8051 microcontrollers are mature microcontrollers that have been used for quite long time and they still are popular in many applications. So here it is a standard temperature sensor project which has some educational aspects. As you know AT89S52 microcontroller has no ADC interface so there is an external ADC used which is built from scratch. In few words it is a counting type ADC which is based on DAC where digital to analog conversion is continued until match is found with comparator. So building such device is more fun.
In this project a NTC type thermal resistor is used which value is red by custom ADC and value is displayed on 3 digit 7 segment LED indicator which is also driven dynamically. So again this is simple interesting and well explained project to move on.