Fire Alarm Circuit Using IC 555 Timer & Thermistor

Overview
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. The thermistor offers a low resistance at high temperatures and high resistance at low temperatures. This phenomenon is employed here for sensing the fire.
You can check this similar project:
Fire Detector using Flame Sensor and Arduino
Bill of Materials
The following are the components required for making this 555 Fire Alarm Project.
S.N.
Components
Description
Quantity

Resistor
470 Ω

Resistor
33 K Ω

Resistor
560 Ω

Resistor
47 KΩ

Resistor
2.2 KΩ

Resistor
10 K Ω

Capacitor
10 µF, 16V (Electrolytic Capacitor)

Capacitor
0.04 µF (Ceramic Disc)

Capacitor
0.01 µF (Ceramic Disc)

NE555
555 Timer IC

BC548
NPN Transistor

BC558
PNP Transistor

SL100/2N3904
NPN Transistor

1N4001
Rectifier Diode

Thermistor
10 KΩ

Speaker
8 Ω, 1W

Fire Alarm Circuit Using IC 555 Timer IC
The Fire Alarm Circuit diagram is given below. The IC NE555 is configured as a free running oscillator at audio frequency. Check
NE555 Datasheet
to understand this.
The transistors BC548 and BC558 drive 555 Timer IC. The output of 555 IC via Pin 3 is coupled to the base of transistor SL100. The same transistor drives the speaker to generate an alarm sound. The frequency of NE555 depends on the values of resistances R5 and R6 and capacitance C2.
When the thermistor becomes hot, it gives a low-resistance path for the positive voltage to the base of transistor Q1 through diode 1N4001 and resistance R2. Capacitor C1 charges up to the positive supply voltage and increases the time for which the alarm is ON. The larger the value of C1, the larger the positive bias applied to the base of transistor Q1. As the collector of Q1 is coupled to the base of transistor Q2, the transistor Q2 provides a positive voltage to the reset pin of NE555.
Resistor R4 is selected so that NE555 keeps inactive in the absence of the positive voltage. Diode 1N4001 stops discharging of capacitor C1 when the thermistor is in connection with the positive supply voltage cools out and provides a high resistance path. It also inhibits the forward biasing of transistor Q1.