Alarm circuit

Systems ApproachBurglar Alarm Project
PROBLEM
SITUATION
DESIGN
BRIEF
SYSTEM
ANALYSIS
SPECIFICATION
SOLUTION
Resource
Selection
Build & Test
Computer
Simulation
DATA SHEETS
Evaluation

Problem SituationBurglar Alarm Project
PROBLEM
SITUATION
An Alarm company is developing a simple low cost security system for use in a new block
of small flats
The pressure mat gives a high (1) output when stood on.
The alarm “arm switch” is incorporated into the the front door key lock. Locking the door
will set the alarm giving signal high (1).
A Reed switch and magnet are mounted on the door and frame and gives a low(0)
signal when the door is opened
The electronic control sub-system is based on a combinational logic circuit which
operates the alarm via a transistor

Design BriefBurglar Alarm Project
Design an electronic logic control sub-system to process the signal from the
Inputs in order to operate the Alarm buzzer as described above
DESIGN
BRIEF

Systems Approach
PROCESSINPUT OUTPUT
Any device or system can be analysed in
terms of INPUT, PROCESS & OUTPUT
The universal System Diagram
Burglar Alarm Project

The universal System Diagram
Burglar
Alarm
INPUT OUTPUT
A) Arm Key Activated
B) Burglar Through Door
C) Burglar Through Window
Z) SCARY SOUND ENERGY

The system diagram can be broken down into a sub
system diagram:

A
B
C
Z
SCARY
SOUND
ENERGY
Arm Key Activated
Burglar Thru Door
Burglar Thru
Window
System Boundary
Subsystem
Inputs
Subsystem
Outputs
Process
Subsystems
Reed
Switch N/C
Micro
Switch N/O
Pressure
Switch N/O
Inverter
OR
AND LATCH DRIVER
BUZZER
Sub System Diagram
Manual Reset
D Push Button
N/O
System
OUTPUTS
System
INPUTS

The System Must:
1) Detect a Burglar entering through the door or the window
2) Enable owner to disable it to go about their business
without setting it off
3) It must give a continuous audible warning to deter
burglars, and should be reset by owner
4) It should be low cost, reliable and durable
5) Sensor should be unobtrusive
SPECIFICATION

Solution
SIMULATION LINK

Solution
A
(Arm Key)
B
(Reed Switch)
C
(Pressure Mat)
Buzzer
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 1
1 0 1 1
1 1 0 0
1 1 1 1 SIMULATION LINK

Subsystem INPUT Devices
Micro
Switch N/O
Reed
Switch N/C
Pressure
Switch N/O
Selection of Resources
For detecting pressure under mat when
burglar comes through window
For detecting when door is locked
“Arming” the alarm
For detecting when the door is open.
(This switch is high when the door is closed)

Subsystem PROCESS Devices
Integrated circuits consist of plastic cases filled with electronic circuitry.
There are many resistors, transistors and other components packed into
the chips. There are literally thousands of ICs on the market, all designed
to do different jobs – logic gates, amplifiers, timers, etc.
In this work we will be using the TTL (transistor transistor logic) range of
chips.
TTL chips require a stable 5V supply to work properly. (Great difficulties
will be met if any other voltage is used.) Any unconnected pins
automatically go to logic 1. In other words, if a wire connected to a pin
is connected to the 0-volt rail (logic 0), it will go to logic 0. If the wire is
disconnected from the 0-volt rail it will go to logic 1. However, it is good
practice to connect pins to „high‟ or „low‟ as needed.
All TTL chips have a four-digit code number, which always starts with 74.
For example, a 7400 is a quad two-input NAND chip.
Although the chip contains complex circuitry, the internal wiring can be
shown as simple logic circuits with the inputs and outputs of each logic
gate shown. This is called a pin-out diagram.
Selection of Resources

Subsystem PROCESS Devices
OR (TTL 7432) NOR (TTL 7402)
AND (TTL 7408) INVERTER (TTL 7408)
Used to detect when the arm key is activated
AND a burglar is in the system
Used to detect when a burglar comes
through the Door OR through the window
2 cross coupled Nor Gates make an SR
FLIP-FLOP Keeping the output high if a
burglar closes the door or steps off the mat
Inverts out put from N/C reeds switch to
detect when Door is open

Subsystem OUTPUT Devices
10K Ohm
Resistor
Protective
Diode
BC 108 NPN
Transistor
Used for switching and driving the
buzzer
This is a protective resistor used at the
transistor base
This will protect the the transistor from back
emf generated by the buzzer when the
Buzzer is reset
For producing Scary Noise Energy at a
noise level of 85 db
BUZZER
6v, 30mA

DATA ON TTL 74HC04 „INVERTER‟ CHIP

DATA ON TTL 74HC32 „OR‟ CHIP

DATA ON TTL 74HC08 „AND‟ CHIP

DATA ON TTL 74HC02 „NOR‟ CHIP

BREAD BOARD

Alarm circuit

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