Simple dusk detector for battery operated Christmas lights.

This is a super-simple circuit that you can use to extend the battery life of Christmas lighting strings. Ideal for garden use where it's more convenient than running cables.
It's basically a waterproof box (a Tupperware style box) containing a battery pack and a light sensor based on any general purpose small transistor you can find. I used a 2N7000 FET but you can also use a BC547, 2N3904 or whatever you can find. In this circuit it has to be an NPN or N-channel transistor.
You'll also need an LDR (Light Dependent Resistor) also sometimes called a Cadmium Sulfide photocell. You probably have a scrap nightlight with a cadmium sulfide cell in it already, just ripe for scavenging. In some instances it may actually be cheaper to buy a product with a photocell in it than just the component on its own!
The circuit is simple. A resistor is connected between positive and the transistors base, then the photocell is also connected from the base to the negative. When there's a modest amount of ambient light the photocell conducts and pulls the base down, keeping the transistor off. When it gets dark the photocell goes high resistance and the transistor turns on. You can adjust the turn-on point by varying the resistor value. When the transistor turns on it pulls the LED strings negative to the battery negative and the other terminal for the LED string is connected to positive via a resistor to limit the current. (10 to 100 ohms depending on how bright you want them.)
Battery life is generally very good with this arrangement. And the circuit is great for the long strings of "copper wire" LEDs where two lacquered copper wires have surface mount LEDs tacked along their length and are dipped in resin.