FIRE ALARM

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 Battery snapper. A switch can be connected between the PCB and one of the batter snapper wire. After completing the circuit, fix IC in the socket observing its orientation similar to IC base. Connect battery. If buzzer sounds, adjust VR1 to set the sensitivity of Thermister so that the Buzzer just stops sounding.  If not adjust VR1 again. Show a lighted match stick near the Thermister (not on it). Buzzer should sound. This indicates fire.