Notes:
Using a photocell such as the ORP12 this circuit activates a relay when the light falls below a preset level.
In bright light the resistance of the ORP12 can be as low as 80 ohm and at 50lux the resistance increases
to over 1 Mohm. The 150k control should provide a wide range for light intensities, if not its value can be
increased.  The output of the op-amp feeds a transistor which controls the relay. The 270k resistor provides
a small amount of hysteresis, so that the circuit switches on and off with slightly different light levels.  This
eliminates  relay chatter ( or a disco effect with your porch light ).  Take great care if you decide to wire the
relay to activate a mains lamp. Make sure the relay contacts provide adequate isolation and have ample
rating for the load.