Day 3: Button Input — Making the Arduino Listen

Hi Anish! So far the Arduino has only been talking (sending power out to LEDs). Today, it starts listening. We are going to connect a button, and the Arduino will light an LED only when you press it. That’s your first real input, and your first piece of code that makes a decision.

What you need today

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• Arduino Uno + USB cable
• Breadboard
• 1 push button (4-legged tactile switch from your kit)
• 1 LED (any color)
• 1 220Ω resistor (for the LED — red-red-brown stripes)
• 1 10kΩ resistor (for the button — brown-black-orange stripes) — this is the pull-down resistor
• 5 jumper wires

First, a quick detour: what is voltage?

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Before we wire a button, it helps to have a picture of what “voltage” even means. Here are two ways to think about it.

Water pipe analogy

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Voltage is like water pressure in a pipe. Imagine two tanks of water:

• One is high up on a hill — lots of pressure, water shoots out fast
• One is on the ground — low pressure, water trickles

The hill-tank pushes water harder. A battery with more volts pushes electricity harder, just like the hill-tank pushes water harder. A 9V battery pushes harder than a 5V one. A dead battery (0 volts) has no push at all — nothing flows.

Electric train track analogy

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Voltage is like how much power a toy train gets from its track:

• 5V = train runs at a nice speed
• 9V = train zips like crazy
• 0V = train doesn’t move

More volts = more push. Simple.

So when the Arduino says 5V, it means “a gentle, steady push of electricity — enough to light an LED or drive a tiny sensor.”

How Arduino reads a button

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A button has only two states: pressed or not pressed. The Arduino reads a pin by measuring its voltage:

• HIGH = about 5 volts = “there’s a push”
• LOW = about 0 volts = “there’s nothing”

So we wire the button so that when it is pressed, the Arduino pin sees 5V (HIGH), and when it is not pressed, the pin sees 0V (LOW).

The tricky part: what happens when the button is not pressed? If the wire is just floating in mid-air, the pin doesn’t see 5V or 0V — it sees random garbage, because the wire picks up electrical noise like an antenna. The Arduino would read HIGH sometimes and LOW sometimes, totally at random. Chaos.

The fix: a pull-down resistor. It “pulls” the pin gently down to 0V whenever nothing else is happening. When the button is pressed, the pin gets pushed up to 5V by a direct wire (much stronger than the weak pull-down), so the pin reads HIGH. When the button is released, the pull-down quietly drags the pin back down to LOW. No more chaos.

Today we use a 10kΩ resistor as our pull-down. “Pull-down” just tells you its job in the circuit.

The circuit

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We will wire two things on the same breadboard:

1. An LED on pin 8 (same as Day 2)
2. A button on pin 7, with a 10kΩ pull-down resistor

graph LR
    subgraph LED_side["LED part"]
        PIN8["Pin 8"] --> R220["220Ω"]
        R220 --> LED_POS["LED long leg"]
        LED_POS --> LED_NEG["LED short leg"]
        LED_NEG --> GND1["GND"]
    end
    subgraph BTN_side["Button part"]
        V5["5V"] --> BTN_A["Button side A"]
        BTN_A -.->|connects when pressed| BTN_B["Button side B"]
        BTN_B --> PIN7["Pin 7"]
        BTN_B --> R10K["10kΩ pull-down"]
        R10K --> GND2["GND"]
    end

Wiring steps — LED first (same as Day 2):

1. Place the LED on the breadboard (long leg and short leg in different columns).
2. Resistor (220Ω) from pin 8 jumper to the long leg of the LED.
3. Jumper from the short leg of the LED to GND.

Wiring steps — Button:

4. Place the button on the breadboard so that it straddles the middle gap. The 4 legs should end up in 4 different columns (2 on top of the gap, 2 below).
5. Jumper from 5V on the Arduino to one side of the button.
6. Jumper from the other side of the button to pin 7 on the Arduino.
7. From that same “other side” column, also plug in the 10kΩ resistor, with its other end going to GND.

The third wire is the trick: the same column of the breadboard now connects to three things at once — the button, pin 7, and the 10kΩ resistor. That is the whole point of a breadboard — one column = one electrical “node”.

