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Microcontroller Based Home SecurityA PROJECT REPORT ON “MICROCONTROLLER BASED HOME SECURITY SYSTEM”
ABSTRACT
Engineering is not only a theoretical study but it is a implementation of all we study for creating something new and making things more easy and useful through practical study. It is an art which can be gained with systematic study, observation and practice. In the college curriculum we usually get the theoretical knowledge of industries, and a little bit of implementation knowledge that how it is works? But how can we prove our practical knowledge to increase the productivity or efficiency of the industry?
Don’t take the chance of becoming victim of burglary, which is often accompanied by violence. Protect our family and valuables with this microcontroller based security system that will let us rest our head knowing that should anyone trying to break into our home, an alarm will go off and the police will be alerted immediately. The transmitter section continuously transmits IR rays which are received by the receiver section. The received signal is further amplified and given to the PLL section, where its frequency is locked to the transmitted frequency. When the IR signal is interrupted, the microcontroller starts working as per the program burnt into the EPROM and control the siren, telephone and cassette player via the respective relays. CONTENTS:
Chapter Chapter: 1. Introduction. Chapter: 2. Circuit description. Chapter: 3. Working of the circuit. Chapter: 4. Used Components. 4.1. Microcontroller (AT89C51) 4.2. NE555 IC. 4.3. MCT2E Optocouploer. 4.4. Regulator (7805, 7809). Chapter: 5. Other Important Used Components. 5.1. BC548 NPN Transistor. 5.2. Relay (12V, 200ohm). Chapter: 6. Applications Summery. Reference.
Chapter: 1 1. Introduction: Protect our family and valuables with this microcontroller based security system knowing that should anyone trying to break into our home, an alarm will go ON and the police will be alerted immediately. The microcontroller based security system consists of transmitter, receiver, phase locked loop and processing section.
The transmitter section continuously transmits IR rays which are received by the receiver section. The received signal is further amplified and given to the PLL section, where its frequency is locked to the transmitted frequency. The transmitter and receiver are arranged such that the transmitted IR rays fall directly onto the phototransistor LI4GI of the receiver. The signal received by T2 is amplified by transistor T3 and operational amplifier µA741 (IC2). Series input resistor R8 and feedback resistor R9 determine the gain of op amplifier IC2. The amplified single so applied to pin 3 of PLLLM567 (IC3) through capacitor C4. ICLM567 is highly stable PLL with synchronous AM lock detection and power output circuitry it is primarily used as frequency decoder which drives a load whenever a sustained frequency falling within its detection band is present in its self biased input. The centre frequency of the determined by external components. In the absence of any input single, the center frequency of PLL’s eternal free running, current control oscillator is determined by resistor R12 abed capacitor C8. Preset VR2 is used for tuning IC3 to the desired center frequency in the 6-10 kHz range, Which should match the modulating frequency of the transmitter? Capacitor C6 and C7 are used as low pass filter. Ned out filter respectively when the received signal is locked to frequency of transmitter signal pin 8 of IC3 goes low and LED 1 glows. Since pin 8 is connected to the base of transistor T4 through R13 its collector voltage rises. As a result T5 is forward biased to energies the relay RL5 the pole and normally closed contact of really contact of RL5 are connected to +5v. When the IR signal is interrupted, the microcontroller starts working as per the program burnt into the EPROM and control the siren, telephone and cassette player via the respective relays. Chapter: 2 2. Circuit Description:
Transmitter Section:
In the transmitter section, NE555(ICI) is wired as an actable multivibrator whose oscillating freq is decided by resistors R1 and R2, preset VR1 and capacitor c1, C3 bypasses the noise to ground, preventing any change in calculated pulse-width.
The out put of ICI is fed to the base of the transistor t1, which drives an IR LED to transmit the modulated IR signal. R4 limits the current flowing through the IR LED. Preset VR1 is used to vary the modulating frequency. · Receiver Section:
The transmitter and receiver are arranged such that the transmitted IR rays fall directly onto the phototransistor LI4GI of the receiver. The signal received by t2 is amplified by transistor t3 and operational amplifier µA741 (IC2). Series input resistor R8 and feedback resistor R9 determine the gain of op amplifier IC2. The amplified single so applied to pin 3 of PLLLM567 (IC3) through capacitor c4.
