Arduino 电容的直接检测：简易触摸传感器

Arduino除了接受数字端口的数字信号，唯一能检测的模拟物理量就是电压。任何模拟传感器的检测值几乎都是通过相关电路转化成电压值，再输入它的模拟端口进行模数转换的。电容值就需要相对更复杂和昂贵的电路转化为电压值，才能被Arduino检测，而对很多物理过程的检测，都可以很方便可靠地通过检测电容值来实现，其中最常用的地方就是触摸传感器。风靡一时的MaKey MaKey就是个例子。这里介绍一个除了一段导线和一个端口，不需要任何元器件的电容检测方法。

这个方法的思路是，首先把一个数字端口设成低电位，并打开arduino的内部上拉电阻，开始计算这个端口到达高电位所需要的时间。而这个时间与此端口的对地电容值有关，电容越大，时间越长。在硬件上只需要在一个端口上连一根导线即可。用手指触摸这段导线的裸露端，就会导致电容变化，arduino可以通过上述方法检测这个变化。如果要增加灵敏度，可以在导线上连一片锡箔。为防止你手上有强静电击穿芯片，可以在锡箔上盖一层薄纸。

使用的代码如下。你可以方便地把它用到你的项目里。程序下载运行后，用手指触摸接在8号口的导线，就可以点亮9号口的led，手指离开，led熄灭。

int ledPin = 9;
int capval;
void setup()
{
pinMode(ledPin, OUTPUT);
Serial.begin(9600);
Serial.println("Touch senser");
}

void loop ()
{
digitalWrite(ledPin,LOW);
capval = readCapacitivePin(8);
Serial.println(capval, DEC);
if (capval > 2) {
// turn LED on:
digitalWrite(ledPin, HIGH);
delay(10);
}
}

// 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;
}

这个主意不是我的原创，readCapacitivePin 函数来自如下地址：
http://playground.arduino.cc/Code/CapacitiveSensor
http://tieba.baidu.com/p/2060482431