In Chapter 3, you created a project that turned on and off an LED every time a button was pushed. In that project, you used the blue LED and the pushbutton on the Netduino itself. Now that you’ve learned about expansion shields, breadboards, and components in Chapter 4, it’s time to use them to connect an external LED and external button to your Netduino. You can also use this exact same code to work with external components.

The original code looked like this:

public static void Main()
{
    // write your code here
    OutputPort led = new OutputPort(Pins.ONBOARD_LED, false);
    InputPort button = 
      new InputPort(Pins.ONBOARD_SW1, false, Port.ResistorMode.Disabled);

    bool buttonState = false;

    while (true)
    {
        buttonState = button.Read();
        led.Write(buttonState);
    }
}

To work with the external LED and pushbutton, you need to change a few lines of code (changes are shown in bold). Open the project again in Visual Studio, then change the line of code that sets up the LED:

OutputPort led = new OutputPort(Pins.GPIO_PIN_D0, false);

Next, change the lines of code that sets up the pushbutton:

InputPort button = 
  new InputPort(Pins.GPIO_PIN_D1, false, Port.ResistorMode.PullUp);

You changed the LED to pin “digital 0” and changed the pushbutton to pin “digital 1.” That’s fairly straightforward. But why did I have you change the ResistorMode for the pushbutton?

Pushbuttons can be wired many different ways. In one configuration, they drive the voltage high when pushed. In another popular configuration (used on the MakerShield), they drive the voltage low when pushed. Since a digital input is neither high nor low by default, you need to provide a default of high so that the MakerShield’s button can drive the signal low when pushed.

By specifying ResistorMode.PullUp, you instruct the Netduino’s microcontroller to put a tiny bit current on the pin, which makes it default to high; when the pushbutton is pressed, it overrides this signal with a strong connection to ground (0V).

Since the pushbutton will give you a reading of false (0V) when it’s pressed, you need to reverse the logic in your code so that the LED lights up when the button is pressed and not vice versa. To do this, you must modify one more line of code, which you’ll find inside the while loop:

buttonState = !button.Read();

The difference in code may seem insignificant and is easy to miss. I had you add an exclamation mark directly in front of the word button. In C# code, an exclamation mark means “not” or “the opposite of” a true/false (Boolean) value. If button.Read() returns false (because it is pushed), you want to invert that so that the button state reads true (indicating that it was pushed).

The final thing you need to do is wire up the pushbutton and LED. To do this, connect a short piece of wire between digital header pin D0 on the MakerShield and the header hole labeled LED1 or LED2 (either one is fine). Also, connect a short piece of wire between digital header pin D1 on the MakerShield and the header hole labeled BTN1. Figure 5-2 shows how to hook things up.

Figure 5-2. MakerShield’s button and LED wired up

Figure 5-3. Wiring the button and LED up on a breadboard

Now run your project as you did in Chapter 3. Press the Start Debugging button in the toolbar at the top of the screen (or press F5).

When you press the pushbutton on your MakerShield, the LED will turn on; release the pushbutton and it will turn off.