ICLM567 is highly stable PLL with synchronous AM lock detection and power output circuitry it is pre merely used as frequency decoder which drives a load whenever a sustained frequency falling within its detection band is present in its self biased input. The centre frequency of the determined by external components. In the absence of any input single, the center frequency of PLL’s eternal free running, current control oscillator is determined by resistor R12 abed capacitor C8. Preset VR2 is used for tuning IC3 to the desired center frequency in the 6-10 kHz range, which should match the modulating frequency of the transmitter? Capacitor C6 and C7 are used as low pass filter. Ned out filter respectively when the received signal is locked to frequency of transmitter signal pin 8 of IC3 goes low and LED 1 glows. Since pin 8 is connected to the base of transistor T4 through R13 its collector voltage rises. As a result T5 is forward biased to energies the relay RL5 the pole and normally closed contact of really contact of RL5 are connected to +5v.
The low order multiplex address and data lines AD0 though AD7 of IC4 are connected to the EPROM (IC5) through the latch(IC6), while its high order address line A8 through A10 are directly connected to the EPROM. Address lines A0 through A7. Are separated from data lines D0 through D7 by latch enable single.
Address latch – enable pin 30 of the microcontroller is connected to latch enable pin 11 Ic6. When ale high the latch us transparent. The output changes according the input data when ALE goes low, the low order address is latched at the input of IC6.
Data lines D0 throughD7 of microcontroller are connected to dated lines of IC5 and IC7 each. Chip sleets signal for IC5 is generated by RD and IO/M lines with the help of nand gate. The inverted IO/M signal provides CS signal through IC7.
IC AT89C51 is general purpose programmable device compatible with most microcontrollers. It has three programmable ports, any of which can be ports and the remaining eight bits as port c. The eight bits of ports c can be used as individual bits or grouped in two 4-bits ports namely, c (upper) and c (lower). Ports A and C are configured as input ports and port B is configured as output port A. is used for inter detection,portB for activating the siren, cassette player, telephone cradle switch and redial button and port C for polarity reversal detection.
The circuit for detecting the polarity reversal detection the telephone line is built around optocoupler IC8 and IC9. Normally, TIP is positive with respect to RING lead of telephone line. With the handset in off position a nominal loop current of 10 mA is assumed to flow through the telephone line. Resistor R23 is selected as 120 ohms to develop the voltage of 1.2v. when the the dc lines voltage polarity reversal occurs, optocoupler IC8’s internal LED conducts and LED3 glows to indicate polarity reversal occurs. Simultaneously, optocoupler IC9’s internal LED goes off and its pin 5 (collector) goes high to provide line –reversal sense signal to AT89C51. Fig.3 shows the power supply circuit. The AC mains are stepped down by transformer X1 to deliver a secondary output of 12V AC at 300 ma. The transformer output is rectified by a full-wave bridge rectifier.
Comprising diodes D7 through D10. Capacitor C12 acts as a filter to eliminate ripples. IC10 and IC11 provide regulated 5v and 9V power supplies, respectively. Capacitors C13 and C14 bypass any ripple present in the regulated out-us. Switch S2 acts as an ‘on’/’off’ switch.
