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Fire alarm


Himanshu Choudhary


At times we forget to switch off our heating devices and eventually they get damaged. Therefore there is always a need of a device which can alert by sounding an alarm if the device temperature goes beyond a particular value. Same concept can be used in this project about fire alarm. If a building catches fire then it will raise an alarm and people could evacuate the building. This circuit uses a LM35 temperature sensor along with a LM339 comparator. Interfaced to LM339 are the following components:

1.     Resistors

2.     Capacitors

3.     Transistors

4.     555 Timer

5.     Buzzer (output) 




This project about fire alarm circuit is based around LM35 which is a temperature sensor and could be used to switch an alarm when temperature goes beyond a preset value. In this circuit the output of LM35 is fed to the negative pin of comparator of LM339. The positive input is connected to a preset VR1 of value 10K. This preset is used to set the reference temperature. If output of LM 35 is less than V reference than the output of comparator is low. The IC LM339 has four inbuilt comparators.


The output of comparator is fed to the base of transistor T1(BC547). As the output is low, T1 is in the cut off state and behaves like an open switch. As a result point A is at high potential. This in turn biases the transistor T2, which behaves like a closed switch. The potential of point B is low, which in turn gives a low signal to the reset pin 4 of the IC 555. When the reset pin is low, there is no output at the output pin 3 and hence the buzzer does not sound.

When the temperature exceeds the reference temperature, the output of the comparator becomes high and T1 gets biased. The point A comes at a low potential thereby making the point B at higher potential. Since the point B is connected to the reset pin, it also becomes high and the 555 timer which is configured in the astable mode, starts producing pulses and the buzzer makes a sound to alert the rise in temperature.


Circuit Diagram


Resistor Image

Resistor is a passive component used to control current in a circuit. Its resistance is given by the ratio of voltage applied across its terminals to the current passing through it. Thus a particular value of resistor, for fixed voltage, limits the current through it. They are omnipresent in electronic circuits.

Capacitor image

Capacitor is a passive component used to store charge. The charge (q) stored in a capacitor is the product of its capacitance (C) value and the voltage (V) applied to it. Capacitors offer infinite reactance to zero frequency so they are used for blocking DC components or bypassing the AC signals. The capacitor undergoes through a recursive cycle of charging and discharging in AC circuits where the voltage and curre...

Presets | Variable Resistors
A preset is a three legged electronic component which can be made to offer varying resistance in a circuit. The resistance is varied by adjusting the rotary control over it. The adjustment can be done by using a small screw driver or...
BC547 | Transistor BC547  | PDF Datasheet
Transistor BC547
BC547 is an NPN bi-polar junction transistor. A transistor, stands for transfer of resistance, is commonly used to amplify current. A small current at its base controls a larger current...
LM35 Temperature Sensor Image
LM35 Temperature Sensor

LM35 is a precision IC temperature sensor with its output proportional to the temperature (in oC). The sensor circuitry is sealed and therefore it is not subjected to oxidation and other processes....

Piezo Buzzer
Piezo Buzzer
The piezo buzzer produces sound based on reverse of the piezoelectric effect. The generation of pressure variation or strain by the application of electric potential across a piezoelectric material is the underlying...
555 Timer | IC 555 | NE555
555 Timer IC

555 is a very commonly used IC for generating accurate timing pulses. It is an 8pin timer IC and has mainly two modes of operation: monostable and astable. In monostable mode time delay of the pulses can be precisely controlled by an external resistor and a capacitor whereas in astable mode the frequency & duty cycle are controlled by two external resistors and a...

LM339 | Comparator IC
LM339 IC

LM339 is a comparator IC with four inbuilt comparators. A comparator is a simple circuit that moves signals between the analog and digital worlds. It compares two input voltage levels and gives digital output to indicate the larger one. The two input pins are termed as inverting (V-) and n...

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