The fear of theft and burglary always annoys many people. When lock and keys become less safe, one can seek help of electronic security systems. Such a portable security system is described here




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НазваниеThe fear of theft and burglary always annoys many people. When lock and keys become less safe, one can seek help of electronic security systems. Such a portable security system is described here
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INDEX


Name of Topic Page No.

Components Required 2

Wireless Home Security System 4

Working of the System 5

Sensor and Transmitter 5

Reciever 6

Power Supply 6

Construction 7

Tuning 9

System Components and What they do? 10

How a Wireless System Works? 11

Features 11

Benefits 12

Pin Connection 13

Astable Circuit

Reset Input

Voltage Control Input

Monostable Circuit

More about Triggering

555 as a Transducer Driver

Bibliography


COMPONENTS REQUIRED


Semiconductors:


IC1 NE555 timer

IC2 CD4011 NAND Gate

T1, T2 BC548 npn transistor

T3, T6 BC547 npn transistor

T4 SL100 npn transistor

T5 BF494 npn transistor

D1 1N4001 Diode


Resistors(All ¼ watts +- 5% carbon, unless stated otherwise):


R1 2.2 kilo ohm

R2, R10 1 mega ohm

R3, R11 10 kilo ohm

R4, R12 47 kilo ohm

R5, R8 1 kilo ohm

R6 220 kilo ohm

R7 1.5 m

R9 470 ohm

VR1, VR2 47 kilo ohm preset


Capacitors:


C1, C2, C7 0.1 microF ceramic

C3, C6 220 microF, 25V electrolytic

C4 2.2 nF ceramic

C5 0.2 microF ceramic

C10, C11 0.01 microF ceramic

C9 22 pF button trimmer

C8 1 microF, 6V electrolytric


Miscellaneous:


RL1 9V, 100ohm Relay


WIRELESS HOME SECURITY SYSTEM




The fear of theft and burglary always annoys many people. When lock and keys become less safe, one can seek help of electronic security systems. Such a portable security system is described here.







WORKING OF THE SYSTEM




The system has two main parts: an intruder sensor cum transmitter and a receiver. The intruder sensor reacts to light, sound, and mechanical movements. When the sensor detects a disturbance, the transmitter gets triggered and begins to send wireless signals.


The receiver placed away from the transmitter receives the signals and produces an audio alarm. At the same time, a relay switch is turned on so that other security systems can be energized through this relay.


SENSOR AND TRANSMITTER


The sensor is wired around a 555 timer IC. Any light, sound and mechanical movement can be sensed by the condenser mic, LDR and a leaf switch etc. When any of these happen, transistors T1 and T2 start conducting. This brings down pin 2 of IC1 below 1/3Vcc. The output pin 3 of the IC goes high and it drives transistor T3 and relay RL1.


The positive line to the transmitter is connected through the relay. Two NAND gates (N1, N2) of IC2 (CD4011) are wired as an audio oscillator. The frequencies derived from pin 4 of IC2 are amplified by transistor T4 and applied to the RF section for modulation. Coil L1 together with capacitors C9, C10, resistor R12, and Transistor T5 forms a tuned RF oscillator.


A low frequency oscillator is wired around the remaining two gates of IC2 (N3, N4). Its output drives transistor T6. Since the emitter of T5 is connected to the ground through t6, the transmitter works only when T6 conducts.


The transmitted signals are captured by the receiver and processed to produce sharp beeps. With one metre telescopic antenna, the range of the transmitter is 50 metres.


RECEIVER


The main part of the RF receiver is a radio chip IC3 (ZN 414). A MW antenna coil joined with a 2J gang condenser forms the tuner section. The signals from IC3 are passed to the input of IC4 through preset VR3. These signals are amplified by IC4 (LM2002) and drive a loudspeaker.


IC5 is wired as a monostable latch. The signals obtained from the audio amplifier are rectified by diode D2 and filtered by capacitor C22. The voltage obtained from D2 drives transistor T7. When pin 2 of IC5 is brought to low level, it gets triggered and drives relay RL2.


