Fire alarm

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 fire alarm. If a building catches fire then it will raise an alarm and people could evacuate the building.

This 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.