Trickyrick:
Ok sorry
Yes Arduino Pins
I will use your library on Github
If I look up a wiring diagram for the LCD I seeGND - VCC - Display contrast - register select - read/write - enable - 8 data pins - anode - cathode
is the back light the same as the contrast you say to connect this to a different pin
If I connected the contrast to a PWM then could I in the sketch dim the display until motion is sensed then turn in up for say 5 min
You will need a transistor to control the backlight power.
The Arduino pin controls the transistor and transistor controls the power for the backlight.
It might be easier to use a i2c LCD backpack. The backpack has a potentiometer for contrast control and a backlight circuit to control the backlight.
You can control the LCD and the backlight over i2c using only the 2 i2c pins.
--- bill