Measuring the Capacitance of Electrical Wiring and Humans for Proximity Sensing with Existing Electrical Infrastructure

In a recent series of electric field sensing experiments, a Theremin was used to measure the mutual capacitance between a human being and a length of electrical wiring. The instrument, based on the LM555 circuit, measures the deflections in capacitance due to the proximity of a human. The measurements are repeatable, and the difference in capacitance for a person at one-half meter with a person at one meter is consistent with the difference computed assuming the human acts as a ground plane for the wiring. Much of the current literature in electric field sensing focuses on measures and models of mutual capacitance for humans interacting with plate conductors, especially fingers near touch screens. Our investigation considers conducting wires to allow development of portable, rapidly deployable human proximity sensing systems that exploit existing electrical infrastructure in buildings. The experiment described here demonstrates that sensing with wires is possible at ranges on the order of a meter and provides evidence that modeling the person as a ground plane of finite extent provides a rough estimate of the change in mutual capacitance