This electronic project is a very simple thermometer that is based on the PIC16F84A microcontroller, designed by Microchip.
Why to use a thermometer that is designed using a microcontroller and not a classic analog thermometer? Because you can design a complex solution using few external components, resulting an low cost application that provide a high precision measurement .
This Digital thermometer microcontroller project use watchdog timer function to measure temperature. The WDT on all PIC-micro microcontrollers has a nominal time-out period of 18 ms. The WDT time-out period varies with temperature, VDD and part-to-part process variations.
Without using a separate temperature sensor, it is possible to calculate the temperature with reasonable accuracy using the WDT time-out period.
To translate the environment temperature into an actual reading, the system must be able to do the following:
• Provide a method for establishing time-out to temperature calibration
• Count the number of WDT time-outs for a given period of time
• Equate the number of time-outs to a temperature
The PIC16F84A microcontroller is normally in SLEEP mode, consuming
very little operating current but if any key is pressed, it ‘wakes up’ from SLEEP and updates the WDT count, and checks for additional key presses. If there are none, it returns to SLEEP mode.
The WDT Thermometer has three distinct operating modes:
Display Mode: When the TEMP key is pressed, the system wakes up and the LEDs show the temperature
in degrees Centigrade.
Calibration Mode: This mode creates a set of new calibration values, in addition to those present in the firmware.
For more detail: Digital thermometer with PIC16F84 circuit