Fire Alarm

Simple Fire Alarm Circuits at Low Cost

A fire alarm circuit is a simple circuit that detects a fire and sounds a siren or activates a buzzer. Fire Alarm Circuits are critical equipment for detecting fires in a timely manner and preventing harm to persons and property.

Security systems include fire alarm circuits and smoke sensors, which aid in the detection and prevention of harm. It is mandatory to install fire alarm systems and smoke detectors in commercial buildings such as offices, movie theatres, shopping malls, and other public locations.

There are many expensive and sophisticated Fire Alarm Circuits available as stand-alone devices, however we created five extremely easy Fire Alarm Circuits utilising common components such as the Thermistor, LM358, Germanium Diode, LM341, and NE555.

In the next sections, we’ll look at all of these circuits, their circuit layouts, the components needed for each circuit, and how each circuit works.


  • Circuit 1 Simple Fire Alarm Circuit    
    • Circuit Diagram
    • Components Required
    • Component Description
    • Circuit Design
    • Working of the Simple Fire Alarm Circuit
  • Circuit 2 Simple Fire Alarm Circuit using Thermistor
    • Circuit Diagram
    • Components of Fire Alarm Circuit
    • Circuit Working
  • Circuit 3 Fire Alarm with Siren Sound
    • Circuit Diagram
    • Components Required
    • Working
  • Circuit 4 Fire Alarm Circuit Using LM741
    • Block Diagram of Fire Alarm Circuit Using LM741
    • Circuit Diagram of Fire Alarm Using LM741
    • Circuit working
  • Circuit 5 Fire Alarm Circuit Using Germanium Diode
    • Block Diagram of Fire Alarm Circuit Using Germanium Diode
    • Circuit Diagram of Fire Alarm Using Germanium Diode
    • Circuit Working
  • Applications

Circuit 1 Simple Fire Alarm Circuit

This is a very simple alarm circuit using Thermistor, LM358 Operational – Amplifier and a Buzzer.

Circuit Diagram

The circuit diagram of this simple Fire Alarm Project is shown in the following image.

Fire Alarm Circuits

Components Required

  • 1 x 10 K Thermistor
  • 1 x LM358 Operational Amplifier (Op – Amp)
  • 1 x 4.7 KΩ Resistor (1/4 Watt)
  • 1 x 10 KΩ Potentiometer
  • 1 x Small Buzzer (5V Buzzer)
  • Connecting Wires
  • Mini Breadboard
  • 5V Power Supply

Component Description

Thermistors are Temperature Dependent Resistors, which means that their resistance changes with the ambient temperature. Thermistors are divided into two categories: PTC Thermistor and NTC Thermistor. The letters PTC and NTC stand for Positive Temperature Coefficient and Negative Temperature Coefficient, respectively. The resistance of a PTC Thermistor is directly proportional to the temperature, but the resistance of an NTC Thermistor is inversely proportional to the temperature.

We utilised a 10 K Thermistor with NTC in this project. The 10 K Thermistor has a resistance of 10 K at 250°C. The 10K Thermistor used in this project is seen in the image below.

LM358 Operational Amplifier

A Dual Operational Amplifier (Op – Amp) IC is the LM358. The LM358 IC can be used to implement all of the functional modes of a standard operational amplifier. However, in this project, we’ll use the LM358 Operational Amplifier in Comparator Mode, which compares input signals on inverting and non-inverting terminals and produces appropriate output.

Circuit Design

The Fire Alarm Circuit with Siren Sound has a very simple design. Connect the 10 K Potentiometer to the LM358 Op – Amp’s inverting terminal first. The wiper terminal is linked to Pin 2 of the Op – Amp, while one end of the POT is attached to +5V and the other end is connected to GND.

We’ll now use a 10 K Thermistor and a 10 K Resistor to create a potential divider. The junction point, or output, of this potential divider is connected to the non-inverting input of the LM358 Operational Amplifier.

We have chosen a small, 5V buzzer in this project to make the alarm or siren sound. So, connect the output of the LM358 Op – amp to the 5V Buzzer directly.

Pins 8 and 4 of the LM358 IC i.e. V+ and GND are connected to +5V and GND respectively.

