Course: EEGR 202 Electric Circuits. Fall 2010
PROJECT 1: WHEATSTONE BRIDGE AND LIGHT
SENSOR CIRCUIT
GROUP MEMBERS:
1. _________________
Name 1
_______________
Signature
2. _________________
Name 2
_______________
Signature
Date:
Department of Electrical/Computer Engineering, Morgan State University.
Course: EEGR 202 Electric Circuits. Fall 2010
PART ONE: WHEATSTONE BRIDGE
INTRODUCTION
The purpose of this project will is to use a variable resistor and a Light Dependent Resistor
(LDR) in two different circuits to illustrate the concept of voltage division. In the first
circuit called the Wheatstone bridge, the variable resistor is used to find the value of an
unknown resistor. In the second circuit, a LDR is used to evaluate the appropriate value of
the fixed resistor in order to design a light sensor circuit.
THEORY
You can include figures from the project/Lab handout. However, you have to rewrite
the theory using your own words. All figures should be numbered and referenced to
in your write-up.
Figure#1 - Wheatstone bridge Circuit
In this section, you can introduce the Wheastone bridge circuit and use it to derive the
voltage division equations for Vx and Vy. Indicate what balancing the bridge implies. Note
that all your equations in the theory section should not use actual numbers (you do that in
the analysis section).
Department of Electrical/Computer Engineering, Morgan State University.
Course: EEGR 202 Electric Circuits. Fall 2010
EQUIPMENT
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Mobile Studio Lab
3478A Multimeter
Fluke 45 Dual Display Multimeter
Three Different value Resistors
Variable Resistor
Laptop Computer
PROCEDURE
1. We picked a variable resistor. Then, using the laboratory multimeter, we
measured the resistance between two terminals of all the combinations,
while turning the knob (wiper) all the way to the left or to the right. Our
goal was to understand how to properly connect the potentiometer in
the circuit.
2. We chose three resistors from our lab kit. Then, we picked two resistors
of the same value which were less than 1 K-ohms and built the
Wheatstone bridge circuit as shown in figure #1 to predict the value of
the unknown resistor Rx . note to students: you could also use the
circuit to predict the value of Rv since you know the value of Rx.
In the first case, we chose two equal resistors Ra = Rb.
3. Next, we adjusted the value of the variable resistor Rv until the output
voltage measured between Vx and Vy was zero. Then, we compared the
value of Rv with Rx. note to students: you could also use the circuit
to predict the value of Rv since you know the value of Rx. In that
case, your procedure will be to first choose a value for Rx, then
vary your variable resistor until Vx=Vy. You finally compare the
measured and calculated values of Rv.
4. We verified our answer by using the color code on the unknown resistor
and compared it to the value of the variable resistor Rv that we
measured using the digital multimeter.
5. Next, we chose Ra = 2Rb and repeated steps 2, 3 and 4.
6. Next, we chose Ra = (0.5) Rb and repeated steps 2, 3 and 4.
Department of Electrical/Computer Engineering, Morgan State University.
Course: EEGR 202 Electric Circuits. Fall 2010
7. Finally, we compared the value of Rx and Rv.
RESULTS
The table below represents the values of resistance of the variable resistor with
respect to the terminals and wiper position as calculated:
Table #1: Variable Resistor Measurements
TERMINALS
WIPER POSITION
Left Most
In the Middle
Right Most
X–Y
0.10ohm
0.5183kohm
0.9831kohm
Y–Z
0.9831kohm
0.4656kohm
0.12ohm
X–Z
0.9826kohm
0.9799kohm
0.9821kohm
X-Y terminal is chosen because it has the highest range from left to right.
When Ra = Rb
Ra = Rb = 390 ohms. (Theoretical Value)
When V = 0; Rv = 314.6 ohms.
Using Wheatstone bridge formula => Rx/Ra = Rb/Rv
Rx = (390* 390)/314.6 = 461.7 ohms.
