capsense

That was the coolest and easiest project ever! I have a capacative sensor with my arduino using just a couple wires and a resistor!

Well, actually a few resistors since I didn't have a 10M Ohm resistor hanging around I put a bunch of 1M Ohm resistors in series.

So, you hook up pins 8 and 9 to your breadboard, and then put a 10M Ohm resistor between them. Then you hookup an antenna (in my case a sheet of aluminum foil) to pin 9. It looks something like this:






Then you run the following code on your Arduino (thanks loads to Paul Badger for the code:

// CapSense.pde
// Paul Badger 2007

// Fun with capacitive sensing and some machine code - for the Arduino (or Wiring Boards).
// Note that the machine code is based on Arduino Board and will probably require some changes for Wiring Board
// This works with a high value (1-10M) resistor between an output pin and an input pin.
// When the output pin changes it changes the state of the input pin in a time constant determined by R * C
// where R is the resistor and C is the capacitance of the pin plus any capacitance present at the sensor.
// It is possible when using this setup to see some variation in capacitance when one's hand is 3 to 4 inches from the sensors
// Try experimenting with larger sensors. Lower values of R will probably yield higher reliability.
// Use 1 M resistor (or less maybe) for absolute touch to activate.
// With a 10 M resistor the sensor will start to respond 1-2 inches away

// Setup
// Connect a 10M resistor between pins 8 and 9 on the Arduino Board
// Connect a small piece of alluminum or copper foil to a short wire and also connect it to pin 9

// When using this in an installation or device it's going to be important to use shielded cable if the wire between the sensor is 
// more than a few inches long, or it runs by anything that is not supposed to be sensed. 
// Calibration is also probably going to be an issue.
// Instead of "hard wiring" threshold values - store the "non touched" values in a variable on startup - and then compare.
// If your sensed object is many feet from the Arduino Board you're probably going to be better off using the Quantum cap sensors.

// Machine code and Port stuff from a forum post by ARP  http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1169088394/0#0



int  i;
unsigned int x, y;
float accum, fout, fval = .07;    // these are variables for a simple low-pass (smoothing) filter - fval of 1 = no filter - .001 = max filter

void setup() {
 Serial.begin(9600);

 DDRB=B101;     // DDR is the pin direction register - governs inputs and outputs- 1's are outputs
 // Arduino pin 8 output, pin 9 input, pin 10 output for "guard pin"
 //  preceding line is equivalent to three lines below
 //  pinMode(8, OUTPUT);     // output pin
 //  pinMode(9, INPUT);      // input pin
 //  pinMode(10, OUTPUT);    // guard pin
 digitalWrite(10, LOW);  //could also be HIGH - don't use this pin for changing output though
}

void loop() {
 y = 0;        // clear out variables
 x = 0;

 for (i=0; i < 4 ; i++ ){       // do it four times to build up an average - not really neccessary but takes out some jitter

   // LOW-to-HIGH transition
   PORTB = PORTB | 1;                    // Same as line below -  shows programmer chops but doesn't really buy any more speed
   // digitalWrite(8, HIGH);    
   // output pin is PortB0 (Arduino 8), sensor pin is PortB1 (Arduinio 9)                                   

   while ((PINB & B10) != B10 ) {        // while the sense pin is not high
     //  while (digitalRead(9) != 1)     // same as above port manipulation above - only 20 times slower!                
     x++;
   }
   delay(1);

   //  HIGH-to-LOW transition
   PORTB = PORTB & 0xFE;                // Same as line below - these shows programmer chops but doesn't really buy any more speed
   //digitalWrite(8, LOW);              
   while((PINB & B10) != 0 ){            // while pin is not low  -- same as below only 20 times faster
     // while(digitalRead(9) != 0 )      // same as above port manipulation - only 20 times slower!
     y++;  
   }

   delay(1);
 }

 fout =  (fval * (float)x) + ((1-fval) * accum);  // Easy smoothing filter "fval" determines amount of new data in fout
 accum = fout;   

 Serial.print((long)x, DEC);    // raw data - Low to High
 Serial.print( "   ");
 Serial.print((long)y, DEC);    // raw data - High to Low
 Serial.print( "   ");
 Serial.println( (long)fout, DEC); // Smoothed Low to High
}

It output three numbers, the raw data for low to high, high to low and and a smoothed version of low to high.

Now, I want to make multiple antennas. I wonder how many I can wire together with one Arduino board? Can I run everything off of pin 8?

Will let you know how it goes.