Fire Alarm Using a Thermistor

Author: Pradeep G       Source:EFY

This project can effectively be used as a temperature-sensing fire alarm. Here, an NTC thermistor is used as the temperature sensor. Resistance of the NTC thermistor decreases with an increase in temperature.

Parts List:

Semiconductors:

IC1,IC2-NE555 TImer

T1-BC547 NPN Transistor

LED1-5mm

Resistors(1/4w,+/-carbon):

R1- 22K

R2- 4.7K

R3,R6- 1K

R4,R7- 56K

R5- 680E

R8- 10E

VR1- 100K Pot

Capacitors:

C1,C6- 100uF,25V

C2- 2.2uF,25V

C3,C5- 0.01uF,ceramic disc

C4- 0.047uF, ceramic disc

Miscellaneous:

CON1 - 2 pin connector pin

S1 - on/off switch

NTC1 - 10K NTC Thermister

LS1 - 8E, 0.5W speaker

Circuit and working

  

Circuit diagram of the fire alarm is shown in Fig. 1. It is built around NTC thermistor (NTC1), transistor BC547 (T1), popular NE555 timers (IC1 and IC2), speaker and a few other components.

In this circuit, the two NE555 timers are wired as astable multi-vibrators. IC1 is wired as a low-frequency generator and IC2 as a high-frequency generator.

At room temperature, transistor T1 conducts and keeps reset pin 4 of IC1 at ground level. As a result, both timer ICs are disabled. But when temperature of the sensor goes above 70°C (depending on the thermistor constant [K]), transistor T1 stops conducting. Both NE555 timers oscillate and a beeping sound is heard from the speaker.

Potmeter VR1 is used to set the cut-off/saturation condition of transistor T1, which is related to the resistance of NTC1 at different temperatures. At room temperature, voltage at pin 4 of IC1 remains low. With heating of NTC1, voltage at pin 4 of IC1 becomes high. This enables both the timer ICs to oscillate and produce sound through the speaker. Also, LED1 starts flashing.

The circuit works on 9V regulated power supply. For a louder sound, you may add a speaker driver circuit with matched impedance.




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