Yangon Institute of Education Research Journal 2011, Vol. 3, No. 1
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Design and Construction of a Sensor Circuit of (a counting device)
Using LDR
Khin Tint 1, Mya Aye2, Wint Shwe War Hlaing3
Abstract
This is a small scale design and construction of a circuit that can be made in the laboratory.
And also it intends to be used as a teaching aid of the Physics Academic Subject,
Introduction to Electronics and Semiconductor Physics (Phy-4002) for 4th year B.Ed.
students. This explains how an LDR can be used in simple circuits to control devices
according to the ambient levels of lighting. The 4th year B.Ed. students can learn the basic
design of electronic circuit practically.
This sensor circuit is built by using LDR Light Dependent Resistor, 7447 ICs, 7490 ICs ,
555 IC, resistors (100 ohm, 220 ohm, 10 kilo ohm, 5 kilo ohm, 1 kilo ohm), capacitors
(0.1µF, 10µF) , seven-segment LED displays , 9-volt battery and connecting wires are
required. It counts two digits 00 to 99. So it is applicable to check the entry and exits for
security.
Keywords: sensor circuit, LDR, ICs, resistors, capacitors, LED displays
Introduction
Electronic sensor circuits convert light, temperature, sound, and other signals into a
form that can be processed by electronic circuits. By using solar cells, photo- resistors,
thermistors, and magnet switches. Then sensor circuits can be built so as to respond to
heat, pressure, light, and more.
In this paper a counting device of two digits (0-99) is described. It is built by using
LDR Light Dependent Resistor, 7447 IC, 7490 IC and 555 IC. In this sensor circuit the
following components such as resistors (100 ohm, 220 ohm, 10 kilo ohm, 5 kilo ohm,
1 kilo ohm), capacitors ( 0.1µF, 10 µF) , seven-segment LED displays , 9-volt battery and
connecting wires are required.
This paper explains the design and construction of the sensor circuit and gives the
knowledge on the operation of the devices composed of some counters and decoder
circuits.
Objectives
This paper aims to get the basic idea on how to design and construct an electronic
circuit by using electronic elements such as resistors, capacitors, ICs and LDR. This is a
small scale design and construction of the circuit that can be made in the laboratory as a
practical experiment with B.Ed. students. And also it intends to be used as a teaching aid
of the Physics Academic Subject, Introduction to Electronics and Semiconductor Physics
(Phy-4002) for 4th year B.Ed. students to explain how LDR can be used in simple circuits
to control devices according to the ambient levels of lighting.
1. Lecturer, Department of Educational Psychology, Yangon Institute of Education
2. Daw, Senior Teacher, Basic Education High School, Yartike, Pauk Taw Twonship, Rakhine State
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Types of Sensor circuits
There are many types of sensor circuits; Temperature sensor, atmospheric Pressure
sensor, Soil moisture sensor , atmospheric Humidity sensor , Sunlight sensor, Capacitive
sensors, Inductive sensors, Displacement sensors, Infra red sensors, Magneto resistive
sensors, Ultrasonic sensors, etc... In this paper, optical sensor such as light sensor circuit
using LDR is designed.
Components of Sensor Circuit
Sensors, Amplifier, Oscillators, Decoders, Counters and Flip-flop are most widely
used in digital electronic systems. The components in the sensor circuit are:
1.1
counting device
1.2
LDR light Dependent Resistor
1.3
ICs (7447 IC, 7490 IC, 555 IC)
1.4
Resistors
1.5
Capacitors
1.6
Lamp
1.7
7- segment displays
1.8
9 – volt battery and
1.9
Cables/ connecting Wires.
It can be understood that the operation of these devices composed of some counters
and decoder circuits. Thus we can apply our knowledge on how to design the construction
of this sensor circuit and its application.
Theory Background of Light Sensor Circuit
A Light Sensor generates an output signal indicating the intensity of light by
measuring the radiant energy that exists in a very narrow range of frequencies basically
called "light", and which ranges in frequency from "Infrared" to "Visible" up to
"Ultraviolet" light spectrum. The light sensor is a passive devices that convert this "light
energy" whether visible or in the infrared parts of the spectrum into an electrical signal
output. Light sensors are more commonly known as "Photoelectric Devices" or "Photo
Sensors" because that converts light energy (photons) into electricity (electrons).
