int cols[5] = {9, 10, 11, 12, 13}; //pins of the coloms int rows[5] = {4, 5, 6, 7, 8}; //pins of the rows void setup() { for (int i = 4; i < 14; i++){ //declare pins as outputs pinMode(i, OUTPUT); } for (int i = 0; i < 5; i++){ digitalWrite (cols[i], HIGH); //set all colom-pins to HIGH digitalWrite (rows[i], LOW); //set all row-pins to LOW } } void loop() { //random leds are blinking byte randomCol = random(0,5); digitalWrite (cols[randomCol], LOW); byte randomRow = random(0,5); digitalWrite (rows[randomRow], HIGH); delay(5); //reset of the selected pins digitalWrite (cols[randomCol], HIGH); digitalWrite (rows[randomRow], LOW); delay(2); }