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.