| 摘要 |
A telephone break-in security unit (20) comprises a connection (15) for an incoming telephone line and a connection (16) for an outgoing telephone line, a current threshold comparator (6) arranged for sensing the current in the telephone line and thus for detecting when the receiver of a telephone apparatus connected to the security unit is lifted, a code memory (8) arranged so that any code may be stored and transmitted, a code modulator (18) arranged to apply a coded signal to the incoming connection, and a microprocessor (1). A chief security unit comprises a code demodulator (19) arranged to receive coded signals and a microprocessor programmed in such a manner that it reacts to a signal from the voltage threshold comparator by examining whether or not the use of the telephone line thus detected is authorised by the emission of the approved code. In case of unauthorised use, the line may be blocked by a short-circuit relay (5). |