· Relay connections: The cradle switch in the telephone instrument is a double pole, two-way switch. Replace this cradle switch with the contacts of DPDT relay RL3 as shown in fig.2.Now relay RL3 is
Used to implement the action of lifting the telephone handset. There are four pads on the PCB of the telephone instrument where cradle switch is connected. The two pads which are shorted when the telephone handset is placed on the cradle are connected to the normally closed (N/O) contacts of relay RL3, while the other two pads which are shorted when the handset is off-hook are connected to to the normally o0pen (N/O) contacts of relay RL3. Relay RL2 is connected in parallel to the redial button of the telephone instrument. When relay RL3 emerges to emulate lifting of the handset, relay RL2 is energized to switch on the redial button and the already loaded telephone number of the police station or any other help provider is automatically dialed. Relay RL4 activates the siren whenever the IR signal being received is interrupted iron sounds continuously until the user presses the reset button. Relay RL1 is used to switch on the audio cassette player, in which the user’s residential address and alert message to be conveyed to the police station are prerecorded. The speaker output of the cassette player is connected to the telephone’s microphone to convey the alert message to the police station. The player gets switched off when the message is over. Chapter: 3
3. Working of the Circuit: The transmitting IR LED1 and phototransistor T2 of the receiver are fitted to the gate such the IR rays emitted by the LED directly fall on the phototransistor. The IR LED transmits a train of IR pulses. These pulses are received by the receiver and amplified by IC2. Output pin 8 of the PLL (IC3) is low when the PLL network is locked to the transmitter frequency and relay RL5 energies to make PA line of IC7 low. When someone walks through the gate to enter your home, the transmitted signal is interrupted. Output pin 8 of the PLL network goes high and relay RL5 de-energies to make PA0 line of IC7 high. Now the microprocessor starts working as per the program loaded in the EPROM. Relay RL4 energies to activate the siren. At the same time, relay RL3 energizes to emulate lifting the telephone handset off the cradle to provide the dial tone. After a few seconds, relay RL2 energies to short the redial button contacts. After the loaded number is dialed, it switches off relay RL2. Then relay RL1 turns on the audio player. Here we have provided the same polarity-reversal detection facility so that the audio player turns on only when polarity-reversal is detected. The actual-size, double-size track lay-outs for solder and component sides of the PCB for the 8085 microprocessor-based home security system are shown in figs5 and figs6 , respectively, and their component layout in fig.7.
· Software Program: Fig. shows the flow-chart of the Assembly language program. The device interface IC (IC7) is initialized with control word 99H. Ports A and C of IC7 act as input ports, while port B becomes the output port.
After initialization, the AT89C51 microcontroller reads the status of port A. If port A is high, siren is activated. The telephone goes in off-hook condition and the emergency number is dialed through the redial button. Redial button gets switched off after the number is dialed. Now the microprocessor reads the status of port C and checks for the polarity reversal of the telephone line. When polarity reversal is detected, the audio player turns on to play the message. Otherwise, the process repeats from activation of the siren followed by emergency number dialing and so on. After delivering the message, the player automatically gets turned off. The siren sounds until the reset switch is pressed. Chapter: 4
4. Used Components:
These are important components with is use in this projects. Other components like resistors, capacitors, transistors, inductors used PCB’s etc are not described here. The details of the important IC’s: 4.1. AT89C51: · Features • Compatible with MCS-51™ Products • 4K Bytes of In-System Reprogrammable Flash Memory– Endurance: 1,000 Write/Erase Cycles • Fully Static Operation: 0 Hz to 24 MHz • Three-level Program Memory Lock • 128 x 8-bit Internal RAM • 32 Programmable I/O Lines • Two 16-bit Timer/Counters • Six Interrupt Sources • Programmable Serial Channel • Low-power Idle and Power-down Modes
· Description:
The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4Kbytes of Flash programmable and erasable read only memory (PEROM). The devices manufactured using Atmel’shigh-density nonvolatile memory technology and incompatible with the industry standardMCS-51 instruction set and pin out. The on-chip Flash allows the program memory to be reprogrammed in-system or by a conventional nonvolatile memory programmer. By combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmen AT89C51 is a powerful microcomputer which provides a highly-flexible and cost-effective solution to many embedded control applications. · Pin Configuration:
· Block Diagram:
The AT89C51 provides the following standard features: 4K bytes of Flash, 128 bytes of RAM, 32 I/O lines, two 16-bit timer/counters, a five vector two-level interrupt architecture, a full duplex serial port, on-chip oscillator and clock circuitry. In addition, the AT89C51 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters, serial port and interrupt system to continue functioning. The Power-down Mode saves the RAM contents but freezes the oscillator disabling all other chip functions until the next hardware reset. · Pin Description:
Supply voltage.
