Micro Servo with micro:bit

Learn to use a micro servo with the micro:bit

Written By: Cherie Tan

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Difficulty
Easy
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Steps
11

Introduction

One of the most entertaining things to do with electronics is to make a mechanical device move. A popular device used to make things move is a servomotor or servo for short.

In this guide, learn to connect a micro servo to the micro:bit and program it in MakeCode.

Complete this guide to learn the basics in controlling a servo.

Step 1   Servo

  • Its rotation angle can be set (limited to 180 degrees)
  • While there are many different types of motors, a servo is ideal for pointer-like position-control applications. They are commonly used in robotics, for steering model cars, boats, airplanes and for many other sensor-positioning purposes. Here are some key points on servos:

    It receives control signals from a controller, in this case the micro:bit
  • Its position can be controlled more precisely than a standard DC motor
  • It may move a little beyond 180 degrees, if so, use this servo calibrator program
  • They are rated in kg/cm (kilogram per centimeter) which tells you how much weight your servo motor can lift at a particular distance.
    For example, the TowerPro Analog SG90 servo used in this guide is rated 1.8kg/cm at an operating voltage of 4.8V.

    So it should be able to lift 1.8kg if the load is suspended 1 cm away from the motor shaft.

    The greater the distance, the lesser the weight carrying capacity.
  • It has three wires:

    Ground (Brown) 

    5V Power (Red) 

    PWM Signal (Orange)

    In the following steps, you will learn to control a servo with the micro:bit.

Step 2   Pulse Width Modulation (PWM)

  • The micro:bit by itself is not able to send varying degrees of voltage out through its GPIO pins. So, what if you needed to produce analog effects such as when using a servo? To simulate analog effects, this can be done through pulse width modulation (PWM).
  • What PWM does is it turns a digital pin ON and OFF very quickly, which simulates various voltage levels. 
  • By turning a pin ON and OFF very quickly, this varies the duty cycle of the pin (how much time it is spent in the 'ON' state)
  • Yup, the more time the pin is spent 'ON' over the course of a second, the longer the duty cycle, and higher the simulated voltage.
  • The MakeCode editor has servo-specific blocks that will allow the control of a servo through PWM. The PWM signal is mapped to specific angles from 0 to 180 degrees

Step 3   Power

  • There are two ways to power the micro:bit, by using an external battery pack or with a USB cable
  • However, there is a maximum amount of current that the micro:bit can handle. 
  • Using the micro:bit to power the servo via USB will get you very close to the limit. 
  • To power the servo and make sure you are getting the full amount of torque from the servo, a battery pack is the ideal choice. For greater accuracy, we recommend powering the servo with an operating voltage of 4.8V to 6V.
  • While you could power the micro:bit via 3 volts, and get basic movement going from one position to another, it may not give you a highly accurate angle of rotation. Also, in a real application, it can cause issues with the micro:bit if the servo draws too much current when a load is placed on it.

Step 4   Pin Overview

Step 5   P0 to SIG

Step 6   5V

Step 7   GND to GND

Step 8   MakeCode Example 1

input.onButtonPressed(Button.A, function () {
    pins.servoWritePin(AnalogPin.P0, 180)
    basic.pause(1000)
})
input.onButtonPressed(Button.B, function () {
    pins.servoWritePin(AnalogPin.P0, 0)
    basic.pause(1000)
})
  • The MakeCode editor comes with support for using the servo. We'll program it so that by pressing button A on the micro:bit, the servo will turn a 180 degrees, and so spinning around one way

    We'll also program it so that by pressing button B, it will return to its starting position.
  • Open up the MakeCode editor
  • Click on 'New Project'
  • Copy and paste this code to the Javascript interface

Step 9   MakeCode Example 2

input.onButtonPressed(Button.A, function () {
    while (true) {
        pins.servoWritePin(AnalogPin.P0, 180)
        basic.pause(1000)
        pins.servoWritePin(AnalogPin.P0, 0)
        basic.pause(1000)
    }
})
  • In this second example:

    After pressing button A, the micro:bit will start to turn one way followed by a pause of 1 second or 1000 ms, before going back to its starting position. Then there is another pause of 1 second. This process continues, where the servo moves one way and then the other until the micro:bit's power source is disconnected.
  • Replace the existing code with the following code.

Step 10   MakeCode Example 3

input.onButtonPressed(Button.A, function () {
    pins.servoWritePin(AnalogPin.P0, 180)
    basic.pause(1000)
})
input.onButtonPressed(Button.B, function () {
    pins.servoWritePin(AnalogPin.P0, 0)
    basic.pause(1000)
})
pins.servoSetPulse(AnalogPin.P0, 500)
  • The code in this example are kept exactly the same as MakeCode example 1, with an added 'pins.servosetPulse(AnalogPin.P0, 500)'.
  • Changing the second parameter changes the pulse width to a specified value. The following code sets the servo pulse to 500 microseconds.
  • Replace the previous code with this in the Javascript interface.

Step 11   Upload the code to micro:bit

  • Finally, to upload the code to the micro:bit and test it out, connect the micro:bit to your computer using a microUSB cable
  • Click on the 'Download' button on the bottom left corner in MakeCode editor
  • Once downloaded, you can find the hex file in your 'Downloads' folder.
  • Drag and drop it to the MICROBIT drive
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