Often there will be a need for reading a value that isn't just LOW or HIGH, but something in between. Temperature, light level, humidity, or pressure, all come in a more continuous form than just yes and no. The Magnolia makes a decent enough sensor setup because all its pins are set up for reading analogue values.
Before we get to the code, an important thing to remember. Unlike digitalRead and digitalWrite, the analogueRead doesn't use LOW and HIGH, but a range from values from 0 to 1023, so make sure to keep that in mind.
We're going to be doing a nice and reactive example that uses a potentiometer. Set up a pin for the pot middle pin like this.
int led = A0;
int pot = A1;
void setup()
{
pinMode(led, OUTPUT);
}
Notice we don't set up the input pin. By default, all ATMega328P pins are in input state. You can do it explicitly if you wish (add pinMode(pot, INPUT); to setup()).
Then we need to read the value before we use it in the loop function. Note that value is a local variable, that is, it is created every time the loop function is called. With local variables, we have to explicitely initialise them. In this case, we declare the variable value as an int then give it a value from analogRead straight away.
void loop()
{
int value = analogRead(pot);
// turn the light on for a bit
digitalWrite(led,HIGH);
delay( 10 );
// off time depends on value
digitalWrite(led,LOW);
delay( value );
}
Note, the smaller value is, the shorter the delay, and this the faster the LED will flash.
We need to wire up the LED for output, and the potentiometer for input. Like with the button, we're going to ground the input line, but this time in different way.
Put the potentiometer in the board and temporarily give the legs names PLEFT, PMID and PRIGHT.
Now try the example. The LED light should be flashing. If you turn the knob, the amount of time the LED is on for per cycle will change. Move the knob towards the left pin, and the analogRead will return a smaller value, around 0, so the light will be on most of the time. Turn the knob towards the right pin, and analogRead will return a large value, around the maximum of 1023, and the delay will cause the led to blink about once a second.
This is how we make inputs work with potentiometers (sliders and knobs, and other variable resistors). We ground one side, power the other, then catch a voltage off the middle pin.
What's really happening is that the potentiometer is acting as a dynamic pair of resistors, and they are doing a voltage division. This means that they are splitting the voltage between them by how much resistance each side of the middle pin has. The middle pin acts as a pointer to a continuum of different voltages between the left and right legs. When the middle is all the way over one side, that side has very little resistance, so ends up taking up a very small amount of the voltage. Meaning the middle has a value very similar to the side it is nearest to.
Always ground things that are going to be used as inputs.