Ground. · Port 0 Port 0 is an 8-bit open-drain bi-directional I/O port. As an output port, each pin can sink eight TTL inputs. When 1s are written to port 0 pins, the pins can be used as high impedance inputs. Port 0 may also be configured to be the multiplexed low order address/data bus during accesses to external program and data memory. In this mode P0 has internal pull-ups. Port 0 also receives the code bytes during Flash programming, and outputs the code bytes during program verification. External pull-ups are required during program verification. · Port 1 Port 1 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 1 output buffers can sink/source four TTL inputs. When 1s are written to Port 1 pins they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port 1 pins that are externally being pulled low will source current (IIL) because of the internal pull-ups. Port 1 also receives the low-order address bytes during Flash programming and verification. · Port 2 Port 2 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 2 output buffers can sink/source four TTL inputs. When 1s are written to Port 2 pins they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low will source current (IIL) because of the internal pull-ups. Port 2 emits the high-order address byte during fetches from external program memory and during accesses to external data memory that uses 16-bit addresses (MOVX @ DPTR). In this application, it uses strong internal pull-ups when emitting 1s. During accesses to external data memory that use 8-bit addresses (MOVX @ RI), Port 2 emits the Contents of the P2 Special Function Register. Port 2 also receives the high-order address bits and some control signals during Flash programming and verification. · Port 3 Port 3 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 3 output buffers can sink/source four TTL inputs. When 1s are written to Port 3 pins they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low will source current (IIL) because of the pull-ups. Port 3 also serves the functions of various special features of the AT89C51 as listed below:
Port 3 also receives some control signals for Flash programming and verification. · RST Reset input. A high on this pin for two machine cycles while the oscillator is running resets the device. · ALE/PROG Address Latch Enable output pulse for latching the low byte of the address during accesses to external memory. This pin is also the program pulse input (PROG) during Flash Programming. In normal operation ALE is emitted at a constant rate of 1/6 the oscillator frequency, and may be used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped during each access to external Data Memory. If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the bit set, ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly pulled high. Setting the ALE-disable bit has no effect if the microcontroller is in external execution mode. · PSEN Program Store Enable is the read strobe to external program memory. When the AT89C51 is executing code from external program memory, PSEN is activated twice each machine cycle, except that two PSEN activations are skipped during each access to external data memory. · EA/VPP External Access Enable. EA must be strapped to GND in order to enable the device to fetch code from external program memory locations starting at 0000H up to FFFFH. Note, however, that if lock bit 1 is programmed, EA will be internally latched on reset. EA should be strapped to VCC for internal program executions. This pin also receives the 12-volt programming enable voltage (VPP) during Flash programming, for parts that require 12-volt VPP. · XTAL1 Input to the inverting oscillator amplifier and input to the internal clock operating circuit. · XTAL2 Output from the inverting oscillator amplifier. Unconnected while XTAL1 is driven as shown in Figure 2. There are no requirements on the duty cycle of the external clock signal, since the input to the internal clocking circuitry is through a divide-by-two flip-flop, but minimum and maximum voltage high and low time specifications must be observed. · Idle Mode In idle mode, the CPU puts itself to sleep while all the on chip peripherals remain active. The mode is invoked by software. The content of the on-chip RAM and all the special functions registers remain unchanged during this mode. The idle mode can be terminated by any enabled interrupt or by a hardware reset. It should be noted that when idle is terminated by a hard ware reset, the device normally resumes program execution, from where it left off, up to two machine cycles before The internal reset algorithm takes control. On-chip hardware inhibits access to internal RAM in this event, but access to the port pins is not inhibited. To eliminate the possibility of an unexpected write to a port pin when Idle is terminated by reset, the instruction following the one that invokes Idle should not be one that writes to a port pin or to external memory. · Programming Algorithm: Before programming the AT89C51, the address, data and control signals should be set up according to the Flash programming mode table and Figure 3 and Figure 4. To program the AT89C51, take the Following steps: 1. Input the desired memory location on the address lines. 2. Input the appropriate data byte on the data lines. 3. Activate the correct combination of control signals. 4. Raise EA/VPP to 12V for the high-voltage programming mode. 5. Pulse ALE/PROG once to program a byte in the Flash array or the lock bits. The byte-write cycle is self-timed and typically takes no more than 1.5 ms. Repeat steps 1 through 5, changing the address and data for the entire array or until the end of the object file is reached. · Data Polling: The AT89C51 features Data Polling to indicate the end of a write cycle. During a write cycle, an attempted read of the last byte written will result in the complement of the written datum on PO.7. Once the write cycle has been completed, true data are valid on all outputs, and the next cycle may begin. Data Polling may begin any time after a write cycle has been initiated. · Ready/Busy: The progress of byte programming can also be monitored by the RDY/BSY output signal. P3.4 is pulled low after ALE goes high during programming to indicate BUSY. P3.4 is pulled high again when programming is done to indicate READY. · Program Verify: If lock bits LB1 and LB2 have not been programmed, the programmed code data can be read back via the address and data lines for verification. The lock bits cannot be verified directly. Verification of the lock bits is achieved by observing that their features are enabled.