POWER SUPPLY


Both transmitter and receiver work on AC mains. However, battery changeover facility is provided. The transmitter should be connected to a 9-12 volt power supply system. A 6-9 volt supply is required for the receiver.


CONSTRUCTION


PCB and component layouts are shown in fig.


Use of IC sockets for fixing ICs 1,2 and 4 is recommended. ICs should be fixed to a heat sink.

Metal cabinets are not suitable for both the units.

Antenna coils are to be fixed on ferrite rods.




TUNING


The transmitter may be tuned first by moving the antenna coil and also by adjusting the trimmer (C9). It is better to tune to a deaf spot on a radio dial. Then, adjust the antenna coil and gang condenser of the receiver to get the best possible reception of transmitted signals.


Pot. VR3 is to be adjusted to get the required output level from the loudspeaker. The sensitivity of the 555-based trigger (IC5) greatly depends upon R20 and VR4. These should be carefully selected to trigger the relay only after receiving three or four beep sounds. This is necessary to avoid false triggering due to lightning etc.


IC5 and related components are optional. It is intended for switching additional alarm systems. If this facility is not needed, components D2, T7, T8 and IC5 may be dispensed with.


System Components and What They Do:





A wireless system consists of three main components: an input device, a transmitter, and a receiver. The input device provides the audio signal that will be sent out by the transmitter. It may be a microphone, such as a handheld vocalist's model, or a lavalier "tie-clip" type. With wireless systems designed for use with electric guitars, the guitar itself is the input device.

The transmitter handles the conversion of the audio signal into a radio signal and broadcasts it through an antenna. The antenna may stick out from the bottom of the transmitter or it may be concealed inside. The strength of the radio signal is limited by government regulations. The distance that the signal can effectively travel ranges from 100 feet to over 1,000 feet, depending on conditions.All wireless transmitters require a battery (usually a 9-volt alkaline type) to operate.


The job of the receiver is to pick up the radio signal broadcast by the transmitter and change it back into an audio signal. The output of the receiver is electrically identical to a standard microphone signal, and can be connected to a typical microphone input in a sound system.

How a Wireless System Works




A conventional wired microphone converts sound waves into an electrical audio signal that travels to the sound system through a cable. A wireless microphone system goes one step further, and converts the audio signal created by the microphone to a radio signal which is sent to the sound system through the air by a transmitter. The radio signal is similar to those used by television and FM radio stations. The receiver tuned to the same frequency as the transmitter picks up the radio signal, converts it back into an audio signal, and feeds it to the sound system through a short cable. The receiver is usually located near the rest of the sound system.

Each performer or presenter using wireless at a particular location (a theater, church, or school, for example) must use a system operating on a different frequency. Wireless systems at one location cannot "share" frequencies because they would interfere with each other, just as if two television stations in the same city tried to broadcast on the same channel. If two performers at one location try to use the same frequency at the same time, neither one will be picked up clearly. This potential for interference limits the number of wireless systems that can be used simultaneously at one venue. Reputable manufacturers and dealers of wireless systems can assist with selecting the appropriate frequencies for your needs.


Features:

  • Patented, proprietary technology

  • Comprehensive, redundant coverage

  • No voice channel interference

  • Cost effective

  • Compatible with virtually all equipment

  • Automatic carrier switching

  • Multiple alarm notification methods

  • Web-enabled management

  • Alarm transmission in less than one second

  • Alarm acknowledgement

Daily or weekly tests to be sent directly to your customer


Benefits:


  • Suitable for home or office.

  • Simple to operate using a handy remote control.

  • Cordless detectors enable very simple installation.



Pin connections:



The 555 timer is an extremely versatile integrated circuit which can be used to build lots of different circuits.






