/*
   headset_NPN
   Version 2.0
   Use Arduino to simulate cellphone headset clicks via NPN transistor
   
   The use of Arduino pin D12 is a little tricky.  
   We set it to "input" mode with digitalWrite LOW to make the pin high-resistance, essentially taking it out of the circuit.  
   To drive the transistor gate pin, we leave D12 in "input" mode but connect the internal pull-up resistor with digitalWrite HIGH.
 */
 
#define LEDPIN 13
#define NPNPIN   9
#define openPin(pin)    digitalWrite(pin, LOW)
#define pullupPin(pin)  digitalWrite(pin, HIGH)

void setup() {
  pinMode(NPNPIN, INPUT);  // Set NPNPIN to high resistance input
  pinMode(LEDPIN, OUTPUT);
}

void loop() {
  delay(1500);
  click(40);
}

void click(int length) {
  digitalWrite(LEDPIN, HIGH);
  pullupPin(NPNPIN);  // Connect pull-up resistor; allows "input" pin to act as weak output to drive transistor
  delay(length);
  openPin(NPNPIN);       // Back to high resistance
  digitalWrite(LEDPIN, LOW);
}