PushToTalk.ino

const int ledPin =  13;      // Pin 13: Teensy 3.0 has the LED on pin 13
const int buttonPin = 2;     // Attach a N.O. momentary pushbutton that connects to ground
const int BUTTON_UP = 0;
const int BUTTON_DOWN = 1;

int buttonState = BUTTON_UP;
elapsedMillis sincePress;
void setButtonState(int newButtonState) {
  if (newButtonState != buttonState && sincePress > 100) {
    if (newButtonState == BUTTON_UP) {
      Keyboard.set_modifier(0);
    } else {
      Keyboard.set_modifier(MODIFIERKEY_CTRL | MODIFIERKEY_ALT | MODIFIERKEY_GUI);
    }
    Keyboard.send_now();
    buttonState = newButtonState;
    sincePress = 0;
  }
}

void setup() {
  pinMode(ledPin, OUTPUT);      
  pinMode(buttonPin, INPUT_PULLUP);
}

void loop()
{
  if (digitalRead(buttonPin)) {
    // buttonPin is high due to pullup resistor
    setButtonState(BUTTON_UP);
  } else {
    // buttonPin is low due to pushbutton pressed
    setButtonState(BUTTON_DOWN);
  }
}