My project is progressing quite well, but I am having some issues reading button presses through my MUX.

The Goal

Cycle through twelve buttons and twelve LEDs. When a button is pressed, light up the corresponding LED. I have one MUX for the LEDs which is working great thanks to the stack exchange community. My button MUX is the issue.

The Symptom

I have one button hooked up to the MUX. When it is not pressed, nothing lights up - that's right. But when it is pressed, every LED lights up as it is cycled through. It is as if pressing this button for one of the MUX pins is sending a HIGH signal to all of the MUX pins.

The Pictures


^^ You can see the big honking button in the bottom right. It is connected to 5V and then the MUX pin.


^^ I have two multiplexers. The one on the left is for the LEDs and the one on the right is for the buttons (of which I currently [ha!] have only one).




The Code

// set vars for all the pins
int led_input = 0;
int led_ctrl_1 = 1;
int led_ctrl_2 = 2;
int led_ctrl_3 = 3;
int led_ctrl_4 = 4;

int btn_input = 5;
int btn_ctrl_1 = 6;
int btn_ctrl_2 = 7;
int btn_ctrl_3 = 8;
int btn_ctrl_4 = 9;

// how many MUX pins?
int players = 12;

// function to take player number and convert to control pins
void ledWrite(int led, boolean val){


        digitalWrite(led_input, LOW);


        if((led - 8) >= 0){
            led = led - 8;
            digitalWrite(led_ctrl_4, HIGH);
            digitalWrite(led_ctrl_4, LOW);

        if((led - 4) >= 0){
            led = led - 4;
            digitalWrite(led_ctrl_3, HIGH);
            digitalWrite(led_ctrl_3, LOW);

        if((led - 2) >= 0){
            led = led - 2;
            digitalWrite(led_ctrl_2, HIGH);
            digitalWrite(led_ctrl_2, LOW);

        if((led - 1) >= 0){
            led = led - 1;
            digitalWrite(led_ctrl_1, HIGH);
            digitalWrite(led_ctrl_1, LOW);

        digitalWrite(led_input, HIGH);

// function to take player number and convert to control pins
int btnRead(int btn){

    if((btn - 8) >= 0){
        btn = btn - 8;
        digitalWrite(btn_ctrl_4, HIGH);
        digitalWrite(btn_ctrl_4, LOW);

    if((btn - 4) >= 0){
        btn = btn - 4;
        digitalWrite(btn_ctrl_3, HIGH);
        digitalWrite(btn_ctrl_3, LOW);

    if((btn - 2) >= 0){
        btn = btn - 2;
        digitalWrite(btn_ctrl_2, HIGH);
        digitalWrite(btn_ctrl_2, LOW);

    if((btn - 1) >= 0){
        btn = btn - 1;
        digitalWrite(btn_ctrl_1, HIGH);
        digitalWrite(btn_ctrl_1, LOW);

    return digitalRead(btn_input);

void setup(){

    pinMode(led_input, OUTPUT);  
    pinMode(led_ctrl_1, OUTPUT);  
    pinMode(led_ctrl_2, OUTPUT);  
    pinMode(led_ctrl_3, OUTPUT);  
    pinMode(led_ctrl_4, OUTPUT);  
    pinMode(btn_input, INPUT);  
    pinMode(btn_ctrl_1, OUTPUT);  
    pinMode(btn_ctrl_2, OUTPUT);  
    pinMode(btn_ctrl_3, OUTPUT);  
    pinMode(btn_ctrl_4, OUTPUT);  

void loop(){

    for(int x = 1; x <= players; x++){

        if(btnRead(x) == HIGH){

            ledWrite(x, true);
            ledWrite(x, false);

I know this is a lot to trudge through, and I honestly appreciate any help you are willing to offer. Thanks in advance!


1 Answer 1


All your unused MUX input pins for the button MUX are floating. You will need to tie all of the unused inputs (those that do not have a physical button attached) to ground. Floating inputs are undefined and cause undefined output.

Also, when your attached button is not pressed, its input will also be floating. You can avoid this by connecting the input pin on the MUX to ground using a resistor. That way, when the button is not pressed the input will be low (through the resistor), and when the button is pressed it will be high (through the button connection).

  • Also, floating inputs will generally increase current consumption of CMOS chips.
    – jfpoilpret
    Commented Jun 25, 2014 at 5:05
  • Ah ha! The Digital Read Serial and Input Pullup Serial tutorials make more sense now. If only there was an INPUT_PULLDOWN! I should never assume that a lack of a definite signal means LOW.
    – Rip Leeb
    Commented Jun 25, 2014 at 17:10

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