators

LABORATORY EXPERIMENT #4
“WHEATSTONE BRIDGE AND VOLTAGE COMPARATORS”
Section A. Wheatstone Bridge
Discussion: A Wheatstone bridge is a circuit designed for the precise measurement of an
unknown resistance. An example of such a circuit is illustrated in Figure 1, where two highprecision
resistors ( R 1)
and a variable high-precision resistor ( R ) are placed in a “bridge”
configuration together with an unknown resistor ( u R ). As labeled in the circuit, the voltages 1 V
and V 2 are found from voltage division as follows:
R
1 V1= VS
R1+ R
R
1 V2= VS
R1+ Ru
The voltage at the center of the bridge, labeled V X , is found by Kirchhoff’s Voltage Law (KVL)
to be the difference between V 1 and V 2 . Consider now that the variable resistor is adjusted to
force V X to be zero, this would necessarily imply that Ru= R and the unknown resistance can be
determined. In the following lab exercises, we will use the Wheatstone bridge to determine the
resistance of a “thermistor” for different temperature conditions and then use it to establish an
unknown resistance provided to you by the instructor.
15V
V1 R 1
V2
1
V x
R 1
R Ru
Figure 1. Wheatstone Bridge Circuit
A thermistor is essentially a device whose resistance changes with temperature. Such a
device could be used in a system to monitor the temperature of a critical element or to provide
temperature feedback in an environment control system. For us, it also provides us with another
opportunity to “see” KVL in action!

1. Build the circuit illustrated in Figure 1: Use the +15V supply provided on your protoboard,
10 kΩ 1% resistors for R 1,
the 1% 0.5W resistance substituter box for R , and the
thermistor for R u . Since we want to vary the temperature of the thermistor, use protowire
and alligator clips to extend its reach from the proto-board. Also, set-up the
Keithley DMM to monitor the voltage V X .
2. Set the Resistance Substituter box to 20 kΩ as an initial guess for the unknown
resistance. Before energizing, have the instructor verify your circuit.
INSTRUCTOR VERIFICATION: ___________________
3. Energize the proto-board and observe the DMM measurement. If it is not zero, we will
need to adjust the variable resistor. Begin the adjustment with the left-most digit. A
voltage polarity reversal implies that you’ve gone too far in the opposite direction.
Continue adjusting the variable resistor box digits proceeding to the right until the DMM
measurement is as close to zero as possible. This is the thermistor resistance at room
temperature. Record this resistance and the room temperature below.
R at room temp: _______________ Room temp: ________________
4. Next, obtain from the instructor an ice cube and a Ziploc baggie or some other protective
sheath. Put the ice cube in the baggie and seal it. Press the ice cube against the
thermistor securely and observe the DMM voltage. Adjust the resistor box to zero out the
voltage reading, again beginning with the left-most digit and working rightward. Record
the final resistance and the temperature of ice.
R for ice: _______________ Ice temp: ________________
5. Return the ice to the instructor. Next, tightly hold the thermistor between two of your
fingers so that it warms beyond room temperature. When the temperature stabilizes (i.e.,
the voltage stabilizes), adjust the resistor box so as to null out the voltage once again.
Record the final resistance.
R for fingers: _______________
6. Finally, obtain a heat gun from the instructor. Carefully place the heat gun about three
inches from the thermistor and activate it. After heating it for about 20 seconds, begin
adjusting the resistor box to null out the DMM voltage. Record the final value of
resistance.
R for heat gun: _______________
7. For your lab write-up, comment on the linearity of the resistance versus temperature
characteristics of the thermistor. Use the following table to estimate the temperature of
your fingers and the temperature obtained with the heat gun.
2

Temperature (C) Resistance (kΩ)
30 8.313
35 6.941
40 5.826
45 4.912
50 4.161
55 3.537
60 3.021
65 2.589
70 2.229
75 1.924
80 1.669
85 1.451
90 1.366
95 1.108
100 0.9375
8. De-energize the power supply and remove the thermistor. Obtain an “unknown”
resistance box from the instructor. Use the Wheatstone bridge technique to determine
the unknown resistance. Record the value here and have the instructor verify your
value. De-energize the proto-board. Replace the unknown resistor with the
thermistor. Leave the circuit built on the proto-board as we will revisit it later.
Section B. Voltage Comparators
Unknown Resistance: ________________
INSTRUCTOR VERIFICATION: __________________
Discussion: As the name implies, a voltage comparator is a component that allows us to
determine which of two input voltages is larger. What is inside the comparator is beyond our
introductory level. We prefer to “put it to use” and connect it in with concepts that we are
already know. The instructor should have provided you with an LM311 integrated circuit
voltage comparator. A small circle on the top of the chip indicates pin 1, the remaining pins are
labeled consecutively counterclockwise about the chip. A diagram of the pin-out is given in
Figure 2. Note that pin 8 must be connected to a positive power supply (normally +15V), pin 4
is connected to a negative power supply (normally –15V), and pin 1 is ground with respect to the
comparator. The power supplies are necessary to properly bias the transistors internal to the
comparator and get it to work properly.
How does it work? Simple, if the voltage applied to pin 2 is less than the voltage applied
to pin 3, then the output (pin 7) is internally connected to comparator ground (pin 1). If pin 2
voltage is greater than pin 3 voltage, then the output assumes an open collector condition.
Therefore, as illustrated in Figure 3, we must tie the output to a supply voltage through a “pullup”
resistor in order to establish an output state (note this voltage could be +15V or +5V, for
3

