Fire Alarm Circuit using Thermistor

Fire alarms are a prime necessity in modern buildings and architecture, especially in banks, data centres, and gas stations. They detect fire in the ambience at a very early stage by sensing smoke or heat and raise an alarm, which warns people and gives them sufficient time to take preventive measures. It not only prevents big losses caused by deadly fire but sometimes proves to be a lifesaver. Here we are building a simple fire alarm system using a 555 Timer IC that will sense a fire (temperature rise in the surrounding area) and trigger the alarm.

The key component of the circuit is a thermistor, which has been used as a fire detector or a fire sensor. A thermistor is a temperature-sensitive resistor whose resistance changes according to the temperature; its resistance decreases with the increase in temperature and vice versa. A well-designed fire alarm system circuit can prevent significant property loss and, crucially, save lives. 

In this guide, I will walk you through the construction of a straightforward fire alarm circuit utilising a 555 Timer IC and a BC547 NPN transistor with a 10K NTC thermistor. The contents include a complete circuit diagram, a full component list, step-by-step instructions for completing the circuit assembly, as well as tips for calibrating the circuit. It is one of the most popular beginner-level fire alarm circuit projects.

We have built the circuit using mainly three components, that is, the thermistor, the NPN transistor and the 555 Timer IC. You can find more such simple circuits here in the electronic circuits section. If you want to build an advanced version of this project, also check out Fire and Smoke alarm using Arduino, where we have used the Arduino UNO board to send alerts when there is a fire or smoke detected.  In this project, we build a cost-effective fire alarm circuit using a thermistor as the primary heat sensor. 

Working Concept of the Fire Alarm Circuit

Role of the 555 Timer IC in Astable Mode

Here, the 555 timer IC has been configured in Astable mode so that the alarm (Buzzer) can produce an oscillating sound. In Astable mode, capacitor C charges through resistance R1 and R2, till 2/3 Vcc and discharges through R2 till it reaches to 1/3Vcc. During the charging time, OUT PIN 3 of the 555 IC remains HIGH, and during discharging it remains LOW, that's how it oscillates. We have connected a Buzzer to the OUT pin, so that it produces a beep sound when 555 is high. We can control the oscillation frequency of the alarm by adjusting the value of R2 and/or capacitor C.

Role of the Thermistor as a Fire Sensor

 The fire detector for this fire alarm circuit that utilizes a thermistor is the 10K NTC thermistor. At the ambient (room) temperature of 25°C, the resistance of the thermistor is about 10KΩ. When combined with RV1, this holds the BC547 transistor in a saturated state (ON). The resistor value of the thermistor significantly decreases when the temperature of the room is elevated due to fire. This decreases the base-emitter voltage VBE of the transistor to below ~0.7V, turning off the transistor and thus activating the alarm.

RESET Pin Control Logic

Logic control on Pin 4 (RESET) of the 555 Timer is based on whether or not the transistor is ON (no fire). When the transistor is ON, Pin 4 of the 555 Timer will be pulled to GND through the transistor's collector-emitter path. The LOW state on RESET will disable the timer and cause the buzzer to remain silent. When the transistor turns OFF (fire detected), R3 (100KΩ) will pull RESET HIGH, thus enabling the astable oscillator, thereby sounding the alarm. This is the primary logic for the fire alarm system circuit.

Components Required for the Fire Alarm Circuit

The following table lists every component needed to build this fire alarm circuit board.

Component	Specification / Value	Quantity	Purpose
555 Timer IC	NE555 / LM555	1	Astable oscillator driving buzzer
NPN Transistor	BC547	1	Controls RESET pin based on thermistor state
Thermistor (NTC)	10KΩ at 25°C	1	Fire / heat sensor
Resistor R1	1KΩ	1	555 Timer timing resistor
Resistor R2	4.7KΩ	1	555 Timer timing / discharge resistor
Resistor R3	100KΩ	1	Pull-up to RESET pin
Variable Resistor RV1	1MΩ Potentiometer	1	Sensitivity calibration
Capacitor C	10µF, 16V electrolytic	1	555 Timer timing capacitor
Buzzer	5V–12V piezo or electromagnetic	1	Audible alarm output
Battery	9V PP3	1	Power supply

Fire Alarm Circuit Diagram and Explanation

You can see the circuit diagram of a fire alarm in the above figure. When there is no FIRE, the thermistor remains at 10k ohm. And the transistor remains in the ON state because there is sufficient voltage across the base-emitter of the transistor, which makes it ON. When the Transistor is ON, Pin 4 (RESET) is connected to the Ground, and when the Reset pin is grounded, the 555 IC doesn't operate.

Now, when we start heating the Thermistor through Fire, its resistance starts to decrease, and when its resistance decreases, the voltage at the base of the transistor starts to decrease, and when the voltage becomes less than the operating voltage (base-emitter voltage V_BE) of the transistor, then the transistor becomes OFF. And when the transistor becomes OFF, the reset pin of the 555 timer IC gets a positive voltage through R3, and the 555 IC starts to work and the buzzer beeps.

