Native Capacitive Sensors without additional Hardware

You can create a touch-sensitive input on any of the Arduino's pins. It requires no special hardware, nevertheless a capacitor of 1nF is recommended in line with the pin to decouple 50Hz noises.

Connect a wire or some metallic plate to a pin.

The code works by setting the pin to ground, turning on the internal pull-up resistor, and measuring the time it takes for the pin to return to the HIGH state. If untouched readCapacitivePin returns a low value e.g. "1"; when touched it rises to about 5. By adding some comparision with a threshold you can make it a boolean key input.

DO NOT CONNECT ANY ACTIVE DRIVER TO THE USED PIN !

the pin is toggled to output mode to discharge the port, and if connected to a voltage source, will short circuit the pin, potentially damaging the Arduino and any hardware attached to the pin.

Original code by Mario Becker, Fraunhofer IGD, 2007 http://www.igd.fhg.de/igd-a4

Updated by: Alan Chatham http://unojoy.tumblr.com

With this function, you can call readCapacitivePin( pinNumber) and get a number from 0 to 17 corresponding to the level of capacitance on the pin - the number will be higher when you touch the pin.

// readCapacitivePin
//  Input: Arduino pin number
//  Output: A number, from 0 to 17 expressing
//  how much capacitance is on the pin
//  When you touch the pin, or whatever you have
//  attached to it, the number will get higher
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;
  if ((pinToMeasure >= 0) && (pinToMeasure <= 7)){
    port = &PORTD;
    ddr = &DDRD;
    bitmask = 1 << pinToMeasure;
    pin = &PIND;
  }
  if ((pinToMeasure > 7) && (pinToMeasure <= 13)){
    port = &PORTB;
    ddr = &DDRB;
    bitmask = 1 << (pinToMeasure - 8);
    pin = &PINB;
  }
  if ((pinToMeasure > 13) && (pinToMeasure <= 19)){
    port = &PORTC;
    ddr = &DDRC;
    bitmask = 1 << (pinToMeasure - 13);
    pin = &PINC;
  }
  // 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 the pin to get pulled up
  int cycles = 17;
  for(int i = 0; i < 16; i++){
    if (*pin & bitmask){
      cycles = i;
      break;
    }
  }
  // Discharge the pin again by setting it low and 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;
}