Astable circuits


Astable circuits produce pulses. The circuit most people use to make a 555 astable looks like this:



As you can see, the frequency, or repetition rate, of the output pulses is determined by the values of two resistors, R1 and R2 and by the timing capacitor, C.



The design formula for the frequency of the pulses is:



The HIGH and LOW times of each pulse can be calculated from:









The duty cycle of the waveform, usually expressed as a percentage, is given by:



An alternative measurement of HIGH and LOW times is the mark space ratio:



Before calculating a frequency, you should know that it is usual to make R1=1 k because this helps to give the output pulses a duty cycle close to 50%, that is, the HIGH and LOW times of the pulses are approximately equal.


Remember that design formulae work in fundamental units. However, it is often more convenient to work with other combinations of units:



resistance

capacitance

Period

frequency



F

S

Hz



µF

S

Hz



µF

Ms

kHz



With R values in M and C values in µF, the frequency will be in Hz. Alternatively, with R values in k and C values in µF, frequencies will be in kHz.

Suppose you want to design a circuit to produce a frequency of approximately 1 kHz for an alarm application. What values of R1, R2 and C should you use?

R1 should be 1k, as already explained. This leaves you with the task of selecting values for R2 and C. The best thing to do is to rearrange the design formula so that the R values are on the right hand side:



Now substitute for R1 and f :



You are using R values in k and f values in kHz, so C values will be in µF.

To make further progress, you must choose a value for C. At the same time, it is important to remember that practical values for R2 are between 1 k and 1M. Suppose you choose C = 10 nF = 0.01 µF:



that is:



and:



This is within the range of practical values and you can choose values from the E12 range of 68 k or 82 k. (The E12 range tells you which values of resistor are manufactured and easily available from suppliers.)


A test circuit can be set up on prototype board, as follows:




With the values of R1, R2 and C shown, the LED should flash at around 10 Hz.

What happens if you replace R2 with an LDR or a thermistor? This gives an astable which changes frequency in response to light intensity, or with temperature.








RESET input:


If the RESET input, pin 4, is held HIGH, a 555 astable circuit functions as normal. However, if the RESET input is held LOW, output pulses are stopped. You can investigate this effect by connecting a switch/pull down resistor voltage divider to pin 4:




Here is the circuit on prototype board:



Use the design formula, or the DOCTRONICS component selector program to calculate the frequency of pulses you would expect to obtain with this circuit.

In an electronic die, provided the output pulses are fast enough, it is impossible to 'cheat' by holding down the button for a definite length of time.

Think about how you could use this circuit together with a bistable as part of a burglar alarm. Under normal conditions, the output of the bistable is LOW and the astable is stopped. If the alarm is triggered, the output of the bistable goes HIGH and the pulses start, sounding the alarm.





CONTROL VOLTAGE input:



By applying a voltage to the CONTROL VOLTAGE input, pin 5, you can alter the timing characteristics of the device. In the astable mode, the control voltage can be varied from 1.7 V to the power supply voltage, producing an output frequency which can be higher or lower than the frequency set by the R1, R2, C timing network.

The CONTROL VOLTAGE input can be used to build an astable with a frequency modulated output. In the circuit below, one astable is used to control the frequency of a second, giving a 'police siren' sound effect.




In most applications, the CONTROL VOLTAGE input is not used. It is usual to connect a 10 nF capacitor between pin 5 and 0 V to prevent interference. You don't need to do this in building a test circuit, but this 'bypass' or 'decoupling' capacitor should be included in your final circuit.




Monostable circuits:


A monostable circuit produces a single pulse when triggered. The two questions about monostables you immediately need to ask are:

  • How can the circuit be triggered to produce an output pulse?

  • How is the duration, or period, of the output pulse determined?

The circuit used to make a 555 timer monostable is:




In most applications, the CONTROL VOLTAGE input is not used. It is usual to connect a 10 nF capacitor between pin 5 and 0 V to prevent interference. You don't need to do this in building a test circuit, but this 'bypass' or 'decoupling' capacitor should be included in your final circuit.




