The circuit utilizes a water proof temperature sensor DS18B20 which will be dipped in to the water that you are going to heat, this sensor collects current temperature of water using which Arduino decides when to trigger the relay ON and OFF to attain / maintain your desire temperature.
The circuit will turn OFF the heater once the desire temperature is reached and turns back ON once the temperature fall below an acceptable value, in this way temperature of the water is maintained.
A 5V buzzer is provided to alert the user once the set temperature is reached, you may enable / disable the buzzer using the slide switch provided in the circuit.
S1: Start / long press to stop the heater.
S2: Increment a digit (INC).
S3: Decrement a digit (DEC).
S4: To set and save the temperature value.