This article demonstrates the design, development and operation of the Celsius scale thermometer using LM35 and microcontroller AT89C51. The circuit contains the analog temperature sensor LM35 which has the advantage of providing an output directly proportional to the Celsius temperature, without having the need to be calibrated. It consists of the 8 bit analog to digital converter ADC0808, which uses the successive approximation conversion technique. The ADC0808 is controlled by the microcontroller which processes its output to display the resultant temperature reading. This article gives details about the microcontroller program, circuit design and the circuit operation. Before going to know about this circuit, get an idea about Digital Temperature Sensor.