Patent ID: 7167089

Claim:
A burglar alarm using a power cord to transmit signals, comprising: a main unit internally provided with a power supply voltage regulator circuit for supplying power needed by different sections of said main unit; a CPU circuit for encoding and receiving instructions from an externally connected control circuit, and generating digital codes having a specific frequency from the received signals; an FET driving circuit for amplifying the generated digital codes; and an RLC oscillation circuit for converting the amplified digital code signals into analog signals, which are amplified and then coupled from a primary coil to a secondary coil of a transformer before being carried via said power cord; and a decoding horn internally provided with a power supply voltage regulator circuit for supplying power needed by different sections of said decoding horn; a reference voltage circuit for providing a reference voltage used a said basis in comparison; an RLC oscillation circuit for retrieving analog code signals from the power cord, filtering off the voltage across the power cord, and amplifying the remained analog code signals; a comparison circuit for converting the analog code signals into digital code signals; a CPU circuit for decoding and comparing a received burglar-alarming signal with said reference voltage; and a horn voice driving circuit for driving a horn to make vocal and sounding warnings; whereby said burglar alarm is controllable by either using different frequencies as codes 1 and 0, or using a specific frequency as a base frequency while using different pulse widths as codes 1 and 0, so that said burglar-alarming signals are carriable via a power cord without interference by noises on the power cord.