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93 lines
3.1 KiB
93 lines
3.1 KiB
10 years ago
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The accelerometer
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=================
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Here you will learn how to read the accelerometer and signal using LEDs states like tilt left and tilt right.
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Using the accelerometer
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-----------------------
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The pyboard has an accelerometer (a tiny mass on a tiny spring) that can be used
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to detect the angle of the board and motion. There is a different sensor for
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each of the x, y, z directions. To get the value of the accelerometer, create a
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pyb.Accel() object and then call the x() method. ::
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>>> accel = pyb.Accel()
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>>> accel.x()
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7
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This returns a signed integer with a value between around -30 and 30. Note that
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the measurement is very noisy, this means that even if you keep the board
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perfectly still there will be some variation in the number that you measure.
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Because of this, you shouldn't use the exact value of the x() method but see if
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it is in a certain range.
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We will start by using the accelerometer to turn on a light if it is not flat. ::
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accel = pyb.Accel()
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light = pyb.LED(3)
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SENSITIVITY = 3
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while True:
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x = accel.x()
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if abs(x) > SENSITIVITY:
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light.on()
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else:
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light.off()
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pyb.delay(100)
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We create Accel and LED objects, then get the value of the x direction of the
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accelerometer. If the magnitude of x is bigger than a certain value ``SENSITIVITY``,
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then the LED turns on, otherwise it turns off. The loop has a small ``pyb.delay()``
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otherwise the LED flashes annoyingly when the value of x is close to
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``SENSITIVITY``. Try running this on the pyboard and tilt the board left and right
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to make the LED turn on and off.
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**Exercise: Change the above script so that the blue LED gets brighter the more
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you tilt the pyboard. HINT: You will need to rescale the values, intensity goes
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from 0-255.**
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Making a spirit level
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---------------------
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The example above is only sensitive to the angle in the x direction but if we
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use the ``y()`` value and more LEDs we can turn the pyboard into a spirit level. ::
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xlights = (pyb.LED(2), pyb.LED(3))
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ylights = (pyb.LED(1), pyb.LED(4))
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accel = pyb.Accel()
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SENSITIVITY = 3
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while True:
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x = accel.x()
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if x > SENSITIVITY:
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xlights[0].on()
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xlights[1].off()
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elif x < -SENSITIVITY:
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xlights[1].on()
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xlights[0].off()
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else:
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xlights[0].off()
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xlights[1].off()
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y = accel.y()
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if y > SENSITIVITY:
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ylights[0].on()
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ylights[1].off()
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elif y < -SENSITIVITY:
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ylights[1].on()
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ylights[0].off()
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else:
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ylights[0].off()
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ylights[1].off()
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pyb.delay(100)
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We start by creating a tuple of LED objects for the x and y directions. Tuples
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are immutable objects in python which means they can't be modified once they are
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created. We then proceed as before but turn on a different LED for positive and
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negative x values. We then do the same for the y direction. This isn't
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particularly sophisticated but it does the job. Run this on your pyboard and you
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should see different LEDs turning on depending on how you tilt the board.
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