11

Arduino programming: Sound, envelopes, and interrupts

SCHOOL OF MAKING

something has changed, and the response will be

fast. Or something changed just after the previous

check and won’t now be serviced for a longer

duration. This induces jitter, which is variations in the

delay between when something happens and when

your code can respond to it. Of course, we’re talking

about differences in milliseconds, but it can make

a difference in time-critical situations, or when jitter

can be easily detected such as with strobing lights

or audio playback.

Let’s get started by writing the code for

the interrupt:

const int interruptPin = 2;

const int piezoPin = 3;

unsigned long note_time;

bool trigger = false;

void setup() {

attachInterrupt

(digitalPinToInterrupt(interruptPin),

triggerSound,

CHANGE);

}

All we’re doing in the above chunk is first declaring

a global constant variable to hold the value of the

pin connected to our button, and then using this

value within setup(). We also create an ‘unsigned

long’ variable to hold up to 4 bytes of data with

no negative numbers, which we’ll use to hold a

timestamp, and a bool to hold the press state of the

button. attachInterrupt is the important part, as

this is the Arduino magic that tells your hardware

to automatically launch a function, triggerSound,

when it receives a signal corresponding to the final

argument in the attachInterrupt function call. We’ve

gone for CHANGE, as this triggers the interrupt when

the button is being pressed and released. We could

also have used RISING to trigger the interrupt when

the button is pressed and FALLING when the button is

released, but we can handle both of those states with

CHANGE without using our one remaining interrupt, as

we’ll show. There’s also LOW (and HIGH on selected

‘attachInterrupt’ is the

important part, as this is the

Arduino magic that tells your

hardware to automatically

launch a function

”

”

Below

We salvaged our

speaker from an old

PC, but they’re easy

to find by taking

apart almost anything

that used to make

a sound

Below

Momentary switches

only stay connected

as long as the user

presses the button