>2) The power supply of this project are two 9V batteries.
>
>So I need to find a solution, may be driving the mosfet with bipolar
>transistor?


You will need a pair of transistors, one to turn it on, and another to
turn it off. Then, another to convert from the pic's 0-5V to 0-18V.
Look at the datasheet for a max712 battery charger for ideas,