This is the circuit of a Fire sensor that produces loud beeps when it detects heat from fire. The circuit uses a Thermister as fire sensor. Thermister is a special kind of resistor and its resistance depends on the temperature. In cold, Thermister offers high resistance and its resistance decreases to a few ohms when the temperature near it increases.
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Thermister is connected to the points A and B. When the temperature is normal,
it offers high resistance so that it will not conduct. This gives a high
voltage to the pin 12 of IC1. It is a binary counter that becomes active only
when its reset pin 12 is low. So in the standby state, IC will not work and
alarm generator remains silent. Variable resistor VR1 adjusts the sensitivity
of the Thermister at the particular temperature level. When there is fire in
front of the Thermister, the temperature rise caused by the fire reduces the
resistance of the Thermister and it becomes conducting. This takes the reset
pin 12 of IC1 to ground and it starts oscillating. The oscillations of IC1
generate beeps through the buzzer. LED also blinks indicating the fire. VR2 can
be adjusted to get slow to fast beep rates.
