This document describes a security alarm system that uses various sensors including a light dependent resistor (LDR). The system uses an LDR circuit that triggers an alarm when a person's shadow falls on the LDR, blocking the light. The circuit uses common electronic components like an LDR, timer IC NE555, capacitors, resistors, and a transformer. It provides intrusion detection with benefits of low cost and energy efficiency compared to other security systems.

1. Electronics and Communication Department
A mini-project of Electrical and Electronics Workshop by:
Darshan Parmar-140110111009
Nirav Patel-140110111041
Prerak Trivedi-140110111045

2. INTRODUCTION
A security alarm is a system designed to detect intrusion – unauthorized entry – into a
building or area. Security alarms are used in residential, commercial, industrial, and military
properties for protection against burglary (theft) or property damage, as well as personal
protection against intruders. Car alarms likewise protect vehicles and their contents. Prisons
also use security systems for control of inmates.
These Security Alarms work on various sensors like:
i. Hermetically Sealed Reed Switches
ii. Passive infrared detector
iii. Ultrasonic detectors
iv. Photo-electric beams
v. LDR detectors
Hermetically Sealed Reed Switches
The Hermetically Sealed Reed Switch is a very common type of two piece sensor that
operates with an electrically conductive reed switch that is either normally open or normally
closed when under the influence of a magnetic field as in the case of proximity to the
second piece which contains a magnet. When the magnet is moved away from the reed
switch, the reed switch either closes or opens, again based on whether or not the design is
normally open or normally closed. This creates a fault in the circuit, switching the alarm on.
Passive Infrared Detector
The passive infrared (PIR) motion detector is one of the most common sensors found
in household and small business environments as they offer affordable and reliable
functionality. It works by detecting the heat energy given off by other objects. Frankly, PIR
sensors do not detect motion; rather, they detect abrupt changes in temperature at a given
point. As an intruder walks in front of the sensor, the temperature at that point will rise
from room temperature to body temperature, and then back again. This quick change
triggers the detection and the intrusion can be easily detected.
Ultrasonic Detectors
Using frequencies between 15 kHz and 75 kHz, these active detectors transmit
ultrasonic sound waves that are inaudible to humans. Any motion of human being causes a
change in frequency and thus unwanted intrusion is detected.
Photo-electric beams
Photoelectric beam systems detect the presence of an intruder by transmitting
visible or infrared light beams across an area, where these beams may be obstructed. To
improve the detection surface area, the beams are often employed in stacks of two or more.
However, if an intruder is aware of the technology's presence, it can be avoided. This
technology works best if a fence like construction is created making it impossible for the
intruder to cross without blocking the beam.

3. LDR Security System
LDR security system works on the principle of resistor dependent on light. This clearly
states that if a person’s shadow falls on the LDR, the buzzer buzzes off so that people can be
alarmed.
Now we know that it includes a sensor so no manual operation is required to switch the
device ON or OFF. Since we have used a transformer which can convert AC current to DC
current for a circuit we can provide a switch connection in the house from where a person can
easily switch it ON or OFF at the required time.
This circuit can be fitted easily anywhere as wanted by the user. The LDR must face
against the direction of light so when a person tries to trans-pass the LDR, the person’s shadow
falls on it. This will activate the alarm.
This activation can be signalled in many ways. A few ways are like switching ON and OFF
of security lights in house, buzzing of the buzzer, closing of automatic doors and windows,
activating security cams etc.
We, personally find this LDR useful in many ways. The most important of all is energy
saving and cost effective. This LDR is energy saving as when it is connected to security cameras,
they get activated only when trans-passing is done and thus saving electrical energy. It is cost
effective as making one of this kind requires an amount of approximately ₹ 300 only.
Component Amount
4 Diodes ₹ 8
1 L.D.R ₹ 10
1 I.C. NE555 ₹ 10
1 Capacitor (410 µF) ₹ 15
1 P.C.B. ₹ 40
1 Resistor of 100 kΩ i.e. ORP12 ₹ 05
1 Regulator IC lM7809 ₹ 15
1 Transformer 909 ₹ 65
Other small equipment ₹ 20
Total ₹ 260

