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.
The 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.
Components
Required
R1 1M, R2 100 Ohms VR1 100K, VR2 1M presets
C1 0.22 IC1 CD 4060 Buzzer, D1 LED 16 pin IC base Thermister Battery snapper PCB 9 volt PP3 battery
Assembling
and Testing
Assemble the circuit carefully on the PCB.
Solder Resistors first and then IC base and Presets. Then solder other
components. Observe the polarity of Buzzer and