Working of the Simple Fire Alarm Circuit

We’ll now look at how a simple fire alarm circuit works. The first thing to know is that the 10 K Thermistor is the most important component in detecting the fire. The 10 K Thermistor utilised here is an NTC type Thermistor, as stated in the component description. The resistance of the Thermistor reduces as the temperature rises.

The temperature rises in the event of a fire. The 10 K Thermistor’s resistance will decrease as the temperature rises. The voltage divider’s output will grow as the resistance decreases. Because the voltage divider’s output is connected to the LM358 Op – Amp’s non-inverting input, its value will be greater than that of the inverting input. As a result, the Op – Amp’s output rises, causing the buzzer to sound.

Circuit 2 Simple Fire Alarm Circuit using Thermistor

Circuit Diagram

Components of Fire Alarm Circuit

  • Thermistor
  • Variable resistor(POT)
  •  Diode
  • Capacitor
  • Resistor
  • BC547 Transistor
  • Speaker

Circuit Working

  • A 10k ohm thermistor is used in the circuit. This is an NTC thermistor, which has a lower resistance as the temperature rises.
  • It had a resistance of 10kohms at room temperature.
  • A voltage divider circuit is formed by connecting another resistance to the thermistor and connecting it to the transistor via a diode.
  • When the transistor is grounded, the buzzer activates. The sound of the buzzer grows as the temperature rises.
  • Also, take a look at this interesting article: Circuit for Panic Alarms

Circuit 3 Fire Alarm with Siren Sound

When there is a fire mishap at home, this circuit sounds a siren to inform us. You may have seen fire alarms before, but this one is unique in that it produces a siren sound rather than a buzzer, and it does so with simple components.

We are aware that many integrated circuits can be used to generate the siren effect, but we prefer to use basic electronics components such as resistors, capacitors, and transistors to generate it so that you can clearly understand its internal workings and gain more knowledge by analysing it rather than relying on pre-designed in-circuit solutions.

Circuit Diagram

Components Required

  • 1 x 10K Thermistor
  • 2 x BC547 NPN Transistor
  • 1 x BC107 NPN Transistor
  • 1 x 2N2222 NPN Transistor
  • 1 x 2N2907 PNP Transistor
  • 3 x 4.7KΩ Resistor (1/4 Watt)
  • 1 x 470KΩ Resistor (1/4 Watt)
  • 1 x 56KΩ Resistor (1/4 Watt)
  • 1 x 47KΩ Resistor (1/4 Watt)
  • 1 x 39KΩ Resistor (1/4 Watt)
  • 1 x 22KΩ Resistor (1/4 Watt)
  • 1 x 1KΩ Resistor (1/4 Watt)
  • 1 x 470Ω Resistor (1/4 Watt)
  • 1 x 120Ω Resistor (1/4 Watt)
  • 1 x 10KΩ Potentiometer
  • 1 x 22µF Capacitor (Polarized)
  • 1 x 470nF (0.47µF) Ceramic Capacitor
  • 1 x Buzzer


This circuit uses a thermistor to sense the temperature. When it senses that the temperature of the environment is increasing above a given threshold, then it gives a signal. The temperature at which the circuit detects fire can be adjusted by using the potentiometer arrangement at VR1.

The potentiometer setup provides a high voltage when the temperature rises above the predetermined value. In common emitter mode, this voltage is then applied to the BC547 transistor. It’s an NPN transistor with a wide range of applications. When a high input is delivered to the base, it is turned on. As the collector to emitter voltage decreases when the transistor is turned on, the collector voltage is decreased to a low level. The first transistor’s collector output voltage is sent into the base of the second BC 547 NPN transistor.This transistor is also in common emitter mode, which means that when the temperature threshold is achieved, the output at the collector will grow rapidly. It will switch on the following transistor, BC107, in this state. The siren circuit’s transistor will now operate as a switch. This transistor can handle far more power than the BC547 and is supplied with a heat sink to do so.

When the BC107 transistor is turned on, current flows from the power source to the collector, acting as an electronically controlled switch. When the current is flowing, the siren circuit, which serves as the circuit’s load, is activated. The siren will thereafter be heard through the buzzer. The primary components in producing the siren effect are the capacitors employed in the circuit. The premise behind creating the siren effect is to create an oscillator with an envelope that grows and drops on a regular basis.

