Thanks, that's a good tip to pre-build the circuit to light an led indoors first, like you say, I can use a phone charger to test it all indoors and and in the car also.. I'll get started on a little bit of relay research.
I've only played around with an Arduino to the extent of making a watch winder with a stepper motor and driver board... I'll have to work out the smallest Arduino I can use for this project too.