Mission 22 · Stage 3

Digital Counter

display a counter value

Count 0-9 on a common-cathode 7-segment display controlled via 7 digital pins.

Pin connections

Part 1		Part 2
Arduino pin 2	→	7-segment display seg A
Arduino pin 3	→	7-segment display seg B
Arduino pin 4	→	7-segment display seg C
Arduino pin 5	→	7-segment display seg D
Arduino pin 6	→	7-segment display seg E
Arduino pin 7	→	7-segment display seg F
Arduino pin 8	→	7-segment display seg G
7-segment display COM	→	Arduino GND
7-segment display COM	→	Arduino GND

See it

Light up digits 0–9!

A DIGITS[] table tells each segment ON or OFF — the display counts like a mini scoreboard.

Microwave timers, elevator floors, and digital clocks use 7-segment displays.

The story

The problem

Showing a number needs seven tiny lights controlled together — not one LED.

Think of it like

Like drawing numbers with seven stick pieces — each digit has its own pattern.

Meet the parts

Runs your sketch

Arduino

Brain

Loading part…

Shows digits 0 through 9

7-segment display

Display

Loading part…

How it works

Prepare segment pins

Pins 2–8 become outputs — one wire per segment A through G.

pinMode(SEG_A + i, OUTPUT);

Loop through digits

The for loop tries 0, then 1, then 2… up to 9 — a counter variable built into the loop.

for (int digit = 0; digit <= 9; digit++)

Light the pattern

DIGITS[digit] holds a bitmask; showDigit() turns each segment ON or OFF.

showDigit(digit);

Hold and repeat

Each digit stays visible for 800 ms, then the for loop shows the next one.

delay(800);

Then loop back to step 2

Build the circuit

Follow these steps in order. Match the wires to the colors shown.

1
Place Arduino
Place the Arduino (uno) on the breadboard.
Arduino placed!
Loading part…
2
Place 7-segment display
Place the 7-segment display (seg1) on the breadboard.
Loading part…
3
Connect Arduino pin 2 to 7-segment display (seg1) anode (+)
Connect Arduino pin 2 to 7-segment display (seg1) anode (+).
4
Connect Arduino pin 3 to 7-segment display (seg1) B
Connect Arduino pin 3 to 7-segment display (seg1) B.
5
Connect Arduino pin 4 to 7-segment display (seg1) cathode (-)
Connect Arduino pin 4 to 7-segment display (seg1) cathode (-).
6
Connect Arduino pin 5 to 7-segment display (seg1) D
Connect Arduino pin 5 to 7-segment display (seg1) D.
7
Connect Arduino pin 6 to 7-segment display (seg1) E
Connect Arduino pin 6 to 7-segment display (seg1) E.
8
Connect Arduino pin 7 to 7-segment display (seg1) F
Connect Arduino pin 7 to 7-segment display (seg1) F.
9
Connect Arduino pin 8 to 7-segment display (seg1) G
Connect Arduino pin 8 to 7-segment display (seg1) G.
10
Connect 7-segment display (seg1) COM.2 to Arduino GND
Connect 7-segment display (seg1) COM.2 to Arduino GND.
11
Connect 7-segment display (seg1) COM.1 to Arduino GND
Connect 7-segment display (seg1) COM.1 to Arduino GND.

Try it

Press Run — watch the display count 0, 1, 2… 9.
Serial Monitor prints each digit too!

Peek at code

Segment patterns table

const uint8_t DIGITS[10] = {
  0b0111111,
  0b0000110,
  0b1011011,
  0b1001111,
  0b1100110,
  0b1101101,
  0b1111101,
  0b0000111,
  0b1111111,
  0b1101111,
};

DIGITS[] stores which segments glow for 0–9 — binary patterns like 0b0111111 for zero.

showDigit() helper

void showDigit(int digit) {
  uint8_t pattern = DIGITS[digit];
  for (int seg = 0; seg < 7; seg++) {
    digitalWrite(SEG_A + seg, (pattern >> seg) & 1);
  }
}

Loops seven segments and uses bit math to set each pin HIGH or LOW.

Count 0 to 9

void loop() {
  for (int digit = 0; digit <= 9; digit++) {
    showDigit(digit);
    Serial.println(digit);
    delay(800);
  }
}

loop() runs the for loop forever — display cycles through every digit.

