Our mini-project is to design and check the functionality of ‘SHADOW
INTRUDER ALARM’ circuit. It can detect the movement of any person near it
and triggers the alarm. It can be used at night by shopkeepers to protect the
valuables in their showrooms. It can also be used to provide security at
warehouses (go-downs) where storage and protection of various types of goods
is main concern, and works good for home-security too. A dim lighting in the
room is necessary to detect the moving shadow. Unlike opto-interruption alarms
based on light-dependent resistors (LDRs), it does not require an aligned light
beam to illuminate the photo-sensor.
SR. NO. NAME OF TOPIC PAGE
1. INTRODUCTION 6
2. LIST OF COMPONENTS 7
3. BLOCK DIAGRAM 8
4. CIRCUIT DIAGRAM 9
5. WORKING 10
6. BASIC FUNCTION OF COMPONENTS 11
7. INTRODUCTION TO LDR 13
8. DESIGN CONSIDERATIONS OF A LDR 14
9. APPLICATIONS OF LDR 15
10. ADVANTAGES AND DISADVANTAGES 16
11. RESULT AND CONCLUSION 17
12. REFERENCES 18
LIST OF COMPONENTS
SR. NO. IDENTIFICATION VALUE COMPONENT
1. R1 10 KΩ RESISTOR
2. R2 1 KΩ RESISTOR
3. R3 1 KΩ RESISTOR
4. C1 1000 μF CAPACITOR
5. C2 100 μF CAPACITOR
6. Q1 BC 547 PH TRANSISTOR
7. D1, D2, D3, D4 IN 4007 DIODE
8. LDR - LDR
9. LED1 5 MM RED LED
10. LED2 5 MM GREEN
11. DR1 LT2S-5V DC DPDT RELAY
12. DR2 JQC-3FC 5V DC RELAY
13. REGULATOR IC 7805 REGULATOR
14. BUZZER VK 27 CT -
15. SUPPLY 9V -
16. X’MER 230/12V AC TRANSFORMER
17. S - SWITCH
The circuit can be powered by both battery and the main supply. It there
a is a power cut off from the mains then it automatically switches to battery
mode with the help of the LT2S 5V DPDT replay. The step-down transformer
steps down the voltage from the mains to 12V AC supply. Then a bridge
rectifier is used by the help of 4007 p-n junction diode. A capacitor of 1000µF
is used as a filter to get a steady direct current from the rectifier without any
pulses. A 7805 regulator is used to maintain the voltage to 5V ahead of it in the
circuit. Switch is placed to separate the direct current supply that we receive
from the battery.
The next part of circuit includes the components required for the working of
alarm using the supply from the battery. The DPDT relay switches to battery
mode when the main supply is cut off. This supply is then fed to the next part of
the circuit which is the alarm circuit. There is also a provision of charging the
battery from the mains when the battery is not in use. 2 LEDs are provided in
both the circuits which glow up showing which type of supply is being used by
the alarm circuit.
The alarm circuit includes a LDR, 10KΩ resistor, a BC547 NPN transistor and a
simple relay. When initially no one is standing in front of the LDR the its
resistance remains low, a high voltage passes through the base and emitter of
the npn transistor due to which the relay do not conduct and the alarm remains
off. When someone’s shadow falls on the LDR its resistance increases and a
low voltage passes through the base making the collector of the transistor to
conduct. Hence the relay conducts too and the alarm starts blowing the siren.
BASIC FUNCTIONS OF MAIN COMPONENTS
1. BC 547
BC 547 is mainly used for amplification and
switching purposes. It is a NPN bipolar transistor with
maximum current gain of 800. It is equivalent to the
transistors BC548 and BC549
• Low current (max. 100 mA)
• Low voltage (max. 65V)
2. Resistor The electrical resistor is a element that opposes the flow
of current. It is a passive two terminal electrical component
that implements electrical resistance as circuit element.
The current through the resistor is directly proportional to
the voltage across the resistors’ terminals. It limits the
current flow through the circuit.
A diode is a two terminal electric component with
asymmetric conductance. It is low resistance to current in
one direction and high resistance to the other. The IN 4007 is
a standard switching silicon switching diode. It is one of the
most popular and long-lived switching diodes because of its
dependable specifications and low cost.
• Switching applications up to about100 MHz.
• Reverse recovery time of less than 4 ns.
• Average rectified current in forward direction is 200 mA.
• Maximum direct forward current is 300 mA.
4. Capacitor A capacitor is passive two terminal electrical component
used to store energy electro statically in an electric field. All
capacitors contain at least two electrical conductors
separated by a dielectric. Unlike a resistor a capacitor does
not dissipate energy.
