An ordinary transistor can serve as an uncalibrated temperature sensor for an accurate, PC-compatible thermometer. Unadjusted accuracy is more than 0.5′C over the -50 to +50′C range, and the only limitation on resolution is the time devoted to measurement acquisition. For example, 0.1′C resolution requires less than 1 sec. The thermometer derives all its power from the RS-232C COM port.source

This is a simple serial volt meter circuit.It can measure from 0 to 5 volts DC.The software for the serial voltmeter monitors the serial port for incoming information sent by the PIC micro. Once received, the displayed result is calculated by taking the value received and multiplying it by 0.02. The PIC16C71 A/D converter is the successive approximation type and provides an 8-Bit result. With only an 8-Bit result our range is limited from 0 to 255 that represents from 0 to 5 volts respectively. source