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.
Note
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.