· Chip Erase: The entire Flash array is erased electrically by using the proper combination of control signals and by holding ALE/PROG low for 10 ms. The code array is written with all “1”s. The chip erase operation must be executed before the code memory can be re-programmed. · Reading the Signature Bytes: The signature bytes are read by the same procedure as a normal verification of locations 030H, 031H, and 032H, except that P3.6 and P3.7 must be pulled to a logic low. The values returned are as follows. (030H) = 1EH indicates manufactured by Atmel (031H) = 51H indicates 89C51 (032H) = FFH indicates 12V programming (032H) = 05H indicates 5V programming · Programming Interface Every code byte in the Flash array can be written and the entire array can be erased by using the appropriate combination of control signals. The write operation cycle is self timed and once initiated, will automatically time itself to completion. All major programming vendors offer worldwide support for the Atmen microcontroller series. Please contact your local programming vendor for the appropriate software revision. 4.2. NE555 IC:
· Features: • High Current Drive Capability (200mA) • Adjustable Duty Cycle • Temperature Stability of 0.005%/°C • Timing from μ Sec to Hours. • Turn off Time Less than 2μSec · Applications: • Precision Timing • Pulse Generation • Time Delay Generation • Sequential Timing · Description: The LM555/NE555/SA555 is a highly stable controller capable of producing accurate timing pulses. With monostable operation, the time delay is controlled by one external resistor and one capacitor. With astable operation, the frequency and duty cycle are accurately controlled with two external resistors and one capacitor. Internal Block Diagram:
· Monostable Operation:
· Monoatable Circuit:
Waveforms of Monostable Operation
Resistance and Capacitance vs. Time delay (td) 4.3. MCT2E Optocouploer:
· FEATURES: • UL recognized (File # E90700) • VDE recognized (File # 94766) -Add option V for white package (e.g., MCT2V-M) –Add Option 300 for black package (e.g., MCT2.300)
· Dimension Package (Surface Mount):
• MCT2 and MCT2E are also available in white package by specifying -M suffix, e.g. MCT2M
· APPLICATIONS: • Power supply regulators • Digital logic inputs • Microprocessor inputs
4.4. Voltage Regulator (7805, 7809): · Features: • Output Current up to 1A • Output Voltages of 5, 6, 8, 9, 10, 12, 15, 18, 24V • Thermal Overload Protection • Short Circuit Protection • Output Transistor Safe Operating Area Protection
The MC78XX/LM78XX/MC78XXA series of three terminal positive regulators are available in the TO-220/D-PAK package and with several fixed output voltages, making them useful in a wide range of applications. Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible. If adequate heat sinking is provided, they can deliver over 1A output current. Although designed primarily as fixed voltage regulators, these devices can be used with external components to obtain adjustable voltages and currents. · Internal Block Diagram:
· Typical Applications:
DC PARAMETERS
LOAD REGULATION
Constant Current Regulator Notes: (1) To specify an output voltage. Substitute voltage value for "XX." A common ground is required between the input and the Output voltage. The input voltage must remain typically 2.0V above the output voltage even during the low point on the input ripple voltage. (2) CI is required if regulator is located an appreciable distance from power Supply filter. (3) CO improves stability and transient response · LM78XX (KA78XX, MC78XX) FIXED VOLTAGE REGULATOR (POSITIVE):
(LM7809 Voltage Regulator) · 3-TERMINAL 1A POSITIVE VOLTAGE REGULATORS The LM78XX series of three-terminal positive regulators are available in the TO-220/D-PAK package and with several fixed output voltages, making them useful in a wide range of applications. Each type employs internal current limiting, thermal shut-down and safe area protection, making it essentially indestructible. If adequate heat sinking is provided, they can Deliver over 1A output current. Although designed primarily as fixed voltage regulators, these devices can be used with external components to obtain adjustable voltages and currents. · FEATURES: · Output Current up to 1A · Output Voltages of 5, 6, 8, 9, 10, 11, 12, 15, 18, 24V · Thermal Overload Protection · Short Circuit Protection · Output Transistor SOA Protection
