These are the most basic form of automated lighting and should be avoided if possible. They basically turn a light one when it gets dark outside. An obvious downside to this is that they use electricity unnecessarily. There are more advanced versions that allow control of the time the light comes on for, when dusk is detected.
These are perhaps the cheapest and most readily available deterrent on the market today. They are easy to use and also provide have a convenience element whilst you are still at home. They are available in analogue (dial) or digital (display) form and the latter can be programmed to perform quite complex timing sequences over a 24 hour or 7 day period.
One thing to watch out for with these devices is that they are capable of switching inductive loads such as low energy light bulbs. Many of the on the market in the UK are not.
PIR Activated Lighting
These essentially come in three forms:
First is a fairly standard external lamp with a built-in PIR sensor to automatically switch on the light when someone approaches. These are particularly useful near a front door and allow you to find your keys and the door lock, without having to waste energy by leaving a light on each time you go out. These nearly always feature a light level sensor to ensure they don't switch on the lamp during daylight hours.
Quite often, these are only capable of switching incandescent or filament bulbs and not low-energy bulbs, which is something to watch out for. Generally the faster switch on time of filament bulbs is preferable though.
Second is a PIR activated flood light, used to illuminate large spaces such as driveways and patios.
Careful thought is required before installing a light like this. Because they are so bright, they can also use a lot of electricity. Typically they are rated between 500 and 2000W. If you really do need to install one, they should be angled down to avoid light pollution and annoying your neighbours.
Third is a PIR activated switch unit, which can be used to control existing lamps. This is simply the light level sensing and PIR activated switch elements packaged in a small and tidy plastic case.