6. Movement

6.1. Accelerometer

Mobile phone knows which up to show the images on its screen because it uses an accelerometer. Game controllers also contain accelerometers to steer and move around in games.

The microbit accelerometer measures movement along three axes in milli-g’s.

When the reading is 0 the microbit is “level” along that particular axis.

tilting it in one direction will give a positive reading; in the opposite direction a negative reading is given.

accelerometer.get_x() * X - tilting the left edge down (-) and the right edge down (+). accelerometer.get_y() * Y - tilting the top edge down (-) and the bottom edge down (+). accelerometer.get_z() * Z - moving down (-) and moving up (+).

The code below scrolls the reading for tilting left or right. An * is shown in between readings to make it easier to tell them apart.

from microbit import *

while True:
    display.scroll(accelerometer.get_x(), delay=80)
    display.show('*')

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6.2. Spirit level

For example, here’s a very simple spirit-level that uses get_x to measure

how level the device is along the X axis. | A reading of 20 is small enough to make this very sensitive to a slight tilt.

from microbit import *

while True:
    x_reading = accelerometer.get_x()
    if x_reading > 20:
        display.show("R")
    elif x_reading < -20:
        display.show("L")
    else:
        display.show("-")

Hold the microbit flat it should display -.

Tilt it left and it’ll show L.

Tilt it right and it’ll show R.

There is also a get_y method for the Y axis and a get_z method for the Z axis.

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Tasks

1. Modify the code above to get an x_val, which is “R”, “-” or “L”, then scroll the x_val.

2. Modify the code above to display ‘T’ for tilting the top edge down and ‘B’ for tilting the bottom edge down.

3. Modify the code above to get an y_val, which is “T”, “-” or “B”, then scroll the y_val.

4. Combine the code using x-val and y_val to scroll all 9 possible tilts: ‘RT’, ‘R-’, ‘RB’, ‘LT’, ‘L-’, ‘LB’, ‘-T’, ‘–’, ‘-B’. Do this by first getting the x_val and y_val as before, then scrolling these together by concatenating them. Add a 250ms sleep to pause briefly before the next reading.

Q1

Modify the code above to get an x_val which is “R”, “-” or “L”, then scroll the x_val.

from microbit import *

while True:
    x_reading = accelerometer.get_x()
    if x_reading > 20:
        x_val = "R"
    elif x_reading < -20:
        x_val = "L"
    else:
        x_val = "-"
    display.scroll(x_val)

Q2

Modify the code above to display ‘T’ for tilting the top edge down and ‘B’ for tilting the bottom edge down.

from microbit import *

while True:
    y_reading = accelerometer.get_y()
    if y_reading > 20:
        display.show("B")
    elif y_reading < -20:
        display.show("T")
    else:
        display.show("-")

Q3

Modify the code above to get an y_val, which is “T”, “-” or “B”, then scroll the y_val.

from microbit import *

while True:
    y_reading = accelerometer.get_y()
    if y_reading > 20:
        y_val = "B"
    elif y_reading < -20:
        y_val = "T"
    else:
        y_val = "-"
    display.scroll(y_val)

Q4

Combine the code using x-val and y_val to scroll all 9 possible tilts: ‘RT’, ‘R-’, ‘RB’, ‘LT’, ‘L-’, ‘LB’, ‘-T’, ‘–’, ‘-B’. Do this by first getting the x_val and y_val as before, then scrolling these together by concatenating them. Add a 250ms sleep to pause briefly before the next reading.

from microbit import *

while True:
    x_reading = accelerometer.get_x()
    if x_reading > 20:
        x_val = "R"
    elif x_reading < -20:
        x_val = "L"
    else:
        x_val = "-"
    y_reading = accelerometer.get_y()
    if y_reading > 20:
        y_val = "B"
    elif y_reading < -20:
        y_val = "T"
    else:
        y_val = "-"
    display.scroll(x_val + y_val, delay=80)
    sleep(250)

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Tasks

1. Modify the code above to display and left arrow for tilting left and a right arrow for tilting right.

2. Modify the code above to display an up arrow for tilting the top down and a down arrow for tilting the bottom down.

3. Write code to indicate all 9 possible tilts with 8 different arrows instead of text: ↗, ↘, ↖, ↙, →, ←, ↑, ↓, ‘-’. Use nested if statements by nesting the y code within each branch of the x code. Add a 250ms sleep to pause briefly before the next reading.

Q1

Modify the code above to display a left arrow for tilting left and a right arrow for tilting right.

from microbit import *

while True:
    x_reading = accelerometer.get_x()
    if x_reading > 20:
        display.show(Image.ARROW_E)
    elif x_reading < -20:
        display.show(Image.ARROW_W)
    else:
        display.show("-")

Q2

Modify the code above to display an up arrow for tilting the top down and a down arrow for tilting the bottom down.

from microbit import *

while True:
    y_reading = accelerometer.get_y()
    if y_reading > 20:
        display.show(Image.ARROW_S)
    elif y_reading < -20:
        display.show(Image.ARROW_N)
    else:
        display.show("-")

Q3

Write code to indicate all 9 possible tilts with 8 different arrows instead of text: ↗, ↘, ↖, ↙, →, ←, ↑, ↓, ‘-’. Use nested if statements by nesting the y code within each branch of the x code. Add a 250ms sleep to pause briefly before the next reading.

from microbit import *

while True:
    x_reading = accelerometer.get_x()
    y_reading = accelerometer.get_y()
    if x_reading > 20:
        if y_reading > 20:
            display.show(Image.ARROW_SE)
        elif y_reading < -20:
            display.show(Image.ARROW_NE)
        else:
            display.show(Image.ARROW_E)
    elif x_reading < -20:
        if y_reading > 20:
            display.show(Image.ARROW_SW)
        elif y_reading < -20:
            display.show(Image.ARROW_NW)
        else:
            display.show(Image.ARROW_W)
    else:
        if y_reading > 20:
            display.show(Image.ARROW_S)
        elif y_reading < -20:
            display.show(Image.ARROW_N)
        else:
            display.show("-")
    sleep(200)

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Tasks

1. Combine the two tasks which displayed output for the 9 different tilts and use text if the A-button is pressed and use arrows if the B-button is pressed. Clear the screen if no button is pressed.

Q1

Combine the two tasks which displayed output for the 9 different tilts and use text if the A-button is pressed and use arrows if the B-button is pressed. Clear the screen if no button is pressed.

from microbit import *

while True:
    x_reading = accelerometer.get_x()
    y_reading = accelerometer.get_y()

    if button_a.is_pressed():
        if x_reading > 20:
            x_val = "R"
        elif x_reading < -20:
            x_val = "L"
        else:
            x_val = "-"
        if y_reading > 20:
            y_val = "B"
        elif y_reading < -20:
            y_val = "T"
        else:
            y_val = "-"
        display.scroll(x_val + y_val, delay=80)

    elif button_b.is_pressed():
        if x_reading > 20:
            if y_reading > 20:
                display.show(Image.ARROW_SE)
            elif y_reading < -20:
                display.show(Image.ARROW_NE)
            else:
                display.show(Image.ARROW_E)
        elif x_reading < -20:
            if y_reading > 20:
                display.show(Image.ARROW_SW)
            elif y_reading < -20:
                display.show(Image.ARROW_NW)
            else:
                display.show(Image.ARROW_W)
        else:
            if y_reading > 20:
                display.show(Image.ARROW_S)
            elif y_reading < -20:
                display.show(Image.ARROW_N)
            else:
                display.show("-")

    else:
        display.clear()

    sleep(250)