Photoelectric devices can be grouped into two main categories, those which generate
electricity when illuminated, such as Photo-voltaics or Photo-emissives etc, and those
which change their electrical properties in some way such as Photo-resistors or Photoconductors.
Dependant Resistor or LDR (The Light Dependant Resistor)
As its name implies, the Light Dependant Resistor (LDR) is made from a piece
of exposed semiconductor material such as cadmium sulphide (Cds) that changes its
electrical resistance from several thousand Ohms in the dark to only a few hundred Ohms
when light falls upon it by creating hole-electron pairs in the material. The net effect is an
improvement in its conductivity with a decrease in resistance for an increase in
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illumination. Also, photoresistive cells have a long response time requiring many seconds
to respond to a change in the light intensity.
Materials used as the semiconductor substrate include, Lead Sulphide (PbS), Lead
Selenide (PbSe), Indium Antimonide (InSb) which detect light in the infra-red range with
the most commonly used of all photoresistive light sensors being Cadmium Sulphide
(Cds), Cadmium Sulphide is used in the manufacture of photoconductive cells because its
spectral response curve is quite similar to that of the human eye and can even be
controlled using a simple torch as a light source. Typically then, it has a peak sensitivity
wavelength ( p) of about 560nm to 600nm in the visible spectral range.
Figure 1. The Light Dependant Resistor Cell
The most commonly used photoresistive light sensors are the ORP12 Cadmium
Sulphide photoconductive cell. This light dependant resistor has a spectral response of
about 610nm in the yellow to orange region of light. The resistance of the cell, when
unilluminated (dark resistance), is very high at about 10M 's and which falls about
100 's when fully illuminated (light resistance) is not high. To increase the dark
resistance and therefore reduce the dark current, the resistive path forms a zigzag pattern
across the ceramic substrate. The CdS photocell is a very low cost device often used in
auto dimming, darkness or twilight detection for turning the street lights "ON" and "OFF",
and for photographic exposure meter type applications.
One simple use of a Light Dependant Resistor is as a light sensitive switch as
shown in below.
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Figure 2. A Sample Circuit of a Light Dependant Resistor
LDR Switch
This basic light sensor circuit is of a relay output light activated switch. A potential
divider circuit is formed between the photoresistor, LDR and the resistor R1. When no
light is present i.e in darkness, the resistance of the LDR is very high in the Megaohms
range so zero base bias is applied to the transistor TR1 and the relay is de-energised or
"OFF".
As the light level increases the resistance of the LDR starts to decrease causing the
base bias voltage at V1 to rise. At some point determined by the potential divider network
formed with resistor R1, the base bias voltage is high enough to turn the transistor TR1
"ON" and thus activate the relay which in turn is used to control some external circuitry.
As the light level falls back to darkness again the resistance of the LDR increases causing
the base voltage of the transistor to decrease, turning the transistor and relay "OFF" at a
fixed light level determined again by the potential divider network.
Design and construction of a counting device of a sensor circuit by using LDR
In this Light Sensor Circuit, ICs used in this circuit and Functions of Circuit
Components are explained.
LDR operation relies on the fact that the conductive resistance of a film of
Cadmium Sulphide (Cds) varies with the intensity of light falling on the face of the film.
This resistance is very high under dark conditions and low under bright conditions. Fig (3)
shows the LDR’s circuit symbol and basic construction, which consists of a pair of metal
film contacts separated by a snake-like track of light-sensitive Cadmium Sulphide film;
which is designed to provide the maximum possible contact area with the two metal films.
The structure is housed, in a clear plastic or resin case, to provide free access to external
light.
Practical LDRs are available in a variety of sizes and package styles, the most
popular size having a face diameter of roughly 10 mm. LDRs are sensitive, inexpensive
and readily available devices with power and voltage handling capabilities similar is those
of conventional resistors. Their only significant defect is that they are fairly slow acting,
taking tens or hundreds of milliseconds to respond to sudden changes in light level. Useful
LDR applications include light and dark-activated switches and alarms, light-bean alarms
and reflective smoke alarms, etc.
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Figure 3. LDR’s circuit symbol and basic construction
Decade Up Counter IC 74LS90
As a decade counters the IC 74LS90 is used. IC 74LS90 counter is widely used in
electronic systems to determine in digital electronic devices. They consist of flip-flops
connected so that they toggle when the pulses to be counted are applied to the clock input.
