Buttons¶

The micro:bit has two buttons: labelled A and B.

You can use the buttons to get input from the user. Perhaps you’d like to start or stop your program with a button press or maybe you’d like to know how many times each button has been pressed.

Basic Functions¶

Checking whether a button is pressed¶

Sometimes we just want a program to wait until something happens, for example: we could ask the micro:bit to wait until, say, button A is pressed and then print a message. We could do that like this

from microbit import *

while True:
    if button_a.is_pressed():
        display.scroll("A")

Let’s break this up into parts. The first bit:

while True:

This is a standard way to tell the micro:bit to repeat whatever code follows it forever and ever and ever ….. you get the idea. The next line looks similar to normal English

while True:
    if button_a.is_pressed():
        display.scroll("A")

This means, if button A is pressed then display an A on the LED screen.

Of course, we usually want to do something a bit more complicated than that. There is a way to check two things, we can use an if with an else like this

while True:
    if button_a.is_pressed():
        display.scroll("A")
    else:
        display.scroll(Image.ASLEEP)

This means, if button A is pressed then display an A on the LED screen, otherwise, display Image.ASLEEP.

Counting the number of presses¶

Sometimes you might want to count the number of button presses in a time period. You can do this using the get_presses() method. Here is an example

from microbit import *

    while True:
        sleep(3000)
        count = button_a.get_presses()
        display.scroll(str(count))

The micro:bit will sleep for 3 seconds and then wake up and check how many times button A was pressed. The number of presses is stored in a variable called count. We can’t print numbers directly on the LED screen so we convert count to a string and then display it. Can you think of another way to do this? (Hint: check whether the button has been pressed and add 1 to a counter if it has).

Advanced Functions¶

Checking for both buttons¶

It is possible to check a series of events by using if, elif and else. Say you wanted to check whether button A was pressed or button B was pressed or whether both buttons were pressed at the same time. We could do that like so

from microbit import *

while True:
    if button_a.is_pressed() and button_b.is_pressed():
        display.scroll("AB")
        break
    elif button_a.is_pressed():
        display.scroll("A")
    elif button_b.is_pressed():
        display.scroll("B")
    sleep(500)

Note

The keyword elif just means else if. You can use the longer form else if if you want.

The code above displays the letter corresponding to the button. If both buttons are pressed at the same time it displays AB. It is important to add a sleep function at the end of the loop to properly detect if both a and b is pressed.

Has the button been pressed?¶

The problem with using is_pressed() is that unless you are pressing the button at that precise moment then you won’t detect whether the button was ever pressed or not.

The was_pressed() function is useful is you want to write code that occasionally checks whether the button has been pushed but then goes on to do something else. While the code is doing the something else, it might be the case that the user pushes the button and lets it go and, because you haven’t checked to see if it’s pressed then, you might miss it. Well, the following function will tell you whether a button has been pushed or released since you last called that function - while your code is doing something else. In this way you need never miss a button press again

from microbit import *

while True:
    if button_a.was_pressed():
        display.scroll("A")
    else:
        display.scroll(Image.ASLEEP)
    sleep(1000)

What you’ll see is that the display will show an A for a second if you press the button, and then Image.ASLEEP is displayed. If you press the button while the program is delaying, then the A won’t show up immediately, but they will show up when it next tests to see if the button has been pressed. You’ll see this more clearly if you make the delay bigger.

Now try using button_a.isPressed() instead of button_a.was_pressed(). What you will find is that if you press the button while the code is delaying the micro:bit will never realise that you pressed it at all.

Checking if pin0, pin1 and pin2 has been touched¶

You can also use the pins 0,1 and 2 at the bottom edge of the micro:bit as buttons by touching the 0,1 and 2 pins and GND at the same time. 0,1 and 2 pins correspond to pin0, pin1 and pin2 when called on the program.

from microbit import *

while True:
        if pin0.is_touched():
                display.show(Image.HAPPY)
        else:
                displa.show(Image.SAD)

In this example, the command pin0.is_touched() detects if pin0 and GND was touched at the same time and will display a happy emoji if so.

Exercises¶

Make a simple calculator that will add 2 numbers:

Display a welcome image.  Use the built-in images or you can create one yourself
When button a is pressed, it will increment

2. Create a voting machine where you can select between 2 foods by a press of a button (button a for ham and button b for eggs, for example). To display the current tally, press both a and b at the same time.