INDEX:
PROJECT TYPE: ANALOG
PROJECT NAME: “FIRE ALARM”
TOPICS PAGE .NO.
1.0 ABSTRACT 2
2.0 CIRCUIT DIAGRAM 2
3.0 WORKING OF CIRCUIT 3
4.0 THEORY 4
5.0 APPLICATIONS 6
6.0 REQUIREMENTS 6
7.0 REFERENCES AND APPENDICS 7
8.0 TECHNICAL REFERENCE MANUAL (DATA SHEETS) 8
1.0 Abstract
A home fire alarm system is usually part of a total security system providing
burglary protection in addition to fire protection. Such a system supervises doors, windows,
and spaces within the home for break-in and may provide monitoring services by dialing your
telephone to report a fire or intrusion to a security office, where it will be reported to your
local police or fire department.
These systems consist of a central control panel to which smoke alarms and
heat detectors are connected, along with bells or horns that are activated when the system
triggers an alarm. Other sensors associated with the burglary functions connect to doors and
windows or monitor rooms for motion or body heat. The control panel operates from house
power but also usually contains an emergency battery which can operate the system for about
24 hours during a power outage.
2.0 Circuit Diagram
3.0 Working of the circuit
In this fire alarm circuit, a thermistor works as the heat sensor. When temperature
increases, its resistance decreases, and vice versa. At normal temperature, the resistance of
the thermistor(TH1) is approximately 10 kilo-ohms,which reduces to a few ohms as the
temperature increases beyond 100°C. The circuit uses readily available components and can
be easily constructed on any general-purpose PCB.
Timer IC NE555 (IC1) is wired as an astable multivibrator oscillating in audio
frequency band. Switching transistors T1 and T2 drive multivibrator NE555 (IC1). The
output of IC1 is connected to npn transistor T3, which drives the loudspeaker (LS1) to
generate sound. The frequency of IC1 depends on the values of resistors R5 and R6 and
capacitor C2. When thermistor TH1 becomes hot, it provides a low-resistance path to extend
positive voltage to the base of transistor T1 via diode D1 and resistor R2. Capacitor C1
charges up to the positive voltage and increases the ‘on’ time of alarm. The higher the value
of capacitor C1, the higher the forward voltage applied to the base of transistor T1 (BC548).
Since the collector of transistor T1 is connected to the base of transistor T2, transistor T2
provides positive voltage to reset pin 4 of IC1 (NE555). Resistor R4 is used such that IC1
remains inactive in the absence of positive voltage. Diode D1 stops discharging of capacitor
C1 when the thermistor connected to the positive supply cools down and provides a high
resistance (10-kilo-ohm) path. It also stops the conduction of T1. To prevent the thermistor
from melting, wrap it up in mica tape. The circuit works off a 6V-12V regulated power
supply. LED1 is used to indicate that power to the circuit is switched on.
4.0 Theory
An automatic fire alarm system is designed to detect the unwanted presence of fire
by monitoring environmental changes associated with combustion. In general, a fire alarm
system is either classified as automatically actuated, manually actuated, or both. Automatic
fire alarm systems can be used to notify people to evacuate in the event of a fire or other
emergency, to summon emergency services, and to prepare the structure and associated
systems to control the spread of fire and smoke.
Fire alarm control panel: This component, the hub of the system, monitors inputs and
system integrity, controls outputs and relays information.
Primary Power supply: Commonly the non-switched 120 or 240 Volt Alternating
Current source supplied from a commercial power utility. In non-residential
applications, a branch circuit is dedicated to the fire alarm system and its constituents.
"Dedicated branch circuits" should not be confused with "Individual branch circuits"
which supply energy to a single appliance.
Secondary (backup) Power supplies: This component, commonly consisting of sealed
lead-acid storage batteries or other emergency sources including generators, is used to
supply energy in the event of a primary power failure.
Initiating Devices: This component acts as an input to the fire alarm control unit and
are either manually or automatically actuated.
Notification appliances: This component uses energy supplied from the fire alarm
system or other stored energy source, to inform the proximate persons of the need to
take action, usually to evacuate.
Building Safety Interfaces: This interface allows the fire alarm system to control
aspects of the built environment and to prepare the building for fire and to control the
spread of smoke fumes and fire by influencing air movement, lighting, process
control, human transport and exit.
5.0 Applications
Some of our most common applications involve:
Coating Machines
Flammable Mix Rooms
Electrical Control Rooms
Heat Treating Furnaces
Ductwork
Data And Telecommunications
Clean Rooms
Power Generation
Turbines And Generators
Cable Trays
Conveyors Dust Collectors
6.0 Fire Notice
1. On discovering a fire (no matter how small)
1.1. OPERATE the nearest break glass alarm point.
1.2. DIAL 2100 (if no response dial 9-999 immediately) and give details of the fire.
1.3. CLOSE all doors and windows in the vicinity of the fire.
1.4. PROCEED immediately to the Assembly Point:-
DO NOT USE LIFTS
2. On hearing the fire alarm
2.1.PROCEEED immediately to the Assembly Point.
2.2. CHECK for the presence of known colleagues.
2.3. Do not Re-enter the building until permittted to do so.
2.4. NOTIFY the officer in charge of the first appliance to arrive of any
missing persons and their last known whereabouts.
7.0 References And Appendix
www.electronicsforu.com
www.wikipedia.com
http://www.datasheetcatalog.org/datasheet/philips/NE_SA_SE555_C_2.pdf
