BEng (Hons) in Electrical and Electronic Engineering

1. EEEC6430312
Measurements and Instrumentation
Faculty of Engineering and Computer Technology
Laboratory Manual
Lecturer: Ravandran Muttiah BEng (Hons) MSc MIET
Year/Semester: Year 2 / Semester 1
Academic Session: 2020/2021
The information in this documentis important and should be noted by all students undertaking the
Bachelor of Engineering (Honours) in Electrical and Electronic Engineering
Approved by Coordinator: Endorsed By Dean:
------------------------------------------ __________________

2. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Measurements and Instrumentation 1
Mini Project 1 - Wheatstone Bridge Light Detector
Theory
A Light Dependent Resistor (LDR) is a light-controlled variable resistor. The resistance
of LDR decreases with increasing incident light intensity; in other words, it
exhibits photoconductivity. A LDR can be applied in light-sensitive detector circuits, and
light-activated and dark-activated switching circuits.
A LDR is made of a high resistance semiconductor. In the dark, a LDR can have a
resistance as high as several MΩ, while in the light a LDR can have a resistance as low as
a few hundred Ωs. If incident light on a LDR exceeds a certain frequency, photons
absorbed by the semiconductor give bound electrons conduct electricity, thereby to jump
into the conduction band. The resulting free electrons conduct electricity, thereby
lowering resistance. The resistance range and sensitivity of a LDR can substantially differ
among dissimilar devices. Moreover, unique LDRs may react substantially differently to
photons within certain wavelength bands.
A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has
its own charge carriers and is not an efficient semiconductor, for example, silicon. In
intrinsic devices the only available electrons are in the valence band, and hence the
photon must have enough energy to excite the electron across the entire band gap.
Extrinsic devices have impurities also called dopants added whose ground state energy is
closer to the conduction band; since the electrons do not have as far to jump, lower
energy photons (that is, longer wavelengths and lower frequencies) are sufficient to
trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus
atoms (impurities), there will be extra electrons available for conduction. This is an
example of an extrinsic semiconductor.
Objectives
The objective of this experiment is to become acquainted with Wheatstone bridge circuit.
Demonstrate your ability to design and construct the Wheatstone bridge light detector
circuit and to view the function of the circuit.
The Wheatstone bridge circuit diagram is shown in figure 1 to how you can make a light
detector. LDR can be used for monitoring and measuring the level of light intensity, or
whether a light source is ON or OFF. An LDR is a resistor where the resistance decreases
with the strength of the light.

3. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Measurements and Instrumentation 2
Figure 1: Wheatstone bridge Light Detector
Specification
Operational Amplifier 741
Transistor NPN 2N2222
Light Dependent Resistor ORP 12
Relay 12V
2 Resistors 10 KΩ
Resistor 1 KΩ
Variable Resistor
A typical Cadmium Sulphide (CdS) cell such as the ORP12 light dependent resistor
typically has a resistance of about one MΩ in dark or dim light, about 900 Ω at a light
intensity of 100 Lux (typical of a well lit room), down to about 30 Ω in bright sunlight.
Then as the light intensity increases the resistance reduces. By connecting a light
dependent resistor to the Wheatstone bridge circuit above, the changes in light levels can
be measured.
A
Op- Amp
741
Relay
LDR
ORP 12
𝑅4
10 KΩ
𝑅5
1 KΩ
𝑅3
10 KΩ
𝑉𝑅1
LDR at
Nominal Light
Levels
Out
2N2222
+12 V
D
C
𝑉D
B
A
𝐷1
TR1
−
+
R

4. AIMST University Faculty of Engineering and Computer Technology
BEng (Hons) in Electrical and Electronic Engineering Measurements and Instrumentation 3
The Function of Wheatstone Bridge Light Detector Circuit
When it’s dark, the LDR has high resistance. This makes the voltage at the base of the
transistor too low to turn the transistor ON. Therefore, no current will go from the
collector to the emitter of the transistor. All the current will instead pass through the LDR
and the potentiometer. When it’s light, the LDR has low resistance. This makes the
voltage at the base of the transistor higher. When it High enough to turn the transistor
ON. Because the transistor is turned on, current flows through the transistor. It flows
from the positive battery terminal, through Relay coil, the LED and the transistor down to
the negative battery terminal. This makes the LED light up.
Report
Write a laboratory report on this project.
(1) Explain in detail about the theory of Wheatstone bridge for the designed circuit with
the theoretical calculations of component values.
(2) Discuss the method of productions of printed circuit board layout, fabrications,
assembly of components and the test results.
(3) Prepare slides for presentation and demonstration of this project.