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.
barrier, commonly across a watercourse, to hold back water, often forming a reservoir or lake; dams are also sometimes used to control or contain rockslides, mudflows, and the like in regions where these are common. special type of water-retaining structure is the sectional navigation dam. To erect such a dam, buttresses of steel beams are set up on a flat apron during the summer low-water period. Bridges are laid along the buttresses, and gates of simple design rest against the buttresses.