// This code consists of a procedure called readCapacitivePin
// The procedure takes one input: an Arduino pin number
// The procedure outputs: a number, from 0 to 17, which
// indicates how much capacitance is on the pin.
// When you touch the pin, or anything attached
// to the pin, the number will get higher

#include "pins_arduino.h" // Arduino pre-1.0 needs this

uint8_t readCapacitivePin (int pinToMeasure) {
    // Variables used to translate from Arduino to AVR pin naming
    volatile uint8_t* port;
    volatile uint8_t* ddr;
    volatile uint8_t* pin;
    // Here we translate the input pin number from
    // Arduino pin number to the AVR PORT, PIN, DDR,
    // and which bit of those registers we care about.
    byte bitmask;
    port = portOutputRegister(digitalPinToPort(pinToMeasure));
    ddr = portModeRegister(digitalPinToPort(pinToMeasure));
    bitmask = digitalPinToBitMask(pinToMeasure);
    pin = portInputRegister(digitalPinToPort(pinToMeasure));
    // Discharge the pin first by setting it low and output
    *port &= ~(bitmask);
    *ddr |= bitmask;
    delay(1);
    // Make the pin an input with the internal pull-up on
    *ddr &= ~(bitmask);
    *port |= bitmask;

    // Now see how long it takes for the pin to get pulled up.
    // This manual unrolling of the loop
    // decreases the number of hardware cycles between each read of the pin,
    // thus increasing sensitivity.
    uint8_t cycles = 17;
    if (*pin & bitmask) { cycles = 0;}
    else if (*pin & bitmask) { cycles = 1;}
    else if (*pin & bitmask) { cycles = 2;}
    else if (*pin & bitmask) { cycles = 3;}
    else if (*pin & bitmask) { cycles = 4;}
    else if (*pin & bitmask) { cycles = 5;}
    else if (*pin & bitmask) { cycles = 6;}
    else if (*pin & bitmask) { cycles = 7;}
    else if (*pin & bitmask) { cycles = 8;}
    else if (*pin & bitmask) { cycles = 9;}
    else if (*pin & bitmask) { cycles = 10;}
    else if (*pin & bitmask) { cycles = 11;}
    else if (*pin & bitmask) { cycles = 12;}
    else if (*pin & bitmask) { cycles = 13;}
    else if (*pin & bitmask) { cycles = 14;}
    else if (*pin & bitmask) { cycles = 15;}
    else if (*pin & bitmask) { cycles = 16;}

    // Discharge the pin again by setting it low and setting
    // it to be an output.
    // It's important to leave the pins low if you want to
    // be able to touch more than 1 sensor at a time - if
    // the sensor is left pulled high, when you touch
    // two sensors, your body will transfer the charge between
    // sensors.
    *port &= ~(bitmask);
    *ddr |= bitmask;

    return cycles;
}

This code was written by Mario Becker, Alan Chatham, Paul Stoffregen, and Casey Rodarmor. The original version can be downloaded here: http://playground.arduino.cc/Code/CapacitiveSensor.