This is only an example/test code that I used to see if the i/o expander would still work with the light sensors (which it did).
If anyone needs to make up their own, they may remove all sub-functions and whatever is in the loop function to get it to work.
All you have to do to go to a certain address of the i/o expander is to digitalWrite Apin through Epin to the values of A through E.
(see the beginning of the reads sub-function).

For this i/o expander, we used 4 4051's--three were attached to the first three address lines of the first.

Here is the [[http://www.jaycar.com.au/images_uploaded/CD4051BC.PDF|datasheet]]

*Look at the bottom of the page for an update to the convert subfunction.


  //This is i/o expander has 18 possible outputs--possible 32 if convert is edited
  //it's loop will only go through the lowest 6 addresses and will read the light sensor at each, one at a time
  
  
  int reads(int led);//read led
  int readsavg(int n, int led);//takes average of n readings for led
  int convert(int i);
  
  int anodePin=4;//all led's use this pin for their long leg
  
  int Apin=14;//A0
  int Bpin=15;//A1
  int Cpin=16;//A2
  int Dpin=17;//A3
  int Epin=18;//A4
  int cathodePin0=19;//A5
  
  int A;
  int B;
  int C;
  int D;
  int E;
  
  void setup(){
    _SFR_IO8(0x35) |= 4;
    _SFR_IO8(0x35) |= (1<<4);
    
    pinMode(anodePin, OUTPUT);  //set pins associated with led light sensors to outputs
    pinMode(Apin, OUTPUT);
    pinMode(Bpin, OUTPUT);
    pinMode(Cpin, OUTPUT);
    pinMode(Dpin, OUTPUT);
    pinMode(Epin, OUTPUT);
    pinMode(cathodePin0, OUTPUT);
    
    
    digitalWrite(anodePin, HIGH);
    digitalWrite(cathodePin0,LOW);
    
    digitalWrite(Apin,LOW);
    digitalWrite(Bpin,LOW);
    digitalWrite(Cpin,LOW);
    digitalWrite(Dpin,LOW);
    digitalWrite(Epin,LOW);
    
    Serial.begin(9600);
  }
  void loop(){
    int n=3;
    for(int j=0; j<6; j++){
      int i=readsavg(n,j);
      Serial.print("j is");
      Serial.println(j);
      Serial.print("i is");
      Serial.println(i); 
      delay(1000);
    }
  }
  int reads(int led){
    led=led;
    int val = 0;
    int vcc=anodePin;
    convert(led);//get address of led
    //set multiplexer's to read that address
    digitalWrite(Apin, A);
    digitalWrite(Bpin, B);
    digitalWrite(Cpin, C);
    digitalWrite(Dpin, D);
    digitalWrite(Epin, E);
    
    
    digitalWrite(vcc,   HIGH);
    digitalWrite(cathodePin0, LOW);
    
   /* Serial.print(A);
    Serial.print(B);
    Serial.print(C);
    Serial.print(D);
    Serial.println(E);*/
    
    //already emitting light
    
    delay(50);
    
    //switch potentials -- charge LED to -5V
    digitalWrite(vcc,   LOW);
    digitalWrite(cathodePin0, HIGH);
    
    //measure time for potential to equalize (for cathode to be LOW)
    
    //switch pinmode 
    pinMode(cathodePin0,  INPUT);
    
    //measure time it takes for cathodePin to go to zero
    //this value probably depends on the chip clock or something
    while((digitalRead(cathodePin0) != 0)&&(val<100)){
      delay(1);
      val++;      
    }
    pinMode(cathodePin0,  OUTPUT);
    
    digitalWrite(vcc,   HIGH);
    digitalWrite(cathodePin0, LOW);
    //Serial.println(val, DEC);
    return val;
  }
  
  int readsavg(int n, int led){
    int val=0;
    for(int i=0; i<n; i++){
      val=val+reads(led);//read n times and sum
    }
    val=val/n;//divide by n for average
    return val;
  }
  int convert(int i){
    //converts a decimal number from 0 to 17, inclusively, to binary
    A=i/16;
    if(A==1){
      i=i-16;
      B=0;
      C=0;
      D=0;
      E=i;
    }
    if(A==0){
      B=i/8;
      if(B==1){
        i=i-8;
        C=i/4;
        if(C==1){
          i=i-4;
          D=i/2;
          if(D==1){
            i=i-2;
            E=i;
          }
          if(D==0){
            E=i;
          }
        }
        if(C==0){
          D=i/2;
          if(D==1){
            i=i-2;
            E=i;
          }
          if(D==0){
            E=i;
          }
        }
      }
      if(B==0){
        C=i/4;
        if(C==1){
          i=i-4;
          D=i/2;
          if(D==1){
            i=i-2;
            E=i;
          }
          if(D==0){
            E=i;
          }
        }
        if(C==0){
          D=i/2;
          if(D==1){
            i=i-2;
            E=i;
          }
          if(D==0){
            E=i;
          }
        }
      }
    }
  }
  
**This is an update to the convert subfunction:  I realized that the above chain of if statements was stupid and that there is a much better way to do it if you just put a bit more thought into it.  So here it is:
  void convert(int i){
    //This function handles numbers from 0 to 31
    A=i/16;
    i=i-(A*16);
    B=i/8;
    i=i-(B*8);
    C=i/4;
    i=i-(C*4);
    D=i/2;
    i=(D*2);
    E=i;
  }
  
  //so much nicer right
  
Here is the promised circuits diagram:

[[http://www.jaycar.com.au/images_uploaded/CD4051BC.PDF|{{:classes:designandproto:me155a_groups:demux.jpg?1500x1200|}}]]