Overview

MiniBit is a ready-assembled simple and inexpensive robot for the BBC micro:bit.

It has the following features:

• Ready-assembled*. Just push on the wheels
• Edge connector to easily insert the Microbit
• Micro metal gear motors with fully-enclosed gearbox (no grit or fluff can enter)
• Wire-free battery holder for 3 x AA batteries
• 4 x Smart RGB LEDs (neopixel compatible)
• Integrated Pen holder for 10mm diameter pens (eg. Sharpie felt tips)
• Robust On/Off switch with Blue indicator LED
• Wide chunky wheels with lots of grip
• Metal ball front caster
• Connector for optional ultrasonic sensor or I2C breakouts (fully compatible with Pimoroni’s Breakout Garden range)
• The Microbit pins 0, 1, 2, Gnd and 3V are available for use with croc clips etc.
• Lots of mounting holes to create your own “body” for the robot or additional sensors etc.
• Makecode extension and micropython examples available

* Wheels need pushing on and optional pen-holder needs screwing in if purchased

Assembly Instructions

1. Push on the wheels
2. If you have the pen holder, then use 2 screws to screw the two pillars into the main board from the bottom, then use the remaining 2 screws to screw the top holder into the pillars

Coding Your MiniBit

Microsoft MakeCode

Click any image to enlarge.

To load the extension, select Advanced, then Extensions. Then enter “Minibit” into the search box and press Enter. If that doesn’t find it (there are sometimes earch glitches) you can enter the full URL into the search box: “https://github.com/4tronix/MiniBit”

Once loaded, you will have a MiniBit menu item with 4 sub-folders:

Motor Blocks

The first command “Drive at speed 600” will set both motors to speed 600. If you do nothing more, the MinBit will continue going forward forever.

The speed value can be from -1023 (full speed reverse) to 1023 (full speed forward). Setting the speed to 0 will stop the motors

There is also a block for spinning – left motor and right motor turn at the same speed but in opposite directions.

Both the drive and spin blocks have a paired block that will drive (or spin) for a selected amopunt of time and then stop

There are two ways of stopping. Coasting to a stop or braking. If you set the speed to 0 or use the “stop with no brake” command, then it will stop gently over the coourse of a second or so 9depending on initial speed). If you use the “stop with brake” block (or the drive/spin for a time block) then it will stop almost immediately.

Finally, you can drive each motor individually. For instance if you set the left motor to drive at 600 and the right motor to drive at 1000, then it will perform an arc towards the left

LED Blocks

You can use these blocks to set and clear one or all the LEDs.

Note that the MiniBit defaults to automatically updating the LEDs whenever any change is made see the “more…” section to learn how and why to change this behaviour

The LEDs on the MiniBit are labelled from 0 to 3. Use these numbers in the Makecode blocks to change the colour. eg setting LED 1 to Purple could be done like this:

The default brightness level is 40. This is plenty bright enough for most uses, saves damaging eyes, and reduces battery consumption. If required you can change the brightness from 0 up to 255

Sensor Blocks

Only one sensor in here; the ultrasonic distance sensor. You can get the values to the nearest object in cm, inches or microseconds

More Blocks

These are the advanced usage blocks. Most students will not need to use them.

• Set update mode is used to switch between automatic LED updates or manual LED updates. The default is for automatic updates: every change to the LEDs results in all the LEDs being written to with the updated values. This is easy to understand, but it does mean that when making a lot of changes it can slow things down considerably. If doing that, it is best to use Manual update mode, make all the changes required, then use the show LED changes block to make all the updates in one go.
• Rotate LEDs block will move the colour in LED 0 to LED 1, LED1 to LED2, LED2 to LED3 and LED3 to LED0. If done repeatedly, with a delay between each one, it will show the lED colours rotating around all the 4 LEDs.
• Shift LEDs block will move LED0 to LED1, LED1 to LED2 and LED2 to LED3. It will blank LED0. So all the colours will disappear one at a time from 0 to 3
• You can also create your own colours and replace the fixed list of colours in any command using the convert from red, green, blue block. For example, to set LED0 to a blue-green colour:

Programming in microPython

Driving Motors

The motors use 2 pins each to determine the speed and direction. In microPython we use write_analog ( ) to set the first pin to a value between 0 and 1023 and the second pin to 0 in order to go forward. To reverse, we swap the pins so that the first pin is set to 0 and the second pin is set to the value.

On the MiniBit the left motor uses pins 12 and 8, and the right motor uses pins 16 and 14.

So to move the left motor forwards at speed 600:

pin12.write_analog(600)
pin8.write_digital(0)

And to move the right motor in reverse at speed 450:

pin16.write_digital(0)
pin14.write_analog(450)

To stop with no brake, use write_digital ( ) to set both pins to 0. To stop with brake, set both pins to 1.
eg. stop left motor with coasting and right motor with brake:

pin12.write_digital(0)
pin8.write_digital(0)

pin16.write_digital(1)
pin14.write_digital(1)

So a complete, but fairly useless, program to drive the motors for 2 seconds and then stop quickly, would look like this:

from microbit import *
pin12.write_analog(600)
pin8.write_digital(0)
pin16.write_analog(600)
pin14.write_digital(0)
sleep(2000)
pin12.write_analog(0) # temporary fix for python bug
pin12.write_digital(1)
pin8.write_digital(1)
pin16.write_analog(0) # temporary fix for python bug
pin16.write_digital(1)
pin14.write_digital(1)

Note the 2 lines that write_analog(0) before swapping a pin from analog to digital. These are required until a fix is obtained for the python PWM driver continually updating the pin type to analog

Lighting the LEDs

This uses the standard neopixel code, with the LEDs connected to Pin 13.

At the top of your program add import neopixel then:

leds = neopixel.NeoPixel(13, 4)

leds is then an array of all 4 LEDs. leds[0] refers to the LED 0 and leds[3] refers to LED3. Each element of the array is a set of 3 numbers representing the Red, Green and Blue values (each 0..255) for that LED. So to set LED2 to Blue:

leds[2] = (0, 0, 255)

All this does is update the array. To show the new value of the array, we need to call the show ( ) function as follows:

leds.show ( )

Reading the Ultrasonic Distance Sensor

The ultrasonic sensor breakout is on pin15.

The concept is simple: send an ultrasonic pulse out, then time how long it takes to return. Using the speed of sound and some maths, we can then work out the distance. The following complete program has 2 parts to it: a function sonar ( ) which returns the distance to the object, and the main code in a loop which continually prints the distance. We also need to import the utime library:

from microbit import *
from utime import ticks_us, sleep_us

def sonar():
    pin15.write_digital(1) # Send 10us Ping pulse
    sleep_us(10)
    pin15.write_digital(0)
    pin15.set_pull(pin15, NO_PULL)
while pin15.read_digital() == 0: # ensure Ping pulse has cleared
        pass
    start = ticks_us() # define starting time
    while pin15.read_digital() == 1: # wait for Echo pulse to return
        pass
    end = ticks_us() # define ending time
    echo = end-start
    distance = int(0.01715 * echo) # Calculate cm distance
    return distance

while True:
    display.scroll(sonar())
    sleep(1000)