Getting Started with MicroPython and the SparkFun Inventor's Kit for micro:bit
LightningHawk Experiment 10: Using the Accelerometer
Introduction
In this experiment you will look at combining the use of the accelerometer on the micro:bit to measure the orientation of the micro:bit and using it to control the angle of a servo.
Ready to shake, rattle and roll?
Parts Needed
You will need the following parts:
- 1x Breadboard
- 1x micro:bit
- 1x micro:bit Breakout with Headers
- 1x micro-B USB Cable
- 1x Hobby Servo
- 3x Jumper Wires
Didn't Get the SIK for micro:bit?
If you are conducting this experiment and didn't get the Inventor's Kit, we suggest using these parts:
Introducing the Accelerometer
The accelerometer is a component that you won't find in the kit's bag of parts. Why? Because it is on the micro:bit itself! On the back of the micro:bit you can see a number of small chips. One of them is the accelerometer. The micro:bit has an onboard accelerometer that measures gravitational force. Depending on the version that you have, the accelerometer and compass can be on separate ICs or combined into a single IC.
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| v1.0 w/ Accelerometer on Separate IC | v1.5 w/ Combined Accelerometer and Magnetometer |
An accelerometer is a sensor that measures the gravitational forces pulling on it in all three dimensions of the chip's X, Y and Z axes.
Not only can an accelerometer measure the raw forces pulling on the chip and the object that the chip is sitting on, but it can also detect steps, shakes and other motions that have a specific pattern. On top of that, you can use an accelerometer to simply detect the orientation of the device. Did you ever wonder how your phone knows when you turn it from portrait to landscape? It is all because of the accelerometer in your phone!
Hardware Hookup
Ready to start hooking everything up? Check out the wiring diagram below to see how everything is connected.
| Polarized Components | Pay special attention to the component’s markings indicating how to place it on the breadboard. Polarized components can only be connected to a circuit in one direction. |
Wiring Diagram for the Experiment
Run Your Script
Let's read an accelerometer and control a servo. Type (or copy) the program into your Mu editor, or download all the programs from this GitHub Repository and open the Ex10_usingAccel.py program. Save it, then click the Flash icon to program your micro:bit.
language:python
# SparkFun Electronics
# Experiment 10
# Using the Accelerometer
from microbit import *
class Servo:
def __init__(self, pin, freq=50, min_us=600, max_us=2400, angle=180):
self.min_us = min_us
self.max_us = max_us
self.us = 0
self.freq = freq
self.angle = angle
self.analog_period = 0
self.pin = pin
analog_period = round((1/self.freq) * 1000) # hertz to miliseconds
self.pin.set_analog_period(analog_period)
def write_us(self, us):
us = min(self.max_us, max(self.min_us, us))
duty = round(us * 1024 * self.freq // 1000000)
self.pin.write_analog(duty)
sleep(100)
self.pin.write_digital(0) # turn the pin off
def write_angle(self, degrees=None):
if degrees is None:
degrees = math.degrees(radians)
degrees = degrees % 360
total_range = self.max_us - self.min_us
us = self.min_us + total_range * degrees // self.angle
self.write_us(us)
Servo(pin0).write_angle(0)
while True:
num = accelerometer.get_x()
Servo(pin0).write_angle(num * 180 / 2048 + 90)
Code to Note
Ignoring the servo class code again [1], we've got some new stuff!
accelerometer.get_x()
The accelerometer module comes with five built-in functions and recognizes 11 gestures. To see the full documentation, click here. The accelerometer.get_x() statement will return the acceleration measurement on the x-axis as a number between -1024 and 1024, depending on the direction.
Since the value stored in num needs to translated to an angle between 0-180 from a range between -1024 and 1024 it needs to be scaled. Adding 90 to the scaled value sets a range of angles for the negative accelerometer values from 0-89, and positive accelerometer values from 90-180.
What You Should See
At the beginning of the program the servo should move to 0 degrees and then react to the orientation of the micro:bit. If you hold the micro:bit flat, the servo will be at 90 degrees. Then if you tilt the servo to the left, it will move less than 90 degrees toward the value of 0. If you move it to the right, the servo will move toward 180.
Troubleshooting
This Seems Backward
You may be holding the micro:bit in a different orientation. Flip it around and try again!
Servo Isn't Working
Double check your wiring! Remember, red to 3.3 volts, black to ground, and white to signal.