This security project is able to protect your house balcony or loggia by means of an optical path. This laser beam is almost invisible. The beam or light ray of Laser pointer should be pointed directly to a mirror fixed on the front wall . The reflected light ray should fall on surface of the LDR. The alarm system consists of two main sections : the transmitter (laser pointer) and the receiver (LDR)

1. University of Oum El Bouaghi, Algeria
Areas of interest: architecture, city
planning, SPSS, VBA, practical
electronics.
Adad Mohamed Cherif

2. Nowadays, cities are not safe and therefore, it is necessary to protect your
house and your belongings. There are plenty of thieves interested in breaking
into your home. So, think about implementing your own home security alarm
system. The simple circuit we are going to build up can prevent your home from
burglary while you’re away at night-time.
This homemade circuit offers plenty of other functions. For examples, Light/Dark
detector. An alarm buzzer or an indicator light may be switched on by a room
light when intruder gets into your house or when drawer, coffer containing
valuable items and door of the fridge are open. In all these cases, you need
only section I of the project , fig.1.

3. Fig.1

4. This security project is able to protect your house balcony or loggia by means of
an optical path. This laser beam is almost invisible. The beam or light ray of Laser
pointer should be pointed directly to a mirror fixed on the front wall . The
reflected light ray should fall on surface of the LDR. The alarm system consists
of two main sections : the transmitter (laser pointer) and the receiver (LDR) .
They are fitted in the same project case, fig. 2.
Transmitter/receiver
Mirror
Fig.1

5. LDR stands for Light Dependent Resistor. It is also called a photo
resistor or a photocell. Before we explain how the whole system works,
let’s concentrate on the working principal of the LDR . This circuit is
primarily based on the LDR component, fig.3, fig.4.
The LDR works on the principle of “Photo Conductivity”. whenever
light falls on the surface of the LDR the resistance of the LDR falls
because it is a property of “cadmium sulphide (CdS)”, a semiconductor
material used on the surface. It is known that LDR will have high
resistance in darkness (several hundred kiloohms) and when it is
illuminated with the light , its resistance drops drastically (few ohms)
and so do the voltage across it. The fig. 5 shows the full circuit diagram
of the alarm unit. The circuit consists of two main sections :
daylight/darkness sensor and laser beam transmitter/receiver, fig.6.
Fig.3 -Symbole of LDR Fig.4- External view of
LDR

6. The intruder
Fig.5. The full circuit diagram of Home security alarm
Relay1
Switch1
on/Off
Relay2
Daylight : T1/T2
Darlington is
switshed off
In darkness T1/T2
Darlignton is switched
on

7. Fig.6 - Section I and section II of the circuit

8. This is the circuit diagram of a laser alarm system based on transistors and a
LDRs. The output are relays and flashing light. In other words, It is a dark
activated switch. During the night-time the system is on and in order to sound
off, the light beam must be broken by the intruder .
As you can see this laser alarm system circuit is very simple and can be easily
built on a stripboard. The main components in this project are laser pointer, LDR
and NPN and PNP transistors, relays, siren and flashing light.
Section I of the circuit : daylight/darkness sensor
In general, when the darkness falls on the LDR, the T1 and T2 , connected as
Darlington amplifier, drive the relay 1 which is in turn connects the power
supply to the circuit of the section II. If everything works as expected we
should see LED 2 turned on, the siren sets off and the flashing light starts
blinking, fig.7.
Again, in the darkness or when the light intensity goes below a certain level,
the resistance of the LDR will be high and so do the voltage across it. The
sensitivity of this part of the unit can be adjusted by means of the
potentiometer VR1. In these conditions, according to the diagram below, LDR is
configured with the transistor T1 in such a way that the base (B) of T1 is at
more than 0,6 V , this should enable T1 to conduct (D or the collector is
negative) and so T2 which drives the relay1 after a short time which depends
on the value of the capacitor C1 (220 μF) . Thus, now the relays1 is switching the
supply voltage to the section II based on the transistors T3,T4 et T5.

9. So, the positive voltage is now connected to the capacitor C1 which starts charging
through both the resistors R1 and VR1 after a couple of seconds the base of T1 becomes
positive and the emitter follower Darlington pair will be conducting and power is fed to
the relay1 coil, the normally open relay contacts are now closed to supply power to section
II. This capacitor is useful because the relay will not suddenly switch on and off when the
light intensity is changing rapidly.
Section II is
supplied
with power
via contacts
of relay1
A
D
B
C
Fig. 7 - Section I of the circuit

10. The section II of the circuit is no more connected to the power
supply (9 to 12V). The diode D1 works as a freewheeling diode which
protects T1 and T2 from the reverse voltage peaks of the relay coil.
To set the desired light intensity for switching the relay1, we should
adjust slowly VR1. The relay1 must be either on or off.
It is worth noting that you can insert a resistance of 10 Ω between the
emitter of T2 and the ground. The reason is to stabilise the
temperature of T2.
However, when the surface of the LDR is illuminated (i.e. Daylight),
its resistance drops and so do the voltage across it. The base of T1 is at
0 volt and consequently T1 and T2 are cut off, their collectors are
positive. As a result LED1 and the relay1 is switched off after a short
while fixed again by C1 because the plus of C1 remains positive
because the base of T1 is positive (NPN).

