So we have to carefully adjust the value of Variable resistance RV1 and Thermistor, to make the circuit work properly. In transistor, usually 0.7v voltage is required across the Base and Emitter, to turn it ON. And when transistor becomes OFF, Reset pin of 555 timer IC, gets positive voltage through R3, and 555 IC starts to work and buzzer beeps. 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 Transistor starts to decrease and when the voltage becomes less than the operating voltage (base-emitter voltage V BE) of transistor, then transistor becomes OFF. When the Transistor is ON, Pin 4 (RESET) is connected to the Ground, and when Reset pin is Grounded, 555 IC doesn't operate. And transistor remains at ON state because there is sufficient voltage across the base-emitter of transistor, which makes it ON. When there is no FIRE, thermistor remains at 10k ohm. You can see the circuit diagram of fire alarm in above figure. You can find more such simple circuits here in this electronic circuits section. We have built the circuit using, mainly three components that is, Thermistor, NPN transistor and 555 Timer IC. Thermistor is temperature sensitive resistor, whose resistance changes according to the temperature, its resistance decreases with the increase in temperature and vice versa. The key component of the circuit is Thermistor, which has been used as fire detector or fire sensor. Here we are building one simple fire alarm system with the help of 555 Timer IC, which will sense the fire (temperature rise in surrounding), and trigger the alarm.
It not only prevents a big losses caused by deadly fire but sometimes proves to be life savers. They detects the fire in ambiance at very early stage by sensing smoke or/and heat and raise an alarm which warns people about the fire and furnish sufficient time to take preventive measures. Fire alarms are prime necessities in modern buildings and architectures, especially in banks, data centers and gas stations.
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