Sensors, apparatus, and methods for measuring movements are disclosed. The sensors include input and output windings wound about a common location and an armature is equally positioned relative to both windings movable to vary inductance reactance of the sensor. The mass of the sensor and the ease of movements are such that flexible membranes, such as skin, can be monitored with insignificant interference. The sensor can be included in “Band-aid” bandage arrangement in which the bandage backing can be removed and held in place on skin by the bandage. A monitoring circuit, responsive to the changes in sensor impedance, provides indications of the detected movements. The monitoring circuit includes an arrangement for self-adjusting parameters so that the system can be automatically preset and continually reset. The monitoring circuit includes a power savings arrangement.