Lesson 2 of 7

A button with an interrupt

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// Count button presses with an interrupt; each press toggles the LED const int BUTTON_PIN = 2; const int LED_PIN = 13; const unsigned long DEBOUNCE_MS = 30; volatile bool pressed = false; volatile unsigned long lastChange = 0; int count = 0; bool ledOn = false; // runs on every edge of D2, even while loop() is busy void onChange() { unsigned long now = millis(); if (digitalRead(BUTTON_PIN) == LOW && now - lastChange > DEBOUNCE_MS) { pressed = true; } lastChange = now; } void setup() { Serial.begin(9600); pinMode(BUTTON_PIN, INPUT_PULLUP); // released = HIGH, pressed = LOW pinMode(LED_PIN, OUTPUT); attachInterrupt(digitalPinToInterrupt(BUTTON_PIN), onChange, CHANGE); } void loop() { if (pressed) { pressed = false; count++; ledOn = !ledOn; digitalWrite(LED_PIN, ledOn ? HIGH : LOW); Serial.print("presses: "); Serial.println(count); } }

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A button on D2 counts presses with attachInterrupt() and toggles an LED on D13. The internal pull-up means the button needs no resistor, and a debounce time hides its contact bounce.

No resistor needed for the button
Press it in the figure, or let the script press twice
The interrupt catches a press even while loop() is busy

Pin	Connects to	What it does
`D2`	One side of the button	Input with the internal pull-up, and an interrupt pin.
`D13`	1 kΩ resistor, then the LED	Output: toggled on each press.
`GND`	The button's other side and the LED's short leg	Pressed, the button pulls D2 down to GND.

Wire it in

Sensors and actuators this board drives, with the real connections.

LEDSupply 5 V
Long leg (anode) through a 1 kΩ resistor to D13, short leg to GND. 1 kΩ gives about 3 mA, well inside the R4's 8 mA per pin.
Part pin Board pin
A D13Wire colour orange
K GNDWire colour black
Careful
Never without a resistor: an R4 pin is good for 8 mA, and a bare LED would try to pull far more.
Push buttonSupply 5 V
Between D2 and GND with pinMode(2, INPUT_PULLUP): it reads LOW while pressed. D2 and D3 also take attachInterrupt().
Part pin Board pin
1 D2Wire colour yellow
3 GNDWire colour black

All 6 parts on the Arduino UNO R4 page

Connects to

What it does

D2

One side of the button

Input with the internal pull-up, and an interrupt pin.

D13

1 kΩ resistor, then the LED

Output: toggled on each press.

GND

The button's other side and the LED's short leg

Pressed, the button pulls D2 down to GND.

Wire it in

Sensors and actuators this board drives, with the real connections.

LEDSupply 5 V

Long leg (anode) through a 1 kΩ resistor to D13, short leg to GND. 1 kΩ gives about 3 mA, well inside the R4's 8 mA per pin.

Part pin	Board pin
A	D13Wire colour orange
K	GNDWire colour black

Careful

Never without a resistor: an R4 pin is good for 8 mA, and a bare LED would try to pull far more.

Push buttonSupply 5 V

Between D2 and GND with pinMode(2, INPUT_PULLUP): it reads LOW while pressed. D2 and D3 also take attachInterrupt().

Part pin	Board pin
1	D2Wire colour yellow
3	GNDWire colour black

All 6 parts on the Arduino UNO R4 page

A button with an interrupt

Try it yourself

Serial monitor

Checks

Break it

Inputs

Parameters

Wire it in

A button with an interrupt

Try it yourself

Serial monitor

Checks

Break it

Inputs

Parameters

Wire it in