11. The T3 et T4 are preamplifier transistors with high gain. When the intruder interrupts the ray of light
falling on surface of the LDR, the potential at the junction of voltage divider circuit ( F) rises and
therefore T5 transistor will activate, in the same time, the thyristor ( which it should in turn switch on
the flashing light), the LED 2 and the relay2 which should connect the power supply to the siren . T5
which drives the relay2 and the thyristor after a predetermined amount of time which depends on
the value of the capacitor C1 (220μF) . So the alarm will only continue to sound during a couple of
minutes , so that the householder would be alerted while the neighbours would not be disturbed .
This delay can be increased by the use of higher values of the C1 : 470 μF , 1000 μF or more.
The sensitivity of this part of the unite can be adjusted by means of the potentiometer VR2. To
prevent relay chatter, a positive feedback should be applied to the base of T3 through R5 of 100k. The
diode D2 works as a freewheeling diode, fig8. From the circuit of section I
F
Fig. 8 -
Section II of
the circuit
Section II of the circuit laser beam transmitter/receiver.

12. For any reason if we want to hold on the alarm , we can use a relay with two contacts RL1 and RL2,
instead of one contact. The goal is to achieve a self-latching property . When the transistor T5 has its
base strapped to negative rail (T4 is conducting ), nothing happened to relay2. However, when its base
is negative (it means T4 is non-conducting), the collector current of T2 will energise relay2 and contacts
RL1 and RL2 will close . The RL2 contacts latch the relay2 on and RL1 contacts operate the alarm which
continues to sound until the unit is switched off . Only one wiring is added to the unit between A and
B via RL2. D2 , called freewheeling diode , across coil, protects T5 from the back e.m.f of the relay2
coil at switch-off., fig .9.
Switch1
on/off
Fig.9 -Self-latching

13. • In Darkness
T1 and T2 are on, LED1 and relay1 are on .
 If laser beam is broken then :
 T3 is on, T4 is off, T5 is on, thyristor is on
(flashing light on), LED2 is light and relay2 is
energised (Siren on).
• In Daylight
T1 , T2 and LED1 are off . Therefore, section II is
switched off because the power supply has been
removed. Everything is switched off:
 LED2, flashing light, relay2 and siren .

14. Fig.10 and fig.11 show the matrix stripboard for the unit . The construction
of the unit is straight forward and should not present any problems . It will be
necessary to have LDR1 mounted where it will receive the daylight. The two
sections of the unit are housed in a small plastic project case with the LEDs
displayed on both sides . It is necessary for the LDRs to be fitted inside the
plastic case inside a black short plastic tube (about 10 mm diameter ). Also, it is
interesting to mount each section of the unit on separate stripboard. This
allows to check very easily the location of the components.
It is essential that all components such as transistor, thyristor, LED, diode,
capacitor, siren and flashing light must be connected to the correct polarity.
Connecting all theses components the wrong way will prevent the circuit to
work properly.
This system is obviously not perfect in that it is possible for an intruder to
enter the house and switch off the alarm before it sounds. To avoid this, the
alarm should be installed in such a way that it would be impossible for anyone
to switch off the unit.
The LEDs are fitted on the stripboard but attached firmly to the case.

15. Section I of the circuit
The LED1 and the LDR are not fitted on the stripboard. Both must be fixed on the
outside face of the project box.
Fig.10

16. Section II of the circuit
The LED2 and the LDR2 are not fitted on the stripboard. Both must be fixed on
the outside face of the project box.
Fig.11

17. Fig.12 - Both sections are housed inside the project plastic case

18. External view I
Fig. 13 - The circuit inside
(section I et II) the plastic case
LDR2 Laser pointer
Switch on/off
LED1
LED1
LDR1 LED2
LED1
LDR1
LDR1
Laser pointer
Laser pointer
Section II
Section I
« Internal View II »
« Internal View III»

19.  Resistors
LDR1, LDR2
R1 22kΩ
R2 1kΩ
R3 1kΩ
R4 1.5kΩ
R5 100k Ω
R6 220Ω
R7 100Ω
R8 100Ω
R9 1kΩ
R10 1kΩ
R11 470Ω
 Potentiometers
VR1 1MΩ
VR2 470kΩ
 Capacitors
C1, C2 220 μF
 Semiconductors
T1, T2, T5 2N1893
T3, T4 BC548
D1, D2 1N4007
LED1 yellow
LED2 Green
 Switch On/off
 Relays
Relay1, Relay2 12V coil (400 Ω)
 Sounder
siren 12V
 flashing light 12V
 Laser pointer 3V
 Miscellaneous
Small plastic case, matrix stripboard , 9 à 12V
transformer power supply, wire.

20. Université d’Oum El Bouaghi
Adad Mohamed Cherif