When I first received this item, it was actually much smaller than I expected, but that is actually a plus because it can be installed more discreetly. Once you have the Mi Home app on your phone and you own the Xiaomi Gateway (needed to operate the Sensor) then the possibilities of what you can do with it are infinite with other Xiaomi devices. If the sensor detects movement (or doesn't detect movement for a set period of time) it can trigger just about anything - lights, sockets, Air purifier, fan, alarm, and a lot more. It can, therefore, be easily used for security reasons. Once you connect the Sensor to the Gateway, the Gateway suddenly springs to life and gives instructions on how to set up (in Chinese). The unit contains a small watch-style battery already installed when you receive it, accessible but unscrewing the base. Note that the sensor is of no use alone, but has to be linked to other devices through the app/gateway.Requires Xiaomi Gateway for access to wifi. Instructions are only in Chinese, but it is not hard to work out.