Hello,
If you want to put an Arduino outside, there aren't a lot of options for permanent power. Often you could find a exterior light fixture. You can use one of these to adapt the fixture into a power outlet:
I've got some cheap automatic light controllers that fit between the bulb and the bulb holder and switch the bulb on and off according to ambient light levels. Perhaps you could repurpose one of these as a project enclosure. I don't know what's inside, but it may even be that they already contain a SMPS - if not, I think they're big enough to hold one, together with a small PCB for your project.
PeterH:
I've got some cheap automatic light controllers that fit between the bulb and the bulb holder and switch the bulb on and off according to ambient light levels. ... I don't know what's inside, but it may even be that they already contain a SMPS
Why would they include a SMPS?
The control device is a TRIAC, the few milliamps required to control it is generally derived from a series capacitor. You also need a small resistor in series with the capacitor - 100 ohms or so - to protect the Zener (shunt regulator) from the charging current from the capacitor if your device happens to be initially connected at the moment when the mains cycle is at its peak.
It has the capability to detect the light level and control how long the lights are turned on for. I don't know how that's implemented, and perhaps it involves some digital logic - in that case, it would need a logic level power supply. This is ignorant speculation, which is why I qualified it with "It may be". In any case, the most important thing here is the physical housing.
