I have connected positive wire of 18v battery supply to the transistor(2N3904) collector. The base of the transistor have connected to the digital output pin 7 then the emitter connect to the negative wire of dc cable to 12v heating element. The positive wire of dc cable then connect back to the battery negative wire.
In the i have force to stop running for the transistor been burn.
I plan to use arduino to control the transistor when is the time to allow the supply to the heater when the condition is meet.
Addition i added the temperature sensor(lm35dt) to sense the water temperature condition.