i am currently doin my as level coursework and for part of it i need to work out what size resistor to use in a potential divider but i have no idea how anyone that could help i would be really grateful,,
Well one part of the potential divider will be an LDR. You'll need to know some values from the LDR and how the NOT gate operates before you can work out what value of resistor you wish to place in the potential divider. The equation for the voltage output of a potential divider is given in your AS textbooks as:
P.S. I would place a Variable Resistor (VR) in place of a fixed resistance, so it can be altered to suit the requirments. You'll still need to work out your VR range though, but you'll gain the advantage of making adjustments to the amount of light needed.