This is a very simple to implement Temperature Sensor. It uses LM35DT as a  semiconductor temperature sensor which operates with a +5 volt DC.                               

It produces an analog output voltage, proportional to the change in  surrounding temperature in Celsius scale (2mv/C). The analog output of the  sensor is then passed to the ADC0804 IC which produces an 8-bit binary output  (digital output) correspoding to the analog input voltage. The digital output  from ADC is then used to glow the LED which indicates the high/low logic (LED  ON: Logic 0, LED OFF: Logic 1).                               

The output of the ADC can be interfaced to a 7-segment diaply using a  7-segment driver or the digital output can be interfaced to a PC /  microcontroller. The bottom portion of the schematic shows a fixed and a  variable power supply which inputs 220 volts AC from the wall outlet in your  house, the transformer then steps-down it to 18 volts AC (9-0-9 centre-tapped),  which is then converted to DC using bridge rectifier.                               

The fixed regulator IC (7805) produces a +5 volts regulated output which is  used to operate the Sensor and the ADC0804 IC. It also outputs a variable  voltage controlled by a 5K variable resistor which is used to adjust the scaling  of the ADC0804 (normally for full scale, it is set to 2.5 volts).                               

Further modification may include an automatic control circuitry interfaced  to the ADC which automatically ON/Off the                               

device whose temperature is to be control/monitor. The automatic control  can be achieved by OP-AMP based comparators or using