US Patent 7504817 Stud sensor

A sensing device having a sensing surface for placing against a wall for detecting objects behind a wall lining has first and second sensors, each sensor including at least first and second capacitor plates and associated resistors forming RC circuits having time constants dependent on the adjacent wall material. Detection circuits connected to the RC circuits provide first and second voltage signals having amplitudes dependent on capacitance changes. A processor effects a comparison of the first and second voltage signals, and compares them to establish the magnitude of the voltage difference between the voltage signals, indicating a detected object center position when the difference signal is less than a predetermined value.