Counting is done in binary code, the “high” and “low” states represent the bits “1” and “0”
respectively.
The number of flip-flops used and the way in which they are connected determine
the number of states (called the modulus) and also the specific sequence of states that the
counter goes through during each complete cycle. Counters are classified into two broad
categories accounting the way they are clocked asynchronous and synchronous.
In asynchronous counters, commonly called ripple counters, the first flip-flop is
clocked by the external clocked pulse and then each successive flip-flop is clocked by the
output of the preceding flip-flop. In synchronous counters, the clock input is connected to
all of the flip-flops so that they are clocked simultaneously. Within each of these two
categories, counters are classified primarily by the type of sequence, the number of states,
or the number of flip-flop in the counter.
An up/down counter is one that is capable of progressing in either direction
through a certain sequence. An up/down counter, sometimes called a bidirectional counter,
can have any specified sequence of states. A 4- bit binary counter that advances upward
through its sequence (0,1,2,…..,13,14) and then can be reversed so that it goes through the
sequence in the opposite direction is an illustration of up/down sequential operation. Pin
configuration of the IC 74LS90 is shown in Fig 4. IC 74LS90 counter count every
incoming pulse for one second and its output is in binary format. After one second of
counting the counter should be reset its BCD (Binary Coded Decided) output to initial
state and ready to count next incoming signal.
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Figure 4. Pin configuration of IC 74LS90
74LS47 BCD – To – 7 Segment Decoder / Driver IC
The 74LS47 IC has active – low outputs designed for driving a common – anode
LED display. The 74LS47 IC has three input “control” terminals, these being designated
LI (Lamp Test), BI / RBO and RBI. The LT terminal drives all display outputs on when
the terminal is driven to Logic “0” with RBO terminal open at logic “1”. When the BI /
RBO terminal is pulled low all outputs are blanked; this pin also functions as a ripple
blanking output terminal.
f
g
a
b
c
d
e
Figure 5. Pin configuration of 74LS47 decode/ driver IC
Seven – Segment LED (Light Emitting Diode) Display
A very common requirement in modern electronics is that of displaying
alphanumeric characters. Digital watches, pocket calculators, and digital multimeters and
frequency meters are all examples of devices that use such displays. The best-known type
of alphanumeric indicator is the 7-segment display, which comprises seven independently
accessible photoelectric segments (such as LEDs or liquid crystals, or gas-discharge or
fluorescent elements, etc) arranged in the form shown in Fig 6. The segments are
conventionally notated from a to g in the manner shown in the diagram, and it is possible
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to make them display any number (numeral) from 0 to 9 or alphabetic character from A to
F (in a mixture of upper and lower case letters) by activating these segment in various
combinations.
Figure 6. Standard form and notations of a 7-segment display
Resistor
The resistance of a conductor restricts the flow of changes in a circuit. Devices
making use of this property are called resistors. They are circuit components specially
constructed to have resistances. Resistors are usually made from carbon or alloy such as
nichrome or constantan.
There are two types of resistors.
(i)
Fixed resistor
(ii)
Variable resistor
In this circuit we can use only the fixed resistors. Fixed resistors have fixed
resistance values. A fixed resistor’s resistance always remains the same. There are three
kinds of fixed resistors.
(i)
Carbon film resistor
(ii)
Metal film resistor and
(iii)
Wire-wound resistor.
Capacitors
A capacitor stores electric charge. It does not allow direct current to flow through.
In its simplest form, it consists of two parallel metal plates separated by an insulator called
the dielectric.
The capacitance (C) of a capacitor measures its ability to store charge and is stated
in farads (F). In practice, capacitances range from 1pF to about 150000 F. There are many
kinds of capacitors for the values of capacitance. They are air, ceramic, mica, paper, film
and electronic capacitors.
555 General – purpose Timer
The 555 monolithic timing circuit is a highly stable controller capable of producing
accurate time delays or oscillation. Additional terminals are provided for triggering of
resettling if desired. In the time-delay mode of operation, the time is precisely controlled
by an external resistor and capacitor. For stable operation as an oscillator, the free-running
frequency and duty cycle are both accurately controlled with two external resistors and
capacitors.
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555 IC Timers
555 IC timer is composed of two comparators, a flip-flop, a discharge transistor
and resistive voltage diver (23 transistors, 16 resistors and 2 diodes) depending on the
manufactures and it has 8-pins dual in package. The pin diagram of 555 IC timer is shown
in Fig 7.
