Skip to content
Circuits Gallery
  • Home
  • BasicsExpand
    • Wiring
    • Op-Amp
    • Circuitry
    • Oscilloscope
    • Conductivity
    • Components
  • MicroControllerExpand
    • Arduino
    • PIC
    • Simulation
  • ProjectExpand
    • Using 555 Timer
  • DigitalExpand
    • Communication
  • Submit Project
Circuits Gallery
Home / Project / Simple Fire Alarm Circuit | Breakdown of Thermistor Circuit Diagram

Simple Fire Alarm Circuit | Breakdown of Thermistor Circuit Diagram

Here Circuitsgallery comes with a simple and automatic fire alarm and detection system. This fire detection alarm circuit is based on the thermistor. A simple potential divider arrangement using a thermistor is capable of sensing the temperature (presence of fire) and alerting us with a warning signal.

This thermistor-based simple fire alarm circuit is suitable for your home security systems. Also, school students can do this fire detector circuit as their high school science fair project. Here I will explain in detail about fire alarm circuit wiring with animation/ simulation of a fire alarm.

What is a Thermistor?

 A thermistor is nothing but a variable resistor whose resistance changes with temperature. Fire detection is simply possible via this low-cost alarm circuit.

Circuit Diagram of Simple Fire Alarm

Circuit Diagram of Simple Fire Alarm

Components Required for Making a Simple Fire Alarm Circuit

  1. Power supply
  2. Resistors (1kΩ)
  3. Potentiometer (10KΩ)
  4. Capacitor (10µF, 16V)
  5. BC548 transistor
  6. High resistance thermistor (NTC)
  7. Diode (1N4001)
  8. 6V Buzzer

Working of Fire Alarm Circuit

  • Working is based on the switching property of transistor.
  • The thermistor and R1 forms a potential divider network which drives the transistor.
  • The heart of this circuit is a thermistor. Thermistors are low-cost, easily-available temperature sensors widely used for uncomplicated temperature measurements.
  • Thermistors are temperature sensitive resistors. When temperature increases resistance offered by the thermistor decreases and vice versa. At normal temperature, the resistance of the thermistor is around 10kΩ.
  • All resistors vary with temperature, but the semiconductor materials used for thermistors are especially sensitive to temperature.
  • The transistor is turned ON by the voltage drop across the resistor R1.
  • Consider the temperature of the atmosphere is around 25°C, and then the resistance of thermistor is high so the voltage across the thermistor is also high according to the basic ohm’s law V=IR.
  • The threshold voltage can be set by varying the potentiometer (R1).
  • At this situation voltage across resistor R1 is low and it is not sufficient to turn on the transistor.
  • As temperature rises, the resistance of thermistor decreases so that the drop across the resistor R1 increases which turns ON the transistor.
  • When the transistor is turned ON, the current from Vcc starts to flow via 6V buzzer which produces a beep sound.
  • Diode is used for enabling unidirectional conduction and the capacitor removes sudden transients from the thermistor.

Simulation of the Fire Alarm Project

We have used an LED instead of a buzzer for the simulation of the fire alarm system.

Conclusion

Many devices around us are constantly sensing the rise and fall of temperature. Almost all of them are using the Thermistor as the basis of this detection. Thus knowing its operation is quite pleasing to most electronics DIYers. Building a fire alarm circuit is, hence, much easier if you follow this guide.

Table Of Contents
  1. What is a Thermistor?
  2. Circuit Diagram of Simple Fire Alarm
  3. Components Required for Making a Simple Fire Alarm Circuit
  4. Working of Fire Alarm Circuit
  5. Simulation of the Fire Alarm Project
  6. Conclusion

Subscribe to our newsletter

& plug into

the world of circuits

    A seasoned electronics enthusiast, Charles Clark is a key contributor to Circuits Gallery. From basic components to advanced microcontroller projects, Charles simplifies complex concepts with ease. His writings are a blend of expertise and passion, making electronics accessible to all. Whether it's circuitry or digital communication, Charles is the voice you can trust.

    Facebook Twitter Instagram

    Recent Blogs

    • How to Make A 75 To 300 Ohm Matching Transformer
      How to Make A 75 To 300 Ohm Matching Transformer? | A Step by Step Guide for You
    • How to Download Circuit Wizard? | Procedure for Downloading the Latest Version
    • What Gauge Wire from Battery to Starter
      What Gauge Wire from Battery to Starter? | Choosing the Right Wire Gauge
    • What Does 50 Milliamps Look Like on a Multimeter
      What Does 50 Milliamps Look Like on a Multimeter? | Let’s Find Out
    • How Many Watts Does a 32 Inch TV Use
      How Many Watts Does a 32 Inch TV Use? | Find Out the Required Wattage for Different Brands
    • What Size Conduit for 122 Wire
      What Size Conduit for 12/2 Wire? | Choosing the Right Conduit for Your 12/2 Wire

    Browse Categories

    • Arduino
    • Basics
    • Circuitry
    • Communication
    • Components
    • Conductivity
    • Digital
    • How To
    • MicroController
    • Op-Amp
    • Oscilloscope
    • PIC
    • Project
    • Simulation
    • Using 555 Timer
    • Wiring
    Circuits Gallery
    Join Our Community:

    Quick Links

    • About Us
    • Privacy Policy
    • Terms and Conditions
    • Submit Your Project
    • Contact Us

    Our Story

    Our journey designing innovative devices had immersed us in convoluted electronics. We realized mastery doesn't require elite degrees or industry secrets—just knowledge presented coherently. We became devoted to unraveling even quantum-complex circuits, diagram by diagram, so anyone eager to learn can unlock these secrets. By simplifying electronics fundamentals, we hope to ignite innovation in generations to come.

    © Copyright 2025 Circuits Gallery | All Rights Reserved.

    • Home
    • Basics
      • Wiring
      • Op-Amp
      • Circuitry
      • Oscilloscope
      • Conductivity
      • Components
    • MicroController
      • Arduino
      • PIC
      • Simulation
    • Project
      • Using 555 Timer
    • Digital
      • Communication
    • Submit Project
    Search