This is the cheapest i have seen it in a while.
As per description
Control your outdoor lights automatically by adding a Philips Hue Lights Accessories - Outdoor Sensor to your system.
You can place the sensor anywhere as it is battery powered and completely wireless. Simply walk by to trigger your lights.
Features:
- Wireless installation
- Battery Powered
- High-quality weatherproof material (IP54)
- Low voltage system
Specifications:
- IP rating: IP54
- Minimal battery lifetime: 2 year(s)
- Mounting options: wall
- Sensor depth: 56 mm
- Sensor height: 76 mm
- Sensor width: 76 mm
- Dimensions (W x H x L): 176 x 165 x 75 mm
- Weight: 0.145 kg
I use the indoor sensor (which is cheaper) for already 2 years outdoors (undercover) and works fine :-).