This circuit activates a buzzer and lights an LED when the temperature in its vicinity increases.
IC1 is used as a Monostable timer with the timing components R1 and C1 and with the given values, the output of IC1 remains high for 2 minutes once triggered and then turns low. The trigger pin 2 of IC1 is connected to an NTC (Negative Temperature Coefficient) Thermister. Its resistance decreases when temperature increases. So normally, the trigger pin 2 of the timer remains high through R2. When the temperature increases, the resistance of Thermister decreases and the trigger pin goes to ground potential. This triggers the monostable and its output turns high for 2 minutes and buzzer beeps. The buzzer stops beeping automatically when the timing cycle completes.
Show a lighted match stick or hot soldering iron near the Thermister. Buzzer should beep.

If component removal is required for re assembling, do not bend the excess leads of components before soldering. Cut the excess lead close to the PCB and apply some flux. Apply little solder at the contact point. Do not apply, excess solder or over heat the solder points. PCB tracks will detach. Clean the surface with kerosene, or PCB cleaning fluid to remove flux and dirt.

HEAT SENSOR 1 HEAT SENSOR 1 Reviewed by Babar Ali IET on February 10, 2019 Rating: 5

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