4. Light Dependent Resistor:
As its name implies, the Light Dependent Resistor (LDR) is made from a piece of exposed
semiconductor material such as cadmium sulphide 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 illumination. Also, photo resistive 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 photo resistive light sensors being Cadmium Sulphide (Cds).
Cadmium sulphide is used in the manufacture of photoconductive cells because its spectral
response curve closely matches 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 (λp) of about 560nm
to 600nm in the visible spectral range.
The most commonly used photo resistive light sensor is the ORP12 Cadmium Sulphide
photoconductive cell. This light dependent 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 10MΩ’s which falls to about 100Ω’s when fully illuminated (lit
resistance).

5. IC NE555
The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse
generation, and oscillator applications. The 555 can be used to provide time delays, as an
oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one
package.
Introduced in 1971 by Signetics, the 555 is still in widespread use due to its ease of use,
low price, and stability. It is now made by many companies in the original bipolar and also in
low-power CMOS types. As of 2003, it was estimated that 1 billion units are manufactured every
year.
PINS:

6. TRANSFORMER 909
A transformer is an electrical device that transfers energy between two or more circuits
through electromagnetic induction.
A varying current in the transformer's primary winding creates a varying magnetic flux
in the core and a varying magnetic field impinging on the secondary winding. This varying
magnetic field at the secondary induces a varying electromotive force (EMF) or voltage in the
secondary winding. Making use of Faraday's Law in conjunction with high magnetic permeability
core properties, transformers can thus be designed to efficiently change AC voltages from one
voltage level to another within power networks.
The windings are concentrically arranged to minimize the flux leakage. The conducting
material used for the windings depends upon the application, but in all cases the individual
turns must be electrically insulated from each other to ensure that the current travels
throughout every turn.
Regulator IC LM7809
7809 is a voltage regulator integrated circuit(IC) which is widely used in electronic
circuits. It has three pins. The Voltage input and Voltage output pins as well as the ground pin. It
is widely used for voltage regulation purposes.

7. How did We Make A Bridge Rectifier using IC 7809?
A bridge rectifier is an arrangement of four or more didoes in a bridge circuit
configuration that provides the required voltage output in DC to whatever input may be. It can
also be considered as AC to DC Converter.
A Bridge Rectifier provides full wave rectification from a two-wire AC Input, resulting in
a cost effective and light transformer as compared to others.
The speciality of this transformer is that the polarity of the output remains unchanged
regardless of the input.
It is also known as Graetz Bridge in memory of German Physicist Leo Graetz.

8. CIRCUIT DIAGRAM

9. Report
An A.C. current of 230V is directly supplied to the transformer which converts it into
12V A.C. supply which then is transferred to bridge rectifier converting it into 9V D.C. supply.
We have used an LDR to detect the presence of any object. The value of LDR depends on
the brightness of the light falling on it. It has the resistance of 1MΩ when in total darkness and
approx. 5kΩ when in brightness. Here, we have made a potential divider circuit by connecting
LDR and Resistor of 100kΩ in series.
We know that voltage is directly proportional to conductance so the voltage gained will
be more when LDR is getting light. This divided voltage is given to the PIN 2 of IC 555.
IC 555 is popular timer IC which is connected as comparator with pin 6 connected with
positive rail. The output goes 1 or high when the trigger pin (PIN 2) is lower than 1/3rd level of
supplied voltage and 0 or low when the lower pin is 1/3rd above the supplied voltage. Hence,
PIN 2 does not have any intermediate state except for Low state (0) and High State (1).
So a small change in trigger pin (PIN 2) is enough to vary the level in Output pin (PIN 3).
Rest pins i.e. PINS 4, 6 and 8 are connected to positive supply and PIN 1 is grounded.
So whenever a shadow falls on the LDR the voltage across the PIN 2 drops more than
1/3rd of applied voltage which starts the beeping of the buzzer.