A simple capacitive proximity sensor experiment for exploring the effects of body capacitance and earth ground

Capacitive proximity sensors are well-suited for educational projects due to their low cost and simple design. Traditional undergraduate textbooks and lab exercises rarely highlight the fact that the performance of capacitive proximity sensors can be quite sensitive to ground loading. This paper presents a simple classroom demonstration for exploring this topic in detail. The capacitive proximity sensor for this demonstration is a hand-held LCR meter connected to a homemade capacitor composed of two strips of aluminum foil. Students explore the operation of this sensor for two different system ground configurations. In the first case the LCR meter is battery powered (floating ground referenced) and in the second case the LCR meter is powered by AC mains supply (earth ground referenced). When a student positions their hand near the foil strips, the battery-powered sensor measures an increase in capacitance. Conversely, the AC-mains-powered sensor measures a decrease in capacitance. The instructor guides students to discover for themselves the reason for this seemingly puzzling difference by modeling parasitic capacitance and ground loading using simple circuit models.