How do i go about constructing temperature controller?

Question

A temperature controller that keep the temperature of liquid constant between 50 to 80 degree celsius

Answer 1

The range 50 to 80 degrees is above room temperature, so you need an electric heater and a power supply. For small lab samples, a surface mount resistor is a good choice. If precise control is needed, mount a thermocouple as close to the sample as possible and run the output to an A/D converter linked to a computer. A very simple computer, or micro controller will do. I used the original Apple for the job 20 years ago. The computer should switch the heater on at a frequency proportional to the difference between the target and actual temperature. This will give a damped settling to the target temperature and stability to better than 0.5 C. For better stability, include the temperature derivative in your model.

Answer 2

a honeywell thermostat.
a thermometer in the liquid and thermostat in the immediate aera test the 2 in calibration then rig thermostat to control heat source.
you can also rig another to warn if temp dwindles with a honeywell relay control you wire tt to thermostat and bring line voltage to
l1 and l2 run an extension chord to a lamp cut 1 side of chord into switch circuit to activate light as temp drops. when the light turns on your alarm has activated