Digital Thermometer

  
Room temperature plays a vital role in determining human thermal comfort. This digital thermometer is designed to measure room temperature and display it on a LCD screen in both Celsius and Fahrenheit scales. A PIC16F688 microchip is used as the main controller that reads temperature from DS1820, a 3-pin digital temperature sensor from Dallas semiconductors (now Maxim). The sensor is designed to measure temperature ranging from -55 to +125 °C in 0.5 °C increments. The room temperature doesn't go that far but the firmware written for the PIC is able to read and display the entire temperature range of DS1820.
Digital Thermometer - schematic

I have tested it from -4.5°C (my freezer temperature) to 105.5 °C (by bringing a soldering iron tip close to the sensor). If you want to measure your freezer temperature too, don't put the entire unit inside it, as some of the components (like LCD) may not work at that low temperatures. Rather put only the sensor inside the freeze and connect it to the rest of the system through three wires. For a better understanding of how DS1820 sensor works, I recommend to read the datasheet from Maxim website. Remember that DS1820 and DS18B20 (both are temperature sensors) have architectural differences, and so DS18B20 will not work here. This project works with DS1820, and it would work with DS18S20 (later version of DS1820) too by changing the temperature conversion time in the firmware. Read this to find the difference between DS1820 and DS18S20, http://www.maxim-ic.com/datasheet/index.mvp/id/3021 PIC16F688 reads data from DS1820 sensor through RA5 port, and the computed temperature is sent to the LCD through RC0-RC3 ports. It means the data transfer from PIC to LCD is achieved in 4-bit mode. The Register Select (RS) and Enable (E) signals for LCD are provided through ports RC4 and RC5. The Read/Write pin of the LCD is grounded as there is no data read from the LCD in this project. The contrast adjustment of LCD is done with the 10K potentiometer shown in the circuit diagram. The temperature reading from DS1820 is 9-bits which...



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