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.
Code History
Original code by Mario Becker, Fraunhofer IGD, 2007 http://www.igd.fhg.de/igd-a4
Updated by: Alan Chatham http://unojoy.tumblr.com
Updated by Paul Stoffregen: Replaced '328 specific code with portOutputRegister, etc for compatibility with Arduino Mega, Teensy, Sanguino and other boards
Gratuitous optimization to improve sensitivity by Casey Rodarmor.
Updated by Martin Renold: disable interrupts while measuring. This fixes the occasional too-low results.
Updated by InvScribe for Arduino Due.
Updated non-Due code by Gabriel Staples (www.ElectricRCAircraftGuy.com), 15 Nov 2014: replaced "interrupts();" with "SREG = SREG_old;" in order to prevent nested interrupts in case you use this function inside an Interrupt Service Routine (ISR).
Code:
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.
For Regular Arduino
// 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
#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);
uint8_t SREG_old = SREG; //back up the AVR Status Register
// Prevent the timer IRQ from disturbing our measurement
noInterrupts();
// 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. 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;}
// End of timing-critical section; turn interrupts back on if they were on before, or leave them off if they were off before
SREG = SREG_old;
// 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;
}
For Arduino Due
// 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) {
pinMode(pinToMeasure, OUTPUT);
digitalWrite(pinToMeasure, LOW);
delay(1);
// Prevent the timer IRQ from disturbing our measurement
noInterrupts();
// Make the pin an input with the internal pull-up on
pinMode(pinToMeasure, INPUT_PULLUP);
// Now see how long 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 (digitalRead(pinToMeasure)) { cycles = 0;}
else if (digitalRead(pinToMeasure)) { cycles = 1;}
else if (digitalRead(pinToMeasure)) { cycles = 2;}
else if (digitalRead(pinToMeasure)) { cycles = 3;}
else if (digitalRead(pinToMeasure)) { cycles = 4;}
else if (digitalRead(pinToMeasure)) { cycles = 5;}
else if (digitalRead(pinToMeasure)) { cycles = 6;}
else if (digitalRead(pinToMeasure)) { cycles = 7;}
else if (digitalRead(pinToMeasure)) { cycles = 8;}
else if (digitalRead(pinToMeasure)) { cycles = 9;}
else if (digitalRead(pinToMeasure)) { cycles = 10;}
else if (digitalRead(pinToMeasure)) { cycles = 11;}
else if (digitalRead(pinToMeasure)) { cycles = 12;}
else if (digitalRead(pinToMeasure)) { cycles = 13;}
else if (digitalRead(pinToMeasure)) { cycles = 14;}
else if (digitalRead(pinToMeasure)) { cycles = 15;}
else if (digitalRead(pinToMeasure)) { cycles = 16;}
// End of timing-critical section
interrupts();
// 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.
digitalWrite(pinToMeasure, LOW);
pinMode(pinToMeasure, OUTPUT);
return cycles;
}