Burglar alarm operated by Venetian blind - is set by lowering blind and gives alarm if blind moves

摘要

A venetian blind (1) inside a window (5) of a house has a conducting peg (6) on its bottom slat (3'). When the blind is lowered, the peg is inserted between a pair of spring contacts (8), fixed to the window frame, and short circuits them. The contacts are connected (10,12) to a battery (20), relay (14) and alarm master switch (18). With the switch on, the relay is energised, which holds open a contact in the alarm circuit. An intruder who breaks the window (5) has to move the blind to gain entry. This dislodges the peg (6) from the spring contacts (8) and the relay (14) drops out and closes the alarm circuit.