Using PWM to fade a LED in and out is dead-simple. Except that the brightness does not seem to change linearly: It gets bright very fast, and then it nearly does not change anymore.
The fix: linearize it.
Since floating point operations are not that popular on a low power microcontroller as the Arduino, the easiest way to do this is via a lookup table to map 8 bit values for bridgeness into PWM values to drive the LED.
Perl to the rescue!
perl -e 'for(my $i=0;$i<256;$i++){ print int(exp($i/46)),",";}'
Manually change the first one to 0 (0 intensity is supposed to be off).
And now LED fading is easy. The example below shows also how to use TimedAction Library which I personally found very useful if you want to do 2 things independently on the Arduino (in this case: blink one LED and fade another one)
#include
/* pwm9
Fade pin on PIN9 up and down and blink LED13
The circuit:
* LED attached from digital pin 9 to Vcc.
*/
#define ledPWMPin 9 // LED connected to digital pin 9
#define ledBlinkPin 13 // LED PWM
static unsigned char exp_map[256]={
0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
1,1,1,1,1,1,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,3,
3,3,3,3,3,3,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4,4,4,5,5,5,
5,5,5,5,5,6,6,6,6,6,6,6,7,7,7,7,7,7,8,8,8,8,8,8,9,9,
9,9,10,10,10,10,10,11,11,11,11,12,12,12,13,13,13,13,
14,14,14,15,15,15,16,16,16,17,17,18,18,18,19,19,20,
20,20,21,21,22,22,23,23,24,24,25,26,26,27,27,28,29,
29,30,31,31,32,33,33,34,35,36,36,37,38,39,40,41,42,
42,43,44,45,46,47,48,50,51,52,53,54,55,57,58,59,60,
62,63,64,66,67,69,70,72,74,75,77,79,80,82,84,86,88,
90,91,94,96,98,100,102,104,107,109,111,114,116,119,
122,124,127,130,133,136,139,142,145,148,151,155,158,
161,165,169,172,176,180,184,188,192,196,201,205,210,
214,219,224,229,234,239,244,250,255
};
TimedAction timedBlink=TimedAction(1000,blink);
TimedAction timedPWM=TimedAction(100,fade);
void setup() {
pinMode(ledBlinkPin,OUTPUT);
pinMode(ledPWMPin,OUTPUT);
}
// loop() is always called again when it exists
void loop() {
timedBlink.check();
timedPWM.check();
}
// Blinking on PIN13
void blink() {
static int ledState=LOW;
ledState ? ledState=LOW : ledState=HIGH;
digitalWrite(ledBlinkPin,ledState);
}
// Fading on PIN9
void fade() {
static int currentPWM=0;
static int dir=1;
// 0 is supposed to be dark
// Since LED connects to 5V, 0 is bright, so invert
analogWrite(ledPWMPin,255-exp_map[currentPWM]);
currentPWM+=dir;
if (currentPWM > 255 || currentPWM < 0) {
currentPWM-=2*dir;
dir = -dir;
}
}