Capacitive Sensor Change
ΔC = ε0 × A / d
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Formula
Description
Capacitive touch and proximity sensors detect changes in capacitance caused by the presence of a finger, hand, or other conductive object near the sensor electrode. The parallel plate capacitor model shows that capacitance increases with larger electrode area and decreases with greater distance. A human finger introduces a grounded conductor near the sensor pad, increasing the total capacitance by a few picofarads. Capacitive sensing ICs detect these tiny changes using charge transfer, sigma-delta, or relaxation oscillator measurement techniques.
Variables
- ΔC — Capacitance change due to proximity (F)
- ε0 — Permittivity of the gap medium (F/m), typically 8.854 pF/m for air
- A — Effective electrode area (m²)
- d — Distance between finger and electrode (m)
Practical Notes
Typical touch pad capacitance: 5-30 pF. A finger touch adds 1-5 pF. The sensing IC must detect changes of 0.1-1 pF reliably. Keep sensor traces short and away from noisy signals. Use a ground plane on the opposite side of the PCB from the sensor for better shielding. Moisture, temperature, and PCB contamination affect baseline capacitance, requiring auto-calibration.
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