Thermistor based fire alarm circuit
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.
Download PCB and component layout PDFs: click here
Construction and testing
A single-side PCB for the fire alarm circuit is shown in Fig. 2 and its component layout in Fig. 3. Enclose the PCB in a small box in such a way that the thermistor can sense the temperature in case of fire.
Pradeep G. is B.Sc. (Physics) and a regular contributor to international magazines. He is also a small-business owner making school/college projects in south India