instance). That is, when pin 2 is greater than pin 3, the output of the comparator is an open
circuit, so no current flows through the pull-up resistor and the output voltage will equal the
supply voltage connected to the pull-up resistor. If pin 2 is less than pin 3, the output is internally
shorted to ground via pin 1. The operation of the strobe input (pin 6) will be deferred until
another day. Let’s get back to work!
8 V+
3
LM311
2
4 6
Inverting
Input
Output
7
Noninverting Comparator
Input
Ground
1
Strobe
V-
4
1
Ground
2
Input
Input
V-
Figure 2. LM311 Pin-Out and Description
3
2
+15V
8
LM311
4 6
-15V
1
1k
V supply
Output
7
Dual-In-Line Package
3
6 Balance
Strobe
4 5
Balance
Pull-up
Resistor
TOP VIEW
Figure 3. Illustration of Open-Collector Condition
8 V+
7
Output
1. Build the circuit illustrated in Figure 4. To do this efficiently, connect the proto-board
power supply ground to one of the long row of horizontal connected pins at the top.
Insert the LM311 chip then connect the ground row to pin 1 of the LM311. A short
jumper may be used to tie pins 1 and 2 together. Connect pin 8 to the proto-board +15V
supply and pin 4 to the proto-board –15V supply. Connect the output pin (7) to the protoboard
+5V supply via a 1 kΩ 5% resistor. Keep the proto-board de-energized.
2. Connect the Tektronix function generator to the Tektronix scope. Set up the function
generator to produce a 10V pk-pk, 1kHz, sinusoid with no DC offset.
3. DO NOT yet apply the function generator signal to the comparator. First, run connection
leads from CH1 of the scope to where the input will be applied; run connection leads

from CH2 of the scope to the output (pin 7 and ground). Have the instructor verify your
circuit set-up.
Function
Generator
INSTRUCTOR VERIFICATION: ______________________
3
2
5
+15V
8
LM311
4 6
-15V
1
Figure 4. LM311 Test Circuit
+5V
1k
5%
7 Output
4. Energize the proto-board. Connect the function generator signal to pin 3 of the LM311
and to the circuit ground. Press AUTOSET if necessary to obtain a good image on the
scope screen. Sketch the waveforms and confirm that the comparator is performing as
indicated in the previous discussion. If no waveform is found on CH 2, consult the
instructor.
5. Gently pull the DC OFFSET knob outward on the function generator. Using the scope
MEASURE-MEAN feature, add 3.5V of DC offset to the function generator input.
Sketch the resultant CH1 and CH2 waveforms.
6. Next, adjust the DC OFFSET so that the function generator signal has a –3.5V mean
value. Sketch the resultant CH1 and CH2 waveforms. Have the instructor verify your
waveforms.
INSTRUCTOR VERIFICATION: ______________________
7. DO NOT completely disassemble your circuit, simply disconnect your function generator
input and de-energize the proto-board supply. Disconnect the connections to the scope
and set the scope aside.
8. The final exercise will merge the two concepts we’ve investigated thus far. Modify your
current circuit to the one indicated in Figure 5. Note that the jumper between pins 1 and
2 is removed and an LED and resistor have replaced the 1 kΩ pull-up resistor. Assuming

a voltage drop of 2.3V for the LED and a desired current of 20mA, calculate the required
current-limiting resistor.
Current Limit Resistor: ______________
9. Set the variable resistor from the Wheatstone bridge to a value between the resistor
values obtained for the thermistor at room temperature and for the thermistor pressed by
the fingers. Have the instructor verify your set-up.
INSTRUCTOR VERIFICATION: ______________________
10. With the instructor present, energize the proto-board. Note the status of the LED at room
temperature. Compress the thermistor between two of your fingers, note the status of the
LED. Release the thermistor, does it return to the original status? If all has gone well,
we’ve just created a rudimentary temperature monitoring instrument. Explain how the
LED logic could be reversed. Test your theory by modifying the circuit.
11. De-energize the proto-board. Return the thermistor and LM311 comparators to the
instructor. Return the 1% precision resistors, 5% resistors, and LED to the proper bin
drawers. Return all proto-wire to the bin. Place the scope, function generator, protoboard,
wire kits, DMM, and resistance substituter back in the proper storage cabinet.
Adj.
Resistor
10k
1%
+15V
10k
1%
R therm
6
3
2
+15V
8
LM311
4 6
-15V
1
+5V
Figure 5. Visual Temperature Threshold Detection Circuit
R Limit
Green
7 Output