Patent ID: 7504817

Claim:
A sensing device having a sensing surface for placing against a wall for detecting objects behind a wall lining comprising: a first sensor comprising at least first and second capacitor plates and first and second resistors forming first and second RC circuits having first and second time constants dependent on adjacent wall material; a first detection circuit connected to the first and second RC circuits and providing a first voltage signal having an amplitude dependent on a change in the first and second time constants of the first and second RC circuits; a second sensor comprising at least third and fourth capacitor plates and third and fourth resistors forming third and fourth RC circuits having third and fourth time constants; a second detection circuit connected to the third and fourth RC circuits and providing a second voltage signal having an amplitude dependent on a change in the third and fourth time constants; comparison means receiving the first and second voltage signals, and comparing the first and second voltage signals to establish magnitude of voltage difference between the first and second voltage signals, and indicating a detected object center position when the voltage difference is less than a predetermined value.