Show full sketch (7segment.ino)

const int SEG_A = 2;
const uint8_t DIGITS[10] = {
  0b0111111,
  0b0000110,
  0b1011011,
  0b1001111,
  0b1100110,
  0b1101101,
  0b1111101,
  0b0000111,
  0b1111111,
  0b1101111,
};
void showDigit(int digit) {
  uint8_t pattern = DIGITS[digit];
  for (int seg = 0; seg < 7; seg++) {
    digitalWrite(SEG_A + seg, (pattern >> seg) & 1);
  }
}
void setup() {
  for (int i = 0; i < 7; i++) {
    pinMode(SEG_A + i, OUTPUT);
    digitalWrite(SEG_A + i, LOW);
  }
  Serial.begin(9600);
  Serial.println("7-Segment Counter Demo");
}
void loop() {
  for (int digit = 0; digit <= 9; digit++) {
    showDigit(digit);
    Serial.println(digit);
    delay(800);
  }
}

Quick quiz

Q1. Where does repeating work belong?

A. loop() ✓
B. setup()
C. pinMode only

Why: Correct—loop() runs again and again.

Q2. What does the for loop variable digit do?

A. Counts 0 through 9 to pick which number to show ✓
B. Turns off Serial Monitor
C. Only runs in setup()

Why: Correct — digit is the counter that picks which pattern to display.

Code lab — try on your own

Show each digit longer — change delay(800) to delay(1200) in loop().
Hint: Line 32 inside the for loop.
Add a comment on the pattern line explaining it comes from DIGITS[].
Hint: Line 15 inside showDigit().

Code walkthrough

A line-by-line tour of the sketch — the same steps as in Robo Gurukul Studio.

Program overview

Technical

Sketches have globals, then setup() once, then loop() forever.

In this project

Count 0-9 on a common-cathode 7-segment display controlled via 7 digital pins.

Why here

Read from top to bottom. Hover words or lines for help!

const int SEG_A = 2;
const uint8_t DIGITS[10] = {
  0b0111111,
  0b0000110,
  0b1011011,
  0b1001111,
  0b1100110,
  0b1101101,
  0b1111101,
  0b0000111,
  0b1111111,
  0b1101111,
};
void showDigit(int digit) {
  uint8_t pattern = DIGITS[digit];
  for (int seg = 0; seg < 7; seg++) {
    digitalWrite(SEG_A + seg, (pattern >> seg) & 1);
  }
}

setup()

Technical

Runs one time when the board turns on.

In this project

Sets up pins and libraries for 7-Segment Display.

Why here

One-time setup belongs here—not in loop().

void setup() {
  for (int i = 0; i < 7; i++) {
    pinMode(SEG_A + i, OUTPUT);
    digitalWrite(SEG_A + i, LOW);
  }
  Serial.begin(9600);
  Serial.println("7-Segment Counter Demo");
}

loop()

Technical

Runs again and again after setup() is done.

In this project

This is the main action you see in 7-Segment Display.

Why here

Repeating work (blink, read sensors) goes here.

void loop() {
  for (int digit = 0; digit <= 9; digit++) {
    showDigit(digit);
    Serial.println(digit);
    delay(800);
  }
}

Try this: Change numbers in loop(), then compile and run the simulator.

pinMode

Technical

Tells a pin if it listens or drives something.

In this project

Gets the 7-Segment Display circuit ready in the simulator.

Why here

Goes in setup() because we only set pins once at the start.

    pinMode(SEG_A + i, OUTPUT);

digitalWrite

Technical

Turns a pin ON or OFF.

In this project

Controls lights, motors, or buzzers in 7-Segment Display.

Why here

Goes in loop() so it can keep changing while the program runs.

    digitalWrite(SEG_A + seg, (pattern >> seg) & 1);

delay

Technical

Waits for some time. Nothing else runs during the wait.

In this project

Controls speed so you can see 7-Segment Display in the simulator.

Why here

Right after an action that should stay the same for a moment.

    delay(800);

begin

Technical

Starts talking to the computer screen (serial monitor).

In this project

Lets 7-Segment Display print debug messages.

Why here

Goes in setup() once before any Serial.print.

  Serial.begin(9600);

println

Technical

Sends text to the serial monitor and starts a new line.

In this project

Prints one line of output for 7-Segment Display.

Why here

In loop() when each reading should appear on its own line.

  Serial.println("7-Segment Counter Demo");

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