Â­TÂ­here are many different ways to create a motion sensor. For example: 1  It is common for stores to have a beam of light crossing the room near the door, and a photosensor on the other side of the Â­room. 2  When a customer breaks the beam, the photosensor detects the change in the amount of light and rings a bell.
Definition of weir. 1  1 : a fence or enclosure set in a waterway for taking fish. 2  2 : a dam in a stream or river to raise the water level or divert its flow.
Weirs and barrages are different types of ârunâofâriverâ dams, this means that while they raise the water level upstream they create only a small reservoir (âhead pondâ) and cannot effectively regulate downstream flows. A weir is normally a low wall of stone, concrete or wicker.