Pin 1:
This is the ground pin and is connected to ground. The voltage should be
the most negative of any voltage appearing at the other pins.
Pin 2:
This is the trigger input. When a negative going pulse causes the voltage
at this point to fall below one-third of the Vcc, the comparator to which
this input is connected causes the flip-flop to change the state, causing the
output level to switch from low to high.
Pin 3:
This is the output pin. It is capable of sinking or source a load that
requires up is 200mA of current. The output voltage available at this pin
is approximately equal to (Vcc – 1.7V).
Pin 4:
This is the reset pin. It is used to reset the flip-flop that controls the state
of output pin 3. This pin is activated when a voltage level anywhere
between 0 and 0.4 V is applied to the pin. To prevent unwanted resetting
of output. Pin 4 should be connected along with pin 8 to the positive side
of Vcc when not in use.
Pin 5:
This is the control voltage input. If it is not used pin 5, it will be ground.
Though 0.01 µF to 0.1 µF capacitor for immunity to noise.
Pin 6:
This is the threshold input. It resets the flip-flop and consequently drives
the output low if the voltage applied to it rises above two-third of the
value of the voltage applied to pin 8. In addition to the voltage level, a
current of at least 0.1 µA must be supplied to this pin.
Pin 7:
This is the discharge pin. It is connected to the collector of the npn
transistor. The emitter of the transistor is connected to the ground.
Pin 8:
This is the power supply pin and is connected to the positive side of the
power supply. The voltage applied to this pin can vary from 4.5V to 15V.
For commercial devices, the selected device that operates at voltage as
high as 12.4 V are available.
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Figure 7. Pin diagram for 555IC Timer (8 pin DIL)
Operation System
In the digital counter circuit, a digital frequency counter counts the number of
pulses per second, which gives the frequency directly in Hz. In this circuit, any moving
object passes through the LDR that produces a pulse. This pulse is amplified by the
transistor amplifier in IC 3 (555 IC Timer)
The amplified pulse is directly to count and display at the 7-segment LED display
by the use of IC 74LS90 (IC 5) decade counter. The output of IC 2 is connected with the
divide-by-10 connection.
Since the decade counter output is connected to the inputs of IC 4 (IC 74LS47)
decoder/ driver and it will drive to display the 7-segment LED display.
When the 7-segment LED display is 80 counts, IC 6 (555 IC Timer) is alert. We can see
that the complete circuit diagram as shown in Fig 8. The photos of this circuit are also
shown below.
Figure 8. Complete Circuit Diagram
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PHOTOS OF CONSTRUCTED SENSOR CIRCUIT BOARD
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Result and Discussion
A small scale of light sensor circuit is constructed by using the idea of the light
sensor. ICs, resistors, capacitors, seven segment display, 9 volt battery and LDR are used
in this circuit. This design and construction of the circuit are made in the Physics
laboratory (Yangon Institute of Education). It intends to be used as a teaching aid of the
Physics Academic Subject (Phy-4002), Introduction to Electronics and Semiconductor
Physics for 4th year B.Ed. students. The components in the circuits are very common and
easy to get. So the learners can design and construct similar circuits with the knowledge of
basic electronic concept. For the students, it can help to comprehend the basic electronic
concept and uses of ICs.
Acknowledgement
We sincerely express our gratitude to Rector Dr Aung Min, Yangon Institute of Education, for his
permission to present this paper. Many thanks are due to Pro-Rector Dr Pyay Thein ,Yangon Institute of
Education and also Professor Dr Khin Mu Soe , Head of the Department of Physics , Yangon Institute of
Education.
We sincerely express our gratitude to all of our teachers and the staff from
Physics, Yangon Institute of Education.
References
Duncan T 1992 “Adventures with Microelectronics” (London: John Murrary)
Duncan T 1999 “Success In Electronics” (London: John Murrary)
Floyd T L 1997 “Digital Fundamentals” (New Jersey: Prentice-Hall)
Kamichik S 1998 “IC Design Projects” (Indianapolis: Howard W Sams)
Pasil Y 2002 “Electronics Projects 16” (New Delhi: EFY)
http://www.educypedia.be
the Department of
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* Assistant Lecturer, Department of Biology, Yangon Institute of Education
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