The temperature sensor sends an analogue signal to the
PIC microcontroller to set off the alarm when the temperature
reaches 125°F.The design consists of a temperature sensor
(LM-35) whose output voltage is equal to 1°F/10 mv.
Vtemp_threshold = X*1°F/10 mv
125°F = X*1°F/10 mv ; x=1.25 V
Based on these calculations performed in the PIC
microcontroller code the temperature alarm is set off after the
output voltage of the temperature sensor reaching appropriate
threshold voltage. Voltage comparator (LM 393) is used
whose output is connected to microcontroller. The Vin – of
voltage comparator is connected to 1.25V and Vin+ is
connected with temperature sensor schematic.