Abstract: A motion detector comprising a self-excited oscillator operating in the megacycle range which is maintained at a substantially fixed frequency by way of a constant current source. By virtue of the constant current input changes in inductance and capacitance associated with the motion of the intruder are registered as changes in oscillatory amplitude rather than frequency change. This change in amplitude is passed through a low band pass filter, i.e. a filter operating in the range of frequencies normally associated with the motion of a human, and the changes in this filtered signal are then passed through a coupling capacitor to a comparator. In this manner, the DC level, which often drifts in a linear circuit is taken out. The output of the comparator is then applied to a coded transmitter which on the occurrence of an intrusion will set off an alarm at a remotely held receiver.