Abstract: An alarm intrusion system using existing power lines. A sweeping oscillator controlled by a second oscillator having a lower frequency is connected to the power line through a transient clipper and a power line isolator. Connected to the sweeping oscillator and transient clipper is a bandpass filter and automatic gain control circuit, the output of which is evelope detected and fed to a bandpass filter and then to a slope amplifier or differentiator. Positive and negative level detectors responsive to changes of envelope slope are fed to a latching flip-flop via an OR gate. Also fed to the latching flip-flop is a reset pulse derived from shaping the output of the second oscillator. This pulse is also fed to an up/down counter, the direction of the count being determined by the latching flip-flop output. The count is decoded and fed to alarm circuits. The power line is connected to an inductor-capacitor circuit having a predetermined resonant frequency that is altered upon the closing of an intrusion switch.