University of Oum El Bouaghi, Algeria

Areas of interest: architecture, city planning, SPSS, VBA, practical electronics.

Adad Mohamed Cherif

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.

Fig.1

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

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

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

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

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.

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

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).

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.

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

• 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 .

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.

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

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

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

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»

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.

Université d’Oum El Bouaghi

Adad Mohamed Cherif