Check: When the button is NOT pressed, pin 7 is connected only to GND through the 10kΩ resistor → pin 7 sees 0V = LOW. When the button IS pressed, pin 7 is connected directly to 5V → pin 7 sees 5V = HIGH.

The code

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void setup() {
  pinMode(8, OUTPUT);  // LED output (from Day 2)
  pinMode(7, INPUT);   // Button input — NEW! We are listening, not talking
}

void loop() {
  if (digitalRead(7) == HIGH) {
    // Button is pressed
    digitalWrite(8, HIGH);   // LED ON
  } else {
    // Button is not pressed
    digitalWrite(8, LOW);    // LED OFF
  }
}

Click Upload. Now press the button. The LED lights up only while you are holding it down. Let go — it turns off. Press again — on. Release — off.

What is new in today’s code?

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pinMode(7, INPUT)

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On Day 1 and Day 2, every pin we used was an OUTPUT — we sent power out. Today, pin 7 is an INPUT — we are listening, not talking. That one word tells Arduino: “don’t send power out of pin 7, just measure what’s on it.”

digitalRead(7)

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digitalRead is the mirror of digitalWrite. Where digitalWrite puts HIGH or LOW on a pin, digitalRead checks if the pin is currently HIGH or LOW. It gives you back one of those two words.

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digitalRead(7)   // → HIGH  (if button pressed)
digitalRead(7)   // → LOW   (if button not pressed)

if and else — making decisions

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This is the big new idea of Day 3. Until now, our code has just run straight through, top to bottom, no choices. Today we make the Arduino decide.

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if (digitalRead(7) == HIGH) {
  // do this
} else {
  // do that instead
}

In English: “IF the button is being pressed, turn the LED on. OTHERWISE, turn it off.”

Break it down:

• if (...) — “IF this thing in the parentheses is true…”
• == — this is how you ask “are these two things equal?” Note: two equal signs. One equal sign (=) means “copy this value into this variable” (we’ll see that on Day 4). Two equal signs (==) mean “compare these — are they the same?” If you ever mix them up, the code will not do what you expect.
• HIGH — we are comparing the result of digitalRead(7) to the word HIGH.
• { ... } — the curly braces hold the instructions that run if the condition is true.
• else { ... } — a second set of braces with instructions that run if the condition is false.

Only one of the two blocks runs each time the loop goes around. Never both.

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if (condition) {
  // runs ONLY if condition is true
} else {
  // runs ONLY if condition is false
}

This is the most important pattern in programming. Pretty much every app, game, and website in the world is built on top of if statements making decisions millions of times a second.

What is INPUT vs OUTPUT, really?

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You pick the mode once, in setup(), with pinMode. You can’t listen and talk on the same pin at the same time.

Try this

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1. Flip the logic. Change == HIGH to == LOW. Now the LED is on when the button is NOT pressed. Weird, but educational.
2. Pick a different pin for the button — say pin 6. You need to: move the jumper wire from pin 7 to pin 6, and change pinMode(7, INPUT) to pinMode(6, INPUT) and digitalRead(7) to digitalRead(6). Three edits, one move.
3. Two LEDs, one button. Add a second LED on pin 9 (resistor + wire, just like Day 2). Make the button switch which LED is on: when pressed, LED 1 off, LED 2 on; when not pressed, LED 1 on, LED 2 off.

What you learned today

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• What voltage is (a push), and why HIGH = 5V, LOW = 0V
• How a push button works (just two pieces of metal that touch when you press)
• Why a pull-down resistor is needed (to keep the pin from floating)
• pinMode(pin, INPUT) — tell Arduino to listen instead of talk
• digitalRead(pin) — check whether a pin is HIGH or LOW
• if / else — the most important decision-making tool in all of programming
• == (two equal signs) — “are these equal?”

What is next

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Day 4 — we introduce variables (little labeled boxes that hold numbers) and for loops (tell the Arduino to do something N times without typing the same line N times). It is the trick that makes your code smaller and smarter.

See you tomorrow, Anish.