The circuit shown below has to be one of the simplest ever. It senses when it's dark and turns a light on. A MOSFET is used as an electronic switch which is off when its gate is held low by an LDR (Light Dependant Resistor) which has a very low resistance when light shines on it. When darkness falls, the resistance of the LDR goes high and the MOSFET is turned on by a 47K resistor. And that's it! With such a simple circuit, results may vary due to component tolerances. Particularly, LDRs have different resistance ranges depending on their type (and even production run), but in practice, I found that just choosing one with the lowest 'light' resistance and the highest 'dark' resistance did the trick (Silonex NORPS-12 works well)

The advantage of using a MOSFET over a normal bipolar transistor is that in its 'ON' state a MOSFET is pretty much a short circuit and so the lamp will get the full supply voltage across it. A MOSFET also runs cooler as the power dissipation in a near short circuit is minimal. During the switch on period the transistor isn't quite a short circuit yet and so may, depending on the load, get a bit warm and require a small heatsink. When the circuit is off, the only current drawn is through the 47K resistor in series with the LDR, which is less than 0.5mA

The veroboard mounted on the lid of the enclosure. The two black wires are soldered onto the LDR

The finished unit. Entry and exit glands are used to ensure that the whole thing is completely water tight