In a transistor, usually 0.7V voltage is required across the Base and Emitter to turn it ON. So we have to carefully adjust the value of Variable resistance RV1 and the Thermistor, to make the circuit work properly. To do this, remove the thermistor and let RV1 be the grounded one. Now, adjust the value of RV1 to that point where even a slight turning of the RV1 starts the Buzzer. Means from this point, if we decrease the resistance, even very little, the buzzer starts to beep. Now, at this point, connect the thermistor again.

We should also note that we can also build a Fire alarm circuit, using a DR25 germanium diode, as it works as a heat sensor. When the DR25 germanium diode is connected in reverse bias, it has a very high reverse resistance, and it only conducts at more than 70 degrees of room temperature.

555 Timer IC Pin Functions in This Circuit

Pin No.	Pin Name	Connection in This Circuit
1	GND	Ground
2	Trigger	Tied to Pin 6; C (+) terminal
3	Output	Buzzer (+)
4	Reset	Collector of BC547 / R3 pull-up
5	Control Voltage	10nF bypass to GND
6	Threshold	Tied to Pin 2; C (+) terminal
7	Discharge	R1–R2 junction
8	VCC	+9V supply

Thermistor vs. Germanium Diode as Fire Sensor - Comparison

Feature	NTC Thermistor (10K)	DR25 Germanium Diode
Operating principle	Resistance ↓ as temperature ↑	Reverse conduction above ~70°C
Response temperature	Adjustable via RV1	Fixed ~70°C threshold
Sensitivity tuning	Yes — via variable resistor	Limited
Cost	Very low	Very low
Availability	Widely available	Less common today
Best for	General fire alarm circuit projects	Fixed high-temp threshold applications

Key Design Considerations for the Fire Alarm System Circuit

• Frequency adjustment: By adding resistance (R2) and/or capacitance (C) in parallel with the buzzer circuit, it can lower the frequency of the buzzer, whereas removing R2 or C will increase the frequency of the buzzer, which will create a more rapid and urgent tone.
• Power supply: The NE555 can use voltages between 4.5 volts and 15 volts for operation; therefore, a 9-volt battery will work for prototyping, but will need a regulated DC power supply for permanent application.
• PCB circuit board for fire alarm: All components must be soldered to an FR4 single-sided PCB and enclosed in an air-ventilated ABS box; the thermistor must be exposed to the ambient air without being enclosed.
• False alarms from external heating sources: In order to eliminate false alarms from external heating sources, place a small (100 nF) capacitor in parallel with the thermistor to help filter out short-duration transients (for example, when a lighter passes close to the thermistor), thus preventing false triggering.
• Upgrade to original (or Arduino): As an alternative for remote monitoring or notification, replace this circuit with an Arduino-based fire alarm using GSM SMS notification. By upgrading in this manner, the IoT capability is added while maintaining the ability to detect heat from the thermistor.

Frequently Asked Questions - Fire Alarm Circuit

These are the most common questions people ask about building a fire alarm circuit diagram and fire detection circuits.

⇥ Can I build a fire alarm circuit on a PCB board?
Yes. After you have tested your smoke detector prototype in a breadboard environment and confirmed that it functions correctly, the next step is to create a permanent, compact and rugged PCB board that will be used for your smoke detector project only. For example, you will want your thermistor to extend out of your enclosure so that it can adequately measure the temperature of the surrounding environment.

⇥  What is the difference between a smoke detector and a thermistor fire alarm?
A thermistor smoke detector identifies temperature increases created by fire, whereas a traditional smoke detector finds particles from combustion that are suspended in the air. In general, thermistor circuits are more straightforward to build and cheaper to purchase; however, smoke detectors perform better than thermistor circuits when it comes to detecting slow-burning/ smouldering fires before there is a large amount of heat created.

⇥  Can I use a germanium diode instead of a thermistor in a fire alarm circuit?
Yes. A DR25 germanium diode, when biased "backward", will sense heat by conduction greater than about 70ºC (158ºF) and is therefore considered a low-cost thermistor substitute. The remainder of the smoke detector (555 Timer) circuit will remain the same.

⇥ What is the supply voltage for this fire alarm system circuit?
This fire alarm system circuit uses a 9-volt direct current source from a standard PP3 battery. As the 555 Timer IC can operate with a voltage between 4.5 volts & 15 volts, you could also use a regulated 5-volt or 12-volt substitute (an adapter) for a permanent installation requiring continuous power.

⇥ How do I add an Arduino to upgrade this fire alarm circuit?
By replacing the 555 timer with an Arduino UNO board and connecting a thermistor to the Arduino's analog input pin, you can then program the Arduino to trigger an audible siren (buzzer or similar) and use a GSM module to send an SMS when the level of heat detected exceeds the programmed threshold value. This will then create a more sophisticated version of the IoT-enabled fire alarm control circuit, which can communicate wirelessly with the user regardless of their distance from the device.

⇥  What are the common applications of a fire alarm system circuit?
You will frequently find fire alarm control circuits installed in homes, workplaces, banks, data storage centres, gas stations, warehouses, and various types of manufacturing facilities. In most cases, these systems are intended to provide early detection of heat, allowing people to evacuate from dangerous situations and activate suppression (and extinguishing) devices before a larger (or potentially) uncontrollable fire develops.

Emergency Alarm-Based Projects

In our previous projects, we designed different alarm systems such as smoke detectors, water level alarms, and burglar alarms using simple circuits. You can explore these projects further through the links provided below.