This document describes the design and working of a simple fire alarm circuit using a thermistor and IC 555 timer. The circuit uses a thermistor to detect changes in temperature from a fire. When the thermistor heats up from a fire, its resistance decreases which triggers the IC 555 timer to produce an audible alarm from the buzzer. The fire alarm circuit provides fire detection and warning with few components at low cost, making it suitable for home and industrial safety applications.

1. Fire alarm

2. Table of contents
• Introduction
• Block diagram
• Circuit diagram
• Working
• Merits and demerits
• Conclusion
• Reference

3. INTRODUCTION
In this mini-project, we are going to learn how to
make a Simple Fire Alarm Circuit Using IC 555
Timer & Thermistor. Most of the Fire Alarm Circuit
available on the internet is expensive and requires
more components. But here we have presented a
simple and inexpensive mini Fire Alarm project
using a thermistor and 555 Timer IC.The thermistor
is the main component and is used as a
temperature sensor and detects fire.

4. Block diagram
Battery Thermistor BC547 NE555 Buzzer

5. Circuit diagram

6. Working
• When there is no FIRE, thermistor remains at 10k ohm. And
transistor remains at ON state because there is sufficient voltage
across the base-emitter of transistor, which makes it ON. When the
Transistor is ON, Pin 4 (RESET) is connected to the Ground, and
when Reset pin is Grounded, 555 IC doesn’t operate.
• Now when we start heating theThermistor through Fire, its
resistance starts to decrease, and when its resistance decreases,
the voltage at the base ofTransistor starts to decrease and when
the voltage becomes less than the operating voltage (base-emitter
voltageVBE) of transistor, then transistor becomes OFF. And when
transistor becomes OFF, Reset pin of 555 timer IC, gets positive
voltage through R3, and 555 IC starts to work and buzzer beeps.

7. Merits
• Alert people of a potential fire.
• Really important to ensure the safety of
everyone in the premises.
• Warns people about the fire to take
preventive steps

8. Demerits
• The system is essentially useless if the
batteries aren’t charged, since it won’t work
properly.
• False alarms caused by cooking.
• False alarms will lead to some people not
responding to the alarm with the alacrity.

9. Conclusion
• It gives sound from fire alarm when fire
attacks to industries, warehouses,
apartments etc. It is most efficient, simple
and less complex circuit. Designed with less
equipment. It can be used to provide security
at warehouses where storage and protection
of various types of goods is main concern
.Works for home security too.

10. REFERENCES
Linear Integrated circuits using op-amp : Ramakant A
Gayakwad
ATextbook of Applied Electronics : Dr. R.S. Sedha
Proteus Software
Easy Eda : https://easyeda.com/