BLOCK DIAGRAM:
Chapter: 5 5. Other Used Components: 5.1. BC548 NPN Transistor:
This device is designed for use as general purpose amplifiers and switches requiring collector currents to 300 mA. Sourced from Process 10. See PN100A for characteristics. NOTES: 1) These ratings are based on a maximum junction temperature of 150 degrees C. 2) These are steady state limits. The factory should be consulted on applications involving pulsed or low duty cycle operations. · Absolute Maximum Ratings
5.2. Relay (12V, 200 ohm):
A relay is an electrical switch that opens and closes under control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered, in a broad sense, to be a form of electrical amplifier. · Operation:When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force that is half as strong as the magnetic force to its relaxed position. Usually this is a spring, but gravity is also used commonly in industrial motor starters. Relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing. If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a spike of voltage and might cause damage to circuit components. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle. [1] The contacts can be either Normally Open (NO), Normally Closed (NC), or change-over contacts. Normally-open contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive. It is also called Form A contact or "make" contact. Form A contact is ideal for applications that require to switch a high-current power source from a remote device. Normally-closed contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive. It is also called Form B contact or "break" contact. Form B contact is ideal for applications that require the circuit to remain closed until the relay is activated. Change-over contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal. It is also called Form C contact or "transfer" contact. By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation, a light-emitting diode (LED) is used with a photo transistor. APPLICATION’S:
Summary: The microcontroller based security system consists of transmitter, receiver, phase locked loop and processing section. The transmitter section continuously transmits IR rays which are received by the receiver section. The received signal is further amplified and given to t6he PLL section, where its frequency is locked to the transmitted frequency. The transmitter and receiver are arranged such that the transmitted IR rays fall directly onto the phototransistor LI4GI of the receiver. The signal received by t2 is amplified by transistor t3 and operational amplifier µA741 (IC2). Series input resistor R8 and feedback resistor R9 determine the gain of op amplifier IC2. The amplified single so applied to pin 3 of PLLLM567 (IC3) through capacitor c4. ICLM567 is highly stable PLL with synchronous AM lock detection and power output circuitry it is pre merely used as frequency decoder which drives a load whenever a sustained frequency falling within its detection band is present in its self biased input. The centre frequency of the determined by external components. In the absence of any input single, the center frequency of PLL’s eternal free running, current control oscillator is determined by resistor R12 abed capacitor C8. Preset VR2 is used for tuning IC3 to the desired center frequency in the 6-10 kHz range, Which should match the modulating frequency of the transmitter? Capacitor C6 and C7 are used as low pass filter. Ned out filter respectively when the received signal is locked to frequency of transmitter signal pin 8 of IC3 goes low and LED 1 glows. Since pin 8 is connected to the base of transistor T4 through R13 its collector voltage rises. As a result T5 is forward biased to energies the relay RL5 the pole and normally closed contact of really contact of RL5 are connected to +5v. When the IR signal is interrupted, the microcontroller starts working as per the program burnt into the EPROM and control the siren, telephone and cassette player via the respective Reference:
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