Nixie tube with Arduino || Simplest design with Opto

1const int nixie_0 = 2;
2const int nixie_1 = 3;
3const int nixie_2  = 4;
4const int nixie_3 = 5;  
5const int nixie_4 = 6;
6const int nixie_5  = 7;
7const int nixie_6 = 8;
8const int nixie_7 = 9;
9const int nixie_8 =  10;
10const int nixie_9 = 11;
11
12const int row_1 = 0;
13const int row_2 =  1;
14
15const int time_on = 3;
16
17void setup() {
18  pinMode(nixie_0, OUTPUT);
19  pinMode(nixie_1, OUTPUT);
20  pinMode(nixie_2, OUTPUT);
21  pinMode(nixie_3,  OUTPUT);
22  pinMode(nixie_4, OUTPUT);
23  pinMode(nixie_5, OUTPUT);
24  pinMode(nixie_6,  OUTPUT);
25  pinMode(nixie_7, OUTPUT);
26  pinMode(nixie_8, OUTPUT);
27  pinMode(nixie_9,  OUTPUT);
28  pinMode(row_1, OUTPUT);
29  pinMode(row_2, OUTPUT);
30  //Serial.begin(9600);  //should NOT use seiral println, it will effect to output pin D0 & D1
31}
32
33void  loop() {
34  for (int i=0; i<=59; i++){
35    int j = i/10; //second number from  right
36    int k = i%10; //first number from right
37
38    for (int m=0; m<=50;  m++){ //this for loop is used for delay showing two numbers
39      //-----------show  first number
40      off_all();
41      on_number(1,k+2); 
42      delay(time_on);
43
44      //-----------show second number
45      off_all();
46      on_number(0,j+2);
47      delay(time_on);
48    }
49  }
50}
51
52void on_number(int row, int nixie){
53  //void off_all();
54  digitalWrite(row, HIGH);
55  digitalWrite(nixie, HIGH);
56}
57
58void  off_all(){
59  digitalWrite(nixie_0, LOW);
60  digitalWrite(nixie_1, LOW);
61  digitalWrite(nixie_2, LOW);
62  digitalWrite(nixie_3, LOW);
63  digitalWrite(nixie_4,  LOW);
64  digitalWrite(nixie_5, LOW);
65  digitalWrite(nixie_6, LOW);
66  digitalWrite(nixie_7,  LOW);
67  digitalWrite(nixie_8, LOW);
68  digitalWrite(nixie_9, LOW);
69  digitalWrite(row_1,  LOW);
70  digitalWrite(row_2, LOW);
71  delayMicroseconds(420);  //to prevent  "ghost" effect to other tube
72}-------------------------------------------------------------------