LDR Based Wireless Theft Alarm System (Part 12/23)

There are many kinds of anti-theft alarm systems. One such system can be made using Light Dependent Resistors (LDR). An LDR based alarm system can be used to protect lockers or locker rooms. In such a system, the LDR sensor is attached to an alarm mechanism and a light source or laser light source is fixed above the sensor in a closed casing. When a locker or locker room is tried to be accessed unethically, the laser light is activated and it in turns stimulates the LDR sensor to invoke alarm. This project illustrates similar alarm mechanism except that instead of building it on product style casing, the circuit has been made on a breadboard for demonstration purpose.

The project utilizes RF-based wireless communication to alert alarm. The benefit of using RF-based wireless communication in the system is that by this way it becomes difficult to hack the security system. Using a wired connection between the sensor circuit and the alarm circuit is prone to hack as the wiring can be cut to disconnect the alarm circuit with the sensor circuit. A 434 MHz RF module is used in the project to facilitate wireless connectivity across the sensor and the alarm circuit. The sensor circuit is attached to the transmitter section of the RF module while alarm circuit is attached to the receiver section of RF. Instead of using buzzers or similar actuators, LEDs are used in the receiver section to illustrate visual hint of the alarming alert.

The security system illustrated in this project is using LDRs but any other sensor like smoke sensor, magnetic (Hall) sensor or temperature sensor can be used to modify this project to a fire alarm system, motion detection system or heat/fire alert system.

Components Required

Sr. No.	Components Required	Quantity
1	RF Tx module(434Mhz)	1
2	RF Rx module(434Mhz)	1
3	HT12E	1
4	HT12D	1
5	LED	5
6	Resistor – 330Ω (Quarter watt)	4
7	Resistor – 1MΩ (Quarter watt)	1
8	Resistor – 50KΩ (Quarter watt)	1
9	LDR	4
10	Resistor – 10KΩ (Quarter watt)	4
11	Battery-9V	2
12	Breadboard	3
13	Connecting wires

Block Diagram of LDR Based Wireless Theft Alarm

Fig. 1: Block Diagram of LDR Based Wireless Theft Alarm

Circuit Connections

As clear from the block diagram, the project consist of two circuits. The sensor circuit consists of the RF transmitter. The RF transmitter is connected to an antenna at pin 4 of the module and connects with the HT12E encoder IC from pin 2 of the module. The circuit connections of RF transmitter and receiver are according to the basic setup of RF module. The address bits of the HT12E are connected to ground to assign an address byte of 0x00 to the transmitter. The pin 14 of the HT12E is hard-wired to ground to enable continuous transmission of the signal. The LDR sensors are connected at the data pins of the HT12E IC. There are four LDR sensors used in the sensor circuit. These LDR sensors are connected in a pull-down configuration where the LDR sensor is connected between the data pin and the VCC while pull-down resistors are connected between the ground and the data pin junction.

The alarm circuit is basically an RF receiver circuit. It consists of an RF receiver having an antenna attached to its pin 8 and serial output from pin 2 connected to pin 14 of the HT12D decoder IC. The circuit connections of HT12D (and the HT12E) are made as specified by their datasheets. The decoder IC also has all the address bits connected to ground to match the transmitter address 0x00. The data pins of the HT12D are used to interface the LEDs. The LEDs are connected between data pins and ground so that on receiving a HIGH bit they starts glowing as an illustration of alarm. The LEDs can also be replaced with other actuators like DC motor or buzzers. Provided that any other actuator is used instead of LEDs additional circuit to bridge the actuator (Like optocouplers and L293D for DC motor or transistor circuit to connect buzzer) should be used.

How the Circuit Works

The circuit operates when the light is detected by the LDR sensors. A light dependent resistor works on the principle of photo conductivity. Photo conductivity is an optical phenomenon in which the materials conductivity reduce when light is absorbed by the material. When light falls i.e. when the photons fall on the device, the electrons in the valence band of the semiconductor material are excited to the conduction band. These photons in the incident light should have energy greater than the band gap of the semiconductor material to make the electrons jump from the valence band to the conduction band. Hence when light having enough energy is incident on the device more & more electrons are excited to the conduction band which results in a large number of charge carriers. The result of this process is more and more current starts flowing and hence it is said that the resistance of the device has decreased. There are two circuit configurations in which LDR sensors can be connected.

Pull up configuration – In this configuration, the LDR sensor is connected between ground and the output junction and the VCC is provided at the junction via pull-up resistor. In such configuration, the pull-up resistor drops some voltage and rest of the voltage is left to be dropped by the LDR and the output. Hence when there is no light falling on LDR sensor, the LDR has higher resistance and due to parallel connection with load, increased LDR resistance results in lower current draws across it while higher current flows through the load. When light falls on the LDR, its resistance is reduced and current flow through it is increased, resulting into lower current draws at the output. The voltage drop at the output junction remains constant due to the parallel connection between LDR and output/load junction and suitable pull-up resistor should be used according to the load connected to it. This configuration is useful when the current draw at the output has to be changed by light sensitivity. Such configuration is quite useful in activating relays at the output junction.

Circuit Diagram of LDR based Voltage Divider Network used as light sensor in Pull-Up Configuration

Fig. 2: Circuit Diagram of LDR based Voltage Divider Network used as light sensor in Pull-Up Configuration

Pull-down configuration – In this configuration, the LDR sensor is connected between the VCC and output junction and a pull-down resistor is connected between the output junction and ground. In such configuration, the voltage is first dropped by the LDR and then the voltage is dropped across pull-down resistor and output. When no light is falling on LDR, it has higher resistance and more voltage is dropped across it. Therefore, a lower voltage (so a LOW logic) is left to drop at the output junction. When light falls on the LDR, its resistance is reduced and a lower voltage is dropped across it. This leaves a higher voltage (so a HIGH logic) dropped at the output junction.

Circuit Diagram of LDR based Voltage Divider Network used as light sensor in Pull-Down Configuration

Fig. 3: Circuit Diagram of LDR based Voltage Divider Network used as light sensor in Pull-Down Configuration

This configuration is useful when voltage drop at the output junction has to be changed by light sensitivity. Such configuration is useful to drive HIGH or LOW digital logic at the output and is used in this project as well. The pull-down resistor used in the circuit has 10K ohm value.

Due to pull-down configuration of LDR sensors, when the light is not falling on it, a LOW logic is latched at the data pins of HT12E and same is transmitted as the respective data bit over the RF system. When light falls on the LDR sensor, a HIGH logic is latched at the data pins of HT12E and same is transmitted as the respective data bit over the RF system.

In the alarm circuit, the transmitted 4-bit data is detected by the RF receiver and passed serially to the HT12D decoder. The same 4-bit data is reflected at the data pins of the HT12D IC. If on light detection, a HIGH logic is latched at a data bit of HT12E, the same is latched at the corresponding data bit of the HT12D IC. The LEDs are used to show a visual simulation of the alarm system. The LEDs are connected having a cathode connected to ground and anode to the HT12D data pins so that on having a HIGH logic at a data pin, the respective LED gets forward biased and starts glowing. A buzzer or other actuator can also be used in place of LEDs with a relevant bridging circuit.

For building a theft alarm system for this project, a light source can be fixed above the LDRs such that on authorized access to the locker, the light source is not triggered ON but any attempt of improvised access triggers the light source and therefore the alarm. Multiple receiver sections with buzzers and LEDs connected to them can also be used to simultaneously trigger many alarms. Learn more about controlling multiple RF receivers from a universal RF transmitter and try to build an advanced version of the security system demonstrated here.