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How can you sense a closed circuit? I want to have 4 pins with disconnected wires and when touched with a fifth wire play a specific note on a piezo buzzer until disconnected again.

This project loops through several keys and checks whether they are being touched using the Capactive Sensor library. I want to do something similar, but not with capacitive sensing. Alternatively, it may work to have the buzzer always trying to play a note, but only sounds when one of the 4 circuits is close. I'm just not sure how that would work in the code.

Here is their code that uses capactive sensing:

#include <CapacitiveSensor.h>
#include "pitches.h"

#define COMMON_PIN      2    // The common 'send' pin for all keys
#define BUZZER_PIN      A4   // The output pin for the piezo buzzer
#define NUM_OF_SAMPLES  10   // Higher number whens more delay but more consistent readings
#define CAP_THRESHOLD   150  // Capactive reading that triggers a note (adjust to fit your needs)
#define NUM_OF_KEYS     8    // Number of keys that are on the keyboard

// This macro creates a capacitance "key" sensor object for each key on the piano keyboard:
#define CS(Y) CapacitiveSensor(2, Y)

// Each key corresponds to a note, which are defined here. Uncomment the scale that you want to use:
int notes[]={NOTE_C4,NOTE_D4,NOTE_E4,NOTE_F4,NOTE_G4,NOTE_A4,NOTE_B4,NOTE_C5}; // C-Major scale

// Defines the pins that the keys are connected to:
CapacitiveSensor keys[] = {CS(3), CS(4), CS(5), CS(6), CS(7), CS(8), CS(9), CS(10)};

void setup() { 
  // Turn off autocalibrate on all channels:
  for(int i=0; i<8; ++i) {
    keys[i].set_CS_AutocaL_Millis(0xFFFFFFFF);
  }
  // Set the buzzer as an output:
  pinMode(BUZZER_PIN, OUTPUT); 
}

void loop() {    
  // Loop through each key:
  for (int i = 0; i < 8; ++i) {
    // If the capacitance reading is greater than the threshold, play a note:
    if(keys[i].capacitiveSensor(NUM_OF_SAMPLES) > CAP_THRESHOLD) {
      tone(BUZZER_PIN, notes[i]); // Plays the note corresponding to the key pressed
    }
  }
}
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Connect your fifth wire to ground. Set the four inputs to INPUT_PULLUP. The four inputs will default HIGH, and be pulled LOW when connected to the ground wire.

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  • Inversely, you can also set the inputs to INPUT_PULLDOWN and have the fifth wire connected to +5v, which will pull the pin HIGH when connected, and LOW when not.
    – Matt Clark
    Oct 22 '14 at 15:59
  • 1
    Assuming you're not using a AVR of course, since those only have pullups. Oct 22 '14 at 16:33

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