5. LED A light-emitting diode (LED) is a two lead semiconductor
light source that resembles a pn junction diode, expect that
an LED emits light. When an LED’s anode lead by at least
the LED’s forward voltage drop, current flows. Electrons are
able to recombine with holes within the device, releasing
energy in the form of photons. This effect is known as
electroluminescence, and the colour of the light is
determined by the energy gap of the semiconductor.
Transformer is an electrical device that transforms energy
between two or more circuits through electromagnetic
induction. Here this transformer steps down the voltage from
primary to secondary.
7. Buzzer A buzzer is used as an indication to the detection of the
metal. When the frequency of the object matches the
frequency of the tuned circuit then the buzzer starts to sound,
A 3V piezo buzzer is being used in this purpose.
8. Regulator The 7805 is a family of self-contained fixed linear voltage
regulator integrated circuit. These are used in electronic
circuit that requires a regulated power supply due to their
ease of use and low cost. They produce a voltage that is
positive relative to common ground.
9. DPDT Relay A relay is electronically operated switch. Many relays use
electromagnet to mechanically operate as a switch. Relays
are used where it is necessary to control a circuit by a low
power signal, or where several circuits must be controlled by
10. LDR A photoresistor or Light Dependent Resistor (LDR) or
photocell is a light controlled variable resistor. The
resistance of a photo resistor decreases with increasing
incident light intensity; in other words, it exhibits
INTRODUCTION TO LIGHT DEPENDENT
Typical LDR The symbol for a LDR
A photoresistor or light-dependent resistor (LDR) or photocell is a light-
controlled variable resistor. The resistance of a photoresistor decreases with
increasing incident light intensity; in other words, it exhibits photoconductivity.
A photoresistor can be applied in light-sensitive detector circuits, and light- and
dark-activated switching circuits.
A photoresistor is made of a high resistance semiconductor. In the dark, a
photoresistor can have a resistance as high as a few megohms (MΩ), while in
the light, a photoresistor can have a resistance as low as a few hundred ohms. If
incident light on a photoresistor exceeds a certain frequency, photons absorbed
by the semiconductor give bound electrons enough energy to jump into the
conduction band. The resulting free electrons (and their hole partners) conduct
electricity, thereby lowering resistance. The resistance range and sensitivity of a
photoresistor can substantially differ among dissimilar devices. Moreover,
unique photoresistors 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.
DESIGN CONSIDERATIONS FOR A LDR
Photoresistors are less light-sensitive devices than photodiodes or
phototransistors: the two latter components are true semiconductor devices,
while a photoresistor is a passive component and does not have a PN-junction.
The photoresistivity of any photoresistor may vary widely depending on
ambient temperature, making them unsuitable for applications requiring precise
measurement of or sensitivity to light.
Photoresistors also exhibit a certain degree of latency between exposure to light
and the subsequent decrease in resistance, usually around 10 milliseconds. The
lag time when going from lit to dark environments are even greater, often as
long as one second. This property makes them unsuitable for sensing rapidly
flashing lights, but is sometimes used to smooth the response of audio signal
APPLICATIONS OF LDR
Photoresistors come in many types. Inexpensive cadmium sulphide cells can be
found in many consumer items such as camera light meters, street lights, clock
radios, alarm devices, night lights, outdoor clocks, solar street lamps and solar
road studs, etc.
They are also used in some dynamic compressors together with a small
incandescent or neon lamp, or light-emitting diode to control gain reduction. A
common usage of this application can be found in many guitar amplifiers that
incorporate an onboard tremolo effect, as the oscillating light patterns control
the level of signal running through the amp circuit.
The use of CdS and CdSe photoresistors is severely restricted in Europe due to
the RoHS ban on cadmium.
Lead sulphide (PbS) and indium antimonide (InSb) LDRs (light-dependent
resistors) are used for the mid-infrared spectral region. Ge:Cu photoconductors
are among the best far-infrared detectors available, and are used for infrared
astronomy and infrared spectroscopy.
ADVANTAGES & APPLICATIONS OF
• It is very compact in nature. And due to this the weight of the whole
apparatus is very less.
• It is very easy to assemble the alarm. Just place it at a position such the
persons shadow falls directly on it.
• It works on both direct current and alternating current.
• In case it the main supply is cut-off, an auxiliary battery supply is
provided so that it works without any interruptions.
• The auxiliary battery charges itself when the main supply is on, thus
providing an alternative backup at time of cut-off.
• It has a very large and alarming sound.
• It’s other applications are- as counter circuit and an electric bell.
• A source of dim light is always required for it’s functioning.
• It is a conventional type of alarm. Modern alarm consists of infrared
diodes, which are more accurate than this.
Thus the project was successfully completed. The knowledge of working and
basic principles of each component was thoroughly studied and implemented in
this project. A fully working “SHADOW ALARM” mini-project was prepared
• Principles of electronics – V.K.Mehta
• Basic Electrical Engineering – Edward Hughes