Fire Sensing System

Fire is one of the most hazardous natural forces. Sensing fire and fighting it in the early stages can prevent losses to a great extent. Sensing fire electronically has become one of the most reliable fire-fighting techniques today.

Sensing fire needs reliable smoke/ fire sensors. Thermistors can sense fire depending on temperature increase principle. We can also use opto devices to sense smoke. One of the projects published in EFY Vol.6 used Japanese Figaro TGS gas sensors. Such sensors are expensive and are not easily available.

The system presented here uses the most common yet very reliable bimetallic strip of a tube light starter as a heat sensor. The system, besides giving an alarm, also visually indicates the exact position where the fire has taken place. This system becomes very necessary in large multi storeyed buildings, hotels, offices etc.

It is very flexible and am take inputs from any number of sensors. It is very simple to construct and is quite economical loo.

The circuit

The entire system works on a very simple principle. The bimetallic strip acts as a switch to switch on the corresponding latch circuit. Here latching properly is needed so that once the fire is sensed the alarm remains ‘on’ until adequate precautions are taken.

When the bimetallic strip gets heated due to the fire flame, it connects the positive supply line to the input of digital latch circuit, thus latching the latch. The digital latch circuit is built around easily available CMOS inverter

CD4049 ICs,

When sensor A is operated, the input of gate N2 (pin 5) is at logic 1 through the 680-ohm limiting resistor After two inversions, output of N1 (pin 2) is at logic 1 which is fed hack through switching diode 1N4148 Lo the input of N2, thus latching the circuit. LED2 connected across the output of N1 and ground indicates the particular position where sensor A is installed, indirectly indicating the place where the fire has occurred. At the same time, diode D8 conducts and provides base bias 10 transistor Tl and the relay operates the hooter or an electric bell.

The 0,1 nF capacitor at input of latch circuit filters the noise pick-up by long wires leading to sensors, thus preventing any false triggering of the alarm.

Switch S1 acts as a master reset switch, OR logic is implemented at the base of transistor T1 to sense signal from each sensor. In this circuit only six sensors are shown but they can be increased without changing the main circuit.

Power supply

The power supply for the system is very straightforward and simple. No regulator is required as the CMOS IC needs no regulation.

Full-wave bridge and smoothing capacitor form the main components of the power supply. A 9V dry cell is used as battery back-up in case of mains failure. In this case, instead of an electric bell a buzzer operates. RL1 is a DPDT {double-pole, double-throw) relay which connects the 9V battery in the circuit and the buzzer driving transistor. Battery back-up does not affect the remaining operations of the system except the sound of the alarm.


The complete circuit can be constructed on a general-purpose PCB even though the suggested PCH layout is provided for readers’ convenience.

A tube light starter element is taken out of its aluminium/plastic covering and is used as a sensor. With great care the glass bulb covering of the bimetallic strip is to be broken. Only the upper half of the bulb is to be broken so that the bimetallic strip is exposed.

Now the sensor is ready for use. It is to be installed at the nearest point possible where fire may occur so that maximum sensitivity is achieved. The bimetallic strip is basically a fast acting sensor which gets connected within 4 to 5 seconds when in direct contact with a fire flame.

Testing the circuit

After completing the fabrication, light up a candle. Switching on the power supply, bring the candle near the bimetallic strip. Within 3 to 4 seconds the strip gets connected and the particular LED lights up and simultaneously the relay operates. If the circuit operates in the given sequence, the system is ready for installation and use.

The complete system with the power supply and a good metallic cabinet would cost around Rs 150 which is quite reasonable as compared to such systems available in the market.

Readers comments:

While thanking the author for this wonderful project, I request him to inform me:

1. Can I use a failed starter as a fire sensor?

2. How to use a thermistor as a fire sensor4*

3. How to use a loudspeaker instead of buzzer?

In the power supply section I think the author unnecessarily used a relay for battery back-up. Perhaps a single DR50 germanium diode in series with batten,’ would have sufficed.

S. RAVI KUMAR Jagtial (Karimnagar Dist.) The author; Mr Kalpesh Dolwadi, replies:

A failed starter may be used only if the capacitor in parallel with bimetallic strip has failed and not the bimetallic strip itself.

A thermistor (NTC type or even PTC type, as shown below) can be used.

An op-amp may be used for better results, such as sensitivity. Thermistors will need such adjustments.

Using a loudspeaker instead of buzzer will need extra circuit to generate alarm sent through speaker. This will unnecessarily make the circuit more complicated.

Use of DR50 is not feasible as the buzzer ringing circuit has to be connected at the out put at times of power failure. Use of DPDT relay kills two birds with a single stone.

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