Monostable circuits



A monostable circuit produces a single pulse when triggered. The two questions about monostables you immediately need to ask are:

  • How can the circuit be triggered to produce an output pulse?

  • How is the duration, or period, of the output pulse determined?

The circuit used to make a 555 timer monostable is:

As you can see, the trigger input is held HIGH by the 10 k pull up resistor and is pulsed LOW when the trigger switch is pressed. The circuit is triggered by a falling edge, that is, by a sudden transition from HIGH to LOW.

The trigger pulse, produced by pressing the button, must be of shorter duration than the intended output pulse.


The period, , of the output pulse can be calculated from the design equation:




Remember again about compatible measurement units:



resistance

capacitance

period



F

s



µF

s



µF

ms

With R1 = 1 M and C = 1 µF, the output pulse will last for 1.1 s.

You can build a test version of the 555 monostable as follows:





By clicking on the monostable tab, the 555 component selection program can be used to investigate the effect of different R1 and C values:

More about triggering:



For a simple 555 monostable, the trigger pulse must be shorter than the output pulse. Sometimes you want to trigger the monostable from a longer pulse:



The trigger network detects the falling edge at the end of each Vin pulse, producing a short 'spike' which triggers the monostable at the appropriate time. The period of the monostable pulse is shorter than the period of the Vin pulses.

If you want to trigger the monostable from a rising edge, you need to add a transistor NOT gate to the trigger circuit:



If you build these circuits, it is interesting to investigate the action of the trigger network using an oscilloscope.

555 as a transducer driver



A transducer is a subsytem which converts energy from one form into another, where one of the forms is electrical. In an output transducer, for example, electrical energy can be converted into light, sound, or movement.

The output of a 555 timer can deliver more than 100 mA of current. This means that output transducers including buzzers, filament lamps, loudspeakers and small motors can be connected directly to the output of the 555, pin 3.

You can use the 555 as a transducer driver, that is, as an electronic switch which turns the transducer ON or OFF:



This circuit has an inverting Schmitt trigger action. The 'inverting' part of this description means that when Vin is LOW, the output is HIGH, and when Vin is HIGH, the output is LOW.

In a 'Schmitt trigger' circuit there are two different switching thresholds. If Vin is slowly increased starting from 0 V, the output voltage snaps from HIGH to LOW when Vin reaches a level equal to 2/3 of the power supply voltage. Once this level has been exceeded, decreasing Vin does not affect the output until Vin drops below 1/3 of the power supply voltage. (If an input change in one direction produces a different result from a change in the opposite direction, the circuit is said to show hysteresis.)

If a filament lamp is connected between the positive power supply rail and the output, as shown above, current flows through the lamp when the output voltage is LOW. In other words, the lamp lights when the input voltage is HIGH.

If you connect the lamp between the output and 0 V, the circuit will still work, but the lamp will light when the input voltage is LOW:


Note that, in both versions of the circuit pins 2 and 6 are joined together. The circuit can be simplified by omitting the 10  nF bypass capacitor, and will continue to work when the RESET input, pin 4 is left unconnected.

Some people are very fond of this circuit and use it whenever a transducer driver is required. However, with a HIGH/LOW digital input signal the same result can be achieved more obviously and at lower cost using a transistor switch circuit.


Acknowledgement


We sincerely wish to thank our guide, Mr. R. K. Bhatija, without whose help and guidance working on this project was not possible.


We also wish to thank Mr. N. K. Shukla for helping us in making the PCB of the hardware design and providing us useful knowledge regarding PCB manufacturing processes.


Also, we wish to thank other people who directly or indirectly helped us in completion of this minor project and whose names have been inadvertently missed out by us in this acknowledgement.


Richa Srivastava

Sheetal Sharma

Sakshi Jain

REFERENCES

  • Wireless Communications & Networks – William Stallings

  • Papers downloaded from Internet

  • Lab manuals







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