FIRE ALARM
PHYSICS INVESTIGATORY PROJECT
KENDRIYA VIDYALAYA PICKET
V Srivatsan
Class XII ‘A’ (2023-24)
35
CERTIFICATE OF COMPLETION
This is to certify that V Srivatsan of class 12 studying in Kendriya
Vidyalaya Picket has successfully completed the project titled “FIRE
ALARM” under the guidance of Ms. Amulya (PGT Physics) during the
academic year 2023-24 in the partial fulfillment of Physics Practical
Examination conducted by the Central Board of Secondary Education
(CBSE).
___________________________ ___________________________
Sign of Guide Teacher Sign of External Examiner
Date:
ACKNOWLEDGEMENTS
I would like to express my immense gratitude to my physics teacher, Ms.
Amulya (PGT Physics), for the help and guidance that she provided in the
successful completion of this project. I also appreciate the way she made
me explore the possibilities and solve the problems I encountered by
teaching me what I needed to know to do it.
I would also like to thank my parents and other family members for helping
me in doing this project by giving ideas and supporting me all the way.
Lastly, I would like to thank the school management for providing me with
ample infrastructure and encouragement to complete this project.
_____________________
Sign
INDEX
1. Introduction
2. Objective
3. Theory
4. Building a circuit
5. Observations
6. Precautions
7. Result
INTRODUCTION
Fire, one of the primitive sources of energy and one of the primary causes
of destruction in any era, can be created by various methods in the modern
world. There are several types of fires which need to be put out in specific
ways based on how they were caused. For example, fires caused by oil
cannot be put out by water. Thus, fire extinguishers commonly have
chemicals which stop one or more of the 3 main factors which keep a fire
going: Fuel, Heat (High Temperature) and Oxygen.
Ever since humans started adopting electricity for their basic needs, it has
become increasingly important for us to be wary of the risks which go with
it. Yes, we have measures in place to prevent such accidents, but they
cannot be eliminated. Even if we take all sorts of preventive measures, at
some point, the wires get worn out, the equipment gets faulty, and lack of
constant checking results in accidents.
An extremely common disaster occurring in the case of electricity are
“electrical” fires. These are fires caused by faulty or worn-out electrical
sockets and/or appliances. It is not practical to prioritize putting out such
fires, as they spread and grow rapidly. And since these fires are more
common in apartments, we need a way to alert all the residents to evacuate.
Until about a decade ago, we used to have manual alarms which used to go
off when someone pulled the level or pressed a button. Once it was set off,
the whole building would have sprinkler systems in the roof which would
activate and put out any fires. But this was not a highly effective way as the
alarm cannot be raised if the fire were in the room itself. Another fault in
the system was that it could be abused for giving false alarms.
OBJECTIVE
We need an alarm system that is automatic and effective. This means that
the alarm needs to be triggered only when a fire occurs. Granted, it is not
always possible to decide whether there is a fire or not, but typically, fires
are accompanied by two changes in the environment: Heat and smoke. Let
us evaluate how probable a fire is when each of these changes are seen
along with our own requirements.
Smoke is not an effective indicator as smoke rising from fires cannot be
differentiated from smoke from other “regular” uses. Granted, we can use
chemicals in the alarm systems which can react with specific components in
the smoke which turns it into a conductor, but as we discussed before, fire
can be caused by various substances. Thus, it calls for various reactants in
the alarm system as well.
This leaves us heat. Now, it is not possible to expose a circuit to heat and
have it in working condition. As we know, resistance increases with an
increase in temperature. Hence, we need to isolate the alarm system
circuitry from the heat while using the heat itself to trigger it. We also need
to set up a threshold for the minimum temperature of the environment for it
to be considered a fire. This is because we have fireplaces in western
homes, and in Indian tradition, we perform a homan which involves
burning wood and using ghee as fuel while keeping it under control.
Summarizing our requirements, we need to build a circuit while satisfying
the following conditions:
1. The circuit needs to be triggered only when the surrounding
environment reaches a specific threshold temperature.
2. False negatives should be reduced. This means that the alarm needs to
be triggered at all the times when there is a fire.
3. The circuitry itself needs to be isolated or kept away from probable
areas of fire
4. No sources of failure like fluctuations, power loss, etc.
For this project, we shall only focus on requirements 1 and 2, as the other
two are case-specific.
THEORY
Considering that there will be a fire which can (and mostly will) disrupt the
electrical supply, it can be concluded that it is dangerous to switch on or off
the electric supply in the case of a fire. Thus, we need to make it so that the
circuit is closed yet is only triggered when the temperature crosses a certain
threshold.
Looking at the conditions, it is obvious that we need to use a material
which acts like a semiconductor. Like a semiconductor, we need a material
which needs some initial activation energy to conduct. But this is contrary
to our requirement that the circuit is already closed.
This brings us to the concept of thermistors (thermal resistors), which are
materials whose resistance is dependent on the temperature. There are two
types of thermistors:
Negative Temperature Coefficient (NTC) Thermistors whose
resistance is inversely proportional to temperature
Positive Temperature Coefficient (PTC) Thermistors whose resistance
is directly proportional to temperature
For our use case, we will be using an NTC Thermistor. To reduce the false
negatives, we can use a thermistor which is sensitive after the mentioned
threshold temperature (Platinum and lead alloy). Such thermistors are
called Bead Thermistors and are known to have fast response times. Lastly,
we just need to add a speaker / buzzer to the other end of the circuit so that
it gives out an alarm when it detects a fire.
BUILDING THE CIRCUIT
Aim: To build the circuit of a fire alarm system
Materials Required:
Breadboard Battery
Connecting wires Variable resistor
Resistor: 500 Ω Thermistor: 104 Ω
Transistor Buzzer / Speaker
Matchbox Stopwatch
Procedure:
Take the breadboard and connect all the components as shown in the
schematic below:
Burn a matchstick and bring it close to the thermistor. Note the time taken
for the buzzer to start and the time taken for it to stop after the matchstick is
removed.
OBSERVATIONS
When a matchstick is burnt and brought near the thermistor, a faint buzzing
sound is heard. It takes a little bit of time for the buzzer to get completely
activated (the buzzing sound is at max magnitude). Even after the fire is
removed, the buzzing sound continues for another 15-20 seconds before
gradually getting faint.
The delay in activation and deactivation of the circuit indicates that
although the thermistor offers fast response time, the resistance does not
drop suddenly. Rather, it reduces slowly as the temperature increases.
PRECAUTIONS
Caution is advised in the following parts of the procedure:
♦ While connecting, ensure that the parts which are in series are in
different rows, otherwise it will result in a parallel connection.
♦ Take care that the burning matchstick is not too near the other parts of
the circuit. Otherwise, the wires may get damaged.
♦ Be careful while handling the matchstick. Adult supervision is
advised while doing the activity.
RESULT
We have successfully built a working fire alarm system. The plots below
display the effect of temperature:
Note: The observed sound is just an estimated relative comparison between
the resultant sound and the max sound it achieved.
