Fire Alarm

Simple Fire Alarm Circuits at Low Cost

A fire alarm circuit serves as a straightforward mechanism that identifies fires and triggers a siren or activates a buzzer. These circuits are vital for promptly detecting fires and preventing harm to individuals and property.

Fire alarm systems and smoke detectors are integral components of security systems designed to identify and forestall potential dangers. The installation of fire alarm systems and smoke detectors is obligatory in commercial structures, including offices, movie theaters, shopping centers, and other public venues.

While there are numerous sophisticated and costly standalone fire alarm circuits available, we have developed five exceedingly uncomplicated fire alarm circuits using common components like the Thermistor, LM358, Germanium Diode, LM341, and NE555.

In the subsequent sections, we will delve into these circuits, elucidating their circuit diagrams, the requisite components for each circuit, and elucidating the functioning of each circuit.


  • 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, known as Temperature Dependent Resistors, exhibit a variability in resistance in response to changes in the surrounding temperature. These Thermistors fall into two distinct categories: PTC Thermistors and NTC Thermistors. The acronyms PTC and NTC correspond to Positive Temperature Coefficient and Negative Temperature Coefficient, respectively. In a PTC Thermistor, resistance increases in direct proportion to temperature, whereas in an NTC Thermistor, resistance decreases inversely with temperature.

For this project, we employed a 10 K Thermistor with NTC characteristics. At 250°C, the 10 K Thermistor exhibits a resistance of 10 K. The 10K Thermistor utilized in this project is depicted in the image below.

LM358 Operational Amplifier

The LM358 is an Integrated Circuit (IC) that houses a Dual Operational Amplifier (Op-Amp). This LM358 IC has the capability to fulfill all the operational modes typically associated with a standard operational amplifier. Nonetheless, for the purpose of this project, we will employ the LM358 Operational Amplifier specifically in Comparator Mode. In this mode, the IC compares input signals presented at its inverting and non-inverting terminals and generates a corresponding output.

Circuit Design

The Fire Alarm Circuit with Siren Sound boasts a remarkably straightforward design. Commence by linking the 10 K Potentiometer to the inverting terminal of the LM358 Op-Amp. Next, attach the wiper terminal to Pin 2 of the Op-Amp, with one end of the POT connected to +5V and the other end linked to GND.

We will proceed by utilizing a 10 K Thermistor and a 10 K Resistor to construct a potential divider. The output of this potential divider, situated at the junction point, is then linked to the non-inverting input of the LM358 Operational Amplifier.

For this project, we’ve opted for a small 5V buzzer to produce the alarm or siren sound. Consequently, connect the output of the LM358 Op-Amp directly to the 5V Buzzer.

Lastly, Pins 8 and 4 of the LM358 IC, denoted as V+ and GND, are connected to +5V and GND, respectively.

Working of the Simple Fire Alarm Circuit

Let’s delve into the operation of a basic fire alarm circuit. It’s crucial to understand that the key element in fire detection is the 10 K Thermistor. In this particular setup, the 10 K Thermistor falls into the category of NTC (Negative Temperature Coefficient) Thermistors, as specified in the component description. The resistance of this Thermistor diminishes as the temperature increases.

In the event of a fire, the temperature escalates. Consequently, the resistance of the 10 K Thermistor decreases with the rising temperature. This decrease in resistance results in an increased output from the voltage divider. Since this output from the voltage divider is connected to the non-inverting input of the LM358 Op-Amp, its value surpasses that of the inverting input. Consequently, the Op-Amp’s output ascends, causing the buzzer to activate.

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

This circuit serves as an alert system, producing a siren sound to notify us in the event of a fire incident at home. While you might have encountered fire alarms before, this one stands out as it generates a siren sound rather than a typical buzzer, and it accomplishes this using basic components.

Although numerous integrated circuits are capable of producing the siren effect, we’ve opted to employ fundamental electronic components like resistors, capacitors, and transistors to create it. This choice enables a more transparent understanding of its internal workings and encourages a deeper comprehension through analysis, 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 incorporates a thermistor to monitor the surrounding temperature. When it detects that the temperature is rising above a predefined threshold, it generates a signal. The temperature at which the circuit detects a fire can be adjusted using the potentiometer configuration at VR1.

The potentiometer setup delivers an elevated voltage when the temperature surpasses the predetermined threshold. This voltage is then applied to the BC547 transistor, operating in the common emitter mode. BC547 is an NPN transistor with a wide range of applications. It turns on when a high input is supplied to its base. As the transistor switches on, the collector-to-emitter voltage decreases, resulting in a reduction in the collector voltage. The collector output voltage of the first transistor is conveyed to the base of the second BC547 NPN transistor.

This second transistor also functions in the common emitter mode. When the temperature threshold is met, the collector’s output experiences a rapid increase. In this state, it triggers the following transistor, BC107. The BC107 transistor in the siren circuit now acts as a switch. It can handle significantly higher power compared to the BC547 and is equipped with a heat sink for this purpose.

Upon activation of the BC107 transistor, current flows from the power source to its collector, effectively functioning as an electronically controlled switch. As current flows, the siren circuit, serving as the load for the circuit, is activated. The siren sound is then produced through the buzzer. The capacitors integrated into the circuit play a key role in generating the siren effect. The concept behind creating this siren effect is to generate an oscillator with an envelope that periodically rises and falls.

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 takes on the primary role of fire detection by sensing the rapid increase in room temperature brought about by the heat generated by the fire. It detects this heat and relays the information to the LM741 OP-AMP. The LM741 OP-AMP triggers the NE555 to produce a pulse, subsequently directing it to activate a buzzer.

LM741: The LM741, an operational amplifier, operates by assessing the disparity between two input voltages. It boasts attributes such as high current driving capability, voltage amplification, noise amplification, and a low output impedance. Additionally, the LM741 can serve as a safeguard against short circuits.

Circuit Diagram of Fire Alarm Using LM741

Fire Alarm Circuit Using lm341


  • 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 cost-effective fire alarm circuit, priced at under a hundred rupees, centers around the DR25 (germanium diode) as its most crucial component. The DR25 exhibits a reduction in resistance as the temperature increases, commencing conduction at 70 degrees Fahrenheit. Consequently, the germanium diode serves as a temperature sensor. When the temperature exceeds 70 degrees Fahrenheit, the germanium diode conducts, activating the NE555 timer via a transistor. Upon conduction of the germanium diode, the NE555 is configured as an astable Multivibrator, prompting the buzzer to emit an alarm. This setup ensures that we receive a timely alert, allowing us to respond appropriately to the alarm.

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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