Ok, thanks a lot. Can I add a potentiometer in place of the zener diode to regulate the voltage?

Not really, it needs a Zener to trip at the required voltage, a resistor will just trip straight away. Try a 12V type.

 

Not really, it needs a Zener to trip at the required voltage, a resistor will just trip straight away. Try a 12V type.

A little bit of elaboration will be very helpful