Hi,
I am beginner/novice to electronics. I would like to make a project which would check the temperature of a liquid on going basis. When the liquid temperature goes under a certain range of temperature, it would start heating pad under the liquid holder to increase the liquid’s temperature to the required range of temperature.
Could you please give me your big picture approach how could I achieve this?
Thanks in advance! 
Gokhan
That’s sounds like a thermostat. What’s wrong with using a controllable hotplate, an oven, a boiler, or something like that?
The first step is to identify what you really need. In this case, the temperature range, the type of liquid, can the liquid contact the sensor. Also: why would you expect the temperature of the liquid to change unpredictably? Room temp varies? How tight do you need to control the liquid temperature? If you do closed loop control like you describe, you must count with a time delay between the heater and the sensor. Also think about risks: if you are brewing an explosive, a second temperature limiting device might save your life just in case your electronics fails.
Use a NTC thermistor which is essentially a temperature sensitive resistor
Use this as one of the resistors in a voltage divider circuit
Connect this circuit to an analog read pin on your microcontroller to read voltage.
Plug this voltage into the voltage divider equation and solve for the resistance of the thermistor.
Plug this resistance into the Steinhart-Hart equation to get temperature.
Use if statements in your code to trigger a pin high or low based on the temperature.
Connect this temperature pin to a solid state relay (SSR) input.
Connect an outlet to the SSR output.
This will cycle the outlet ON/OFF which will cycle your heating pad ON/OFF and control temperature.
I’d just buy a PID temperature controller and wire it to the heater controls.