555 Timer ProjectsElectronics Circuits

Fire Alarm Circuit Using 555 Timer IC & Thermistor

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In this blog post, we will discuss how to create a fire alarm circuit using a 555 timer IC and a thermistor. This circuit is very simple to build and can be easily installed at home. Moreover, we will review the circuit diagram and the components needed for this project. Finally, we will go over the wiring and assembly of the circuit.

Fire alarm circuits are very useful in our homes and offices. They help us to be alerted in case of fire. A fire alarm circuit using a 555 timer IC and a thermistor can be used to detect the presence of fire. The thermistor is used to sense the temperature of the surrounding environment.

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When the temperature rises above a certain threshold, the thermistor will conduct electricity and trigger the 555 timer IC. The 555 timer will then generate a series of square waves that can be used to drive an alarm speaker.

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Most fire alarm circuits accessible on the internet are costly and require extra components. However, we have offered a simple, low-cost small Fire Alarm project that uses a thermistor and a 555 Timer IC.

The essential component is the thermistor, a temperature sensor that detects fire. The resistance of the thermistor is low at hot temperatures and high at low temperatures.

List of Components Used

The following are the materials needed to complete this 555 Fire Alarm Project. Reach out using the comment section for component equivalent and substitution.

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Capacitor10 µF, 16V (Electrolytic Capacitor)1
Capacitor0.04 µF (Ceramic Disc)1
Capacitor0.01 µF (Ceramic Disc)1
NE555 555 Timer IC1
2N5551 / BC548NPN Transistor1
2N5401 / BC558PNP Transistor1
BD139 / TIP41NPN Transistor1
1N4001Rectifier Diode1
Thermistor10 KΩ1
Speaker8 Ω, 1W

Fire Alarm Circuit Diagram Using NE555 Timer IC

The schematic of the Fire Alarm Circuit is shown below. The IC NE555 is set as a free-running oscillator at an audio frequency range. To learn more, consult the NE555 Datasheet.

Fire Alarm Circuit Using 555 Timer IC and Thermistor
Fire Alarm Circuit Using 555 Timer IC and Thermistor

Fire Alarm Circuit Operation

For the fire alarm circuit to work, the 2N5551 and 2N5401 transistors must trigger the 555 Timer IC. The 555 IC output (Pin3) is connected to the base of transistor Q3. The transistor also drives the speaker to generate an alarm sound.

The frequency (pitch of the output sound) of NE555 depends on the values of resistances R5 and R6 and capacitance C2.

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When the thermistor heats up, it provides a low-resistance path for the positive voltage to transistor Q1’s base through diode 1N4001 and resistance R2. Capacitor C1 charges to the positive supply voltage and extends the alarm’s active time.

The higher the value of C1, the greater the positive bias applied to the base of transistor Q1. Because the collector of Q1 is tied to the base of transistor Q2, transistor Q2 supplies a positive voltage to the NE555’s reset pin.

Resistor R4 is chosen, so the NE555 remains inactive without a positive voltage. When the thermistor is connected to the positive supply voltage, diode 1N4001 stops draining capacitor C1 and creates a high resistance route. It also prevents transistor Q1 from being forward-biased.


This blog post demonstrated how to build a fire alarm circuit using a 555 timer IC and a thermistor. We also discussed the circuit diagram and the components needed for this project. This circuit is straightforward to make and can be easily installed at home. However, if you have any problem building the project, kindly comment, and I will be glad to assist you.

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Boateng Oduro is passionate about learning new technologies and working on them. He is a teacher and an engineer who loves to read, write, and teach. He's always curious about things and very determined to track the latest technologies and the trends for the future.

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