Circuit 4 Fire Alarm Circuit Using LM741

Here is another small project on fire alarm. When a fire accident is happened in home or office, it will detect the fire and give the alarm.

Block Diagram of Fire Alarm Circuit Using LM741

The thermistor is the principal component that detects a fire by detecting a sudden change in room temperature caused by the fire’s heat. The thermistor will detect heat and transmit the data to the LM741 OP-AMP. The op-amp causes the NE555 to emit a pulse, which is then sent to a buzzer.

LM741: The LM741 is an operational amplifier that operates based on the difference between two input voltages. High current driving, voltage gain, noise amplification, and low output impedance are all features of the LM741. The LM741 can also be used to defend against short circuits.

Circuit Diagram of Fire Alarm Using LM741

Fire Alarm Circuit Using lm341

Circuit working

  • Circuit principle is similar to the first circuit i.e. Thermistor is used to sense the raise in temperature. But it rises only after a fixed temperature.
  • Here op amp acts as non-inverting comparator i.e. Vout is positive only if Vin (voltage at pin 2) < VRef (voltage at pin3).
  • When there is no any fire, voltage at pin 2 of the comparator is greater than the voltage at pin3.
  • When there is no fire resistance of thermistor is 10k. So 10K and 4.7k forms voltage divider circuit.
  • Voltage at pin2 is calculate using formula. V= (100*12) / (100+4.7) =11.4
  • Voltage at pin 3 =50*12/100=6v (Variable pin of the pot is at 50% of total resistance.)
  • When there is any fire thermistor temperature raises and its resistance decreases. So voltage at pin2 starts decreasing. Thus Vout is goes to positive i.e. it is equal to Vcc.
  • Here reference voltage selected is 6v.Fire alarm starts only if the input voltage is less than 6v.To increase the reference voltage decrease the resistance of pot.

Circuit 5 Fire Alarm Circuit Using Germanium Diode

This is a simple fire alarm circuit using Germanium Diode and 555 timer. In this circuit Germanium Diode play very important role in detecting the fire. This circuit is very easy to construct, cost effective and implementable.

Block Diagram of Fire Alarm Circuit Using Germanium Diode

Block Diagram of Fire Alarm Using Germanium Diode

This is a basic fire alarm circuit that costs less than a hundred rupees. The circuit’s most important component is the DR25 (germanium diode), whose resistance decreases as the temperature rises. At 70 degrees, the germanium diode will begin to conduct. As a result, the germanium diode might be used as a heat sensor. When the temperature rises beyond 70 degrees Fahrenheit, the germanium diode conducts, triggering the NE555 timer through a transistor. When the germanium diode conducts, the NE555 is configured as an astable Multivibrator, which causes the buzzer to sound an alarm. So that we can be alerted and respond appropriately to the alarm.Circuit Working

Circuit Diagram of Fire Alarm Using Germanium Diode

Fire Alarm Circuit Using Germanium Diode

Circuit Working

  • The DR25 germanium diode is a heat sensor that conducts when the temperature rises to a specified level. In the circuit, the DR25 is reverse biassed. It will only conduct when the temperature is over 70 degrees Fahrenheit.
  • The DR25 is connected in reverse bias to the transistor, which has a high reverse resistance (more than 10K ohm) and prevents the transistor from turning off, which is attached to the 555 timer’s reset pin. When the transistor is turned off, the 555timer’s reset pin will be at ground level. The 555 timer is set up as an astable Multivibrator in this example.
  • When the temperature rises over 70 degrees, the resistance of the DR25 diode drops to 1 k ohm, causing the transistor to switch off and the reset pin to go high. This will cause the output to be generated at pin 3 and the alarm to sound.
  • Three or more reverse bias diodes connected in parallel and positioned in different rooms can be used. It will detect and sound the alarm if there is a fire.


  • If DR25 germanium diode is available, you can still use AC128, AC188 or 2N360 germanium transistors. Use base and emitter junctions in place of cathode and anode.
  • Diode must be connected to the circuit in reverse bias.


  • Fire Alarm Circuits are very useful in homes, offices, schools, labs, etc. to detect and prevent any disasters due to fire.
  • Fire Alarm Systems can work as a stand – alone devices or be a part of a complex home security system with other security features like smoke detection, intruder alert, motion detection, etc.


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