Using Color Code: Yellow, Purple, Brown, Gold => 47 * 10^1 = 470 ohms
Tolerance = Gold. Therefore 5% tolerance = 5/100*470 = 23.5
Thus, the theoretical resistance should be between 446.5 Rx/Ra = Rb/Rv
Rx = (780 * 390)/620.3 = 490.4 ohms.
When Ra = (0.5) Rb
Ra = 390 ohms and Rb = 780 ohms (Theoretical Values);
At V = 0; Rv = 616.7 ohms.
Using Wheatstone bridge formula => Rx/Ra = Rb/Rv
Rx/Ra = Rb/Rv; Rx = (390 * 780)/616.7 = 471.13 ohms.
Department of Electrical/Computer Engineering, Morgan State University.
Course: EEGR 202 Electric Circuits. Fall 2010
Table #2: Rv (or Rx) measured and Theoretical Values
TERMINALS
Choice of Resistors
Ra=Rb
Ra=0.5Rb
Ra=2Rb
Rv (or Rx)
Theoretical
Rv (or Rx)
Measured
%Error
ANALYSIS/DISCUSSION
This is the most important part of your report, because here, you show that
you understand the experiment beyond the simple level of completing it.
Explain. Analyze. Interpret.
This is the place where you answer any questions posed in the lab
handout.
In the first part of the procedure, we chose Ra=Rb= 390 ohms, and assumed
that Rx = 462 ohms. The theoretical values indicate that Rv should be equal to
315 ohms. We built the circuit and balanced the Wheastone bridge. We then
measured the value of Rv = 314.6 ohms. The results are correct because the
measured and calculated values are very close to each other and the %error is
***%. The complete results are shown in Table#2.
We repeated this procedure with Ra = 0.5 Rb, and assumed that Rx = ….etc
You go through each step of your procedure and explain your results.
CONCLUSION
This lab effectively showed how the Wheatstone bridge provides a mechanism
to calculate an unknown resistance using the known relationships given in the
Wheatstone bridge formula. It demonstrated how to set-up a Wheatstone
bridge and how to manipulate a Wheatstone bridge in a laboratory setting.
Altogether, the Wheatstone bridge is a circuit used to compare an unknown
resistance with a known resistance. Although significant error existed in this
lab, the results still reflect the relationships governing the Wheatstone bridge
sufficiently for understanding in an experimental contextual environment.
Department of Electrical/Computer Engineering, Morgan State University.
Course: EEGR 202 Electric Circuits. Fall 2010
PART TWO:
Light Dependent Resistor (LDR) Light Sensor Circuit-Theory
Temperature Dependent Resistor (TDR) Temperature Sensor Circuit
Keep the same format and repeat all the steps of part I.
INTRODUCTION
The goal of this MS-lab experiment is to experimentally find out how to choose
a “good” or “sensible” value for the fixed resistor R such that the LDR can be
precisely used in a voltage divider circuit to design a light sensor circuit that is
most sensitive to changes in illumination.
THEORY
Include a short theory on how LDRs operate and how Voltage division is
applied here.
Figure#2 – LDR Circuit
Department of Electrical/Computer Engineering, Morgan State University.
Course: EEGR 202 Electric Circuits. Fall 2010
PROCEDURE
Follow the same steps as in Part I and list your steps.
RESULTS
Include all theoretical and measured results in this section using the order listed
in the procedure.
Table #3: LDR Voltage Measurement Values
Fixed resistor
Vout in the light
Vout in the shade Voltage change
R value
100
1
10
100
1M
ANALYSIS/DISCUSSION
This is the most important part of your report, because here, you show that
you understand the experiment beyond the simple level of completing it.
Explain. Analyze. Interpret.
Please refer to the tables with the results values to justify your analysis.
This is the place where you answer any questions posed in the lab
handout.
CONCLUSION
Department of Electrical/Computer Engineering, Morgan State University.