IO: Add an example that shows how to use a digital pin and a capacitor to measure resistive sensors

java/libraries/io/examples/SimpleResistorSensor/SimpleResistorSensor.pde

@@ -0,0 +1,56 @@
+import processing.io.*;
+
+// using a capacitor that gets charged and discharged, while
+// measuring the time it takes, is an inexpensive way to
+// read the value of an (analog) resistive sensor, such as
+// a photocell
+// kudos to ladyada for the original tutorial
+
+// see setup.png in the sketch folder for wiring details
+
+int max = 0;
+int min = 9999;
+
+void setup() {
+}
+
+void draw() {
+  int val = sensorRead(4);
+  println(val);
+
+  // track largest and smallest reading, to get a sense
+  // how we compare
+  if (max < val) {
+    max = val;
+  }
+  if (val < min) {
+    min = val;
+  }
+
+  // convert current reading into a number between 0.0 and 1.0
+  float frac = map(val, min, max, 0.0, 1.0);
+
+  background(255 * frac);
+}
+
+int sensorRead(int pin) {
+  // discharge the capacitor
+  GPIO.pinMode(pin, GPIO.OUTPUT);
+  GPIO.digitalWrite(pin, GPIO.LOW);
+  delay(100);
+  // now the capacitor should be empty
+
+  // measure the time takes to fill it
+  // up to ~ 1.4V again
+  GPIO.pinMode(pin, GPIO.INPUT);
+  int start = millis();
+  while (GPIO.digitalRead(pin) == GPIO.LOW) {
+    // wait
+  }
+
+  // return the time elapsed
+  // this will vary based on the value of the
+  // resistive sensor (lower resistance will
+  // make the capacitor charge faster)
+  return millis() - start;
+}