2026-06-17
Capacitive touch sensors detect a finger's presence by measuring the tiny capacitance change it adds to a copper pad. The pad and the finger form one plate each of a parasitic capacitor, with skin, glove, or panel plastic as the dielectric. Typical untouched pad capacitance is 5-20 pF; a finger adds another 1-5 pF. Your job is to detect that 10-30% shift reliably despite humidity, temperature, and EMI.
The dominant measurement method is charge transfer (used by Atmel/Microchip QTouch, Cypress CapSense, STM32 TSC). A sampling capacitor C_s (10-50 nF) is repeatedly charged from the pad via a CMOS pin toggling between drive-high and high-Z-read states. Count how many transfer cycles N it takes for C_s to cross a threshold voltage:
Quick rule of thumb: sensitivity scales as ΔC/C_pad. To get 1% count resolution, keep C_pad as low as possible — that means short traces (under 100 mm), no ground pour directly under the pad, and a series resistor (1-10 kΩ) to limit ESD and slow edges. The series resistor R with pad capacitance C_pad sets an RC time constant; for C_pad = 15 pF and R = 4.7 kΩ, τ = 70 ns, easily handled by a 1 MHz sampling clock.
Real-world example: a kitchen appliance touch panel with a 3 mm acrylic overlay. Pad is a 12 mm × 12 mm copper square on the PCB back-side. Hatched ground (50% fill, 0.2 mm trace / 0.5 mm gap) surrounds the pad at 2 mm spacing — this drains accumulated charge and shields against 50/60 Hz mains pickup without killing sensitivity. A self-capacitance scan runs every 20 ms. Firmware applies a slow baseline tracker (1-minute time constant) so humidity drift doesn't trigger false touches, plus a fast detection filter with hysteresis: trigger at 80% of baseline-minus-delta, release at 90%.
Common mistakes: running pad traces parallel to switching power supplies (couples noise directly into the count), forgetting that wet fingers and water droplets look identical to your sensor (use mutual capacitance for water rejection), and placing the LED backlight directly behind the pad without an isolated driver — PWM dimming hash will swamp your counts.
For multi-pad keyboards, switch to mutual capacitance: drive one electrode (Tx), measure coupling to a perpendicular electrode (Rx). A finger reduces the Tx-Rx coupling by stealing field lines. This is how every smartphone touchscreen works and naturally rejects water shorts.