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Digital Thermometers in this circuit design from microcontroller PIC16F628 of micrichip company.Good point of PIC16F628 such as brown-out reset , low-voltage programming reference voltafe module,USART: Universal Synchronous Asynchronous Receiver Transmitter
PWM (CCP : Capture Compare Pulse-width modulation module
PIC16F628
For completed circuit of Digital Thermometers show in figure 1 . It can be display 2 digit which you can applied to 3 digit
For temperature sensor use IC DS1820 which The DS18S20 communicates over a 1-Wire bus. You can download data sheet from hardware Support Data