Reading Analog Sensors - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 58

Conveniently, the map function can handle this automatically; simply swap
the high and low values so that the low value is 255 and the high value is 0.
The map() function creates a linear mapping. For example, if your fromLow and
fromHigh values are 200 and 900 , respectively, and your toLow and toHigh values
are 255 and 0 , respectively, 550 maps to 127 because 550 is halfway between
200 and 900 and 127 is halfway between 255 and 0 . Importantly, however, the
map() function does not constrain these values. So, if the photoresistor does
measure a value below 200 , it is mapped to a value above 255 (because you are
inverting the mapping). Obviously, you don't want that because you can't pass
a value greater than 255 to the analogWrite() function. You can deal with this
by using the constrain() function. The constrain() function looks like this:
output = constrain(value, min, max)
If you pass the output from the map function into the constrain function,
you can set the min to 0 and the max to 255 , ensuring that any numbers above or
below those values are constrained to either 0 or 255 . Finally, you can then use
those values to command your LED! Now, take a look at what that final sketch
will look like (see Listing 3-3).
Listing 3-3: Automatic Nightlight Sketch—nightlight.ino
//Automatic Nightlight
const int RLED=9; //Red LED on pin 9 (PWM)
const int LIGHT=0; //Lght Sensor on analog pin 0
const int MIN_LIGHT=200; //Minimum expected light value
const int MAX_LIGHT=900; //Maximum Expected Light value
int val = 0; //variable to hold the analog reading
void setup()
{
pinMode(RLED, OUTPUT); //Set LED pin as output
}
void loop()
{
val = analogRead(LIGHT); //Read the light sensor
val = map(val, MIN_LIGHT, MAX_LIGHT, 255, 0); //Map the light reading
val = constrain(val, 0, 255); //Constrain light value
analogWrite(RLED, val); //Control the LED
}