🧩 Both pins 3 and 8 are internally the same – you can tie one or both to GND.

Wiring to Arduino Example

// Pin mapping
int a = 7;
int b = 6;
int c = 4;
int d = 2;
int e = 1;
int f = 9;
int g = 10;
int dp = 5;
int common1 = 3;
int common2 = 8;

void setup() {
pinMode(a, OUTPUT); pinMode(b, OUTPUT); pinMode(c, OUTPUT);
pinMode(d, OUTPUT); pinMode(e, OUTPUT); pinMode(f, OUTPUT);
pinMode(g, OUTPUT); pinMode(dp, OUTPUT);
pinMode(common1, OUTPUT); pinMode(common2, OUTPUT);

digitalWrite(common1, LOW); // Common Cathode → GND
digitalWrite(common2, LOW);
}

void loop() {
// Display “8” – all segments ON
digitalWrite(a, HIGH);
digitalWrite(b, HIGH);
digitalWrite(c, HIGH);
digitalWrite(d, HIGH);
digitalWrite(e, HIGH);
digitalWrite(f, HIGH);
digitalWrite(g, HIGH);
digitalWrite(dp, LOW); // decimal point OFF
}

🧠 Add a 220 Ω – 470 Ω resistor in series with each segment line to limit current.