BBC micro:bit
Bitkit - Colour Sensing

Introduction

The micro:car has a colour sensor at the front. This allows you to be a little more inventive with your line following applications.

micro:bit circuit

The image shows the car moving autonomously on one of the tracks that come with the kit. I have used the markers from the kit to colour in some sections of the track. You can see a blue circle, a red rectangle and, just off the bottom of the image, is a green circle.

The following program code uses the Drive and FollowLine procedures previously developed to follow a black line and do something when different coloured sections are reached. When the car reaches a blue circle, it makes a 90° turn to the right. When it reaches the red rectangle, it stops for a few seconds. When it reaches a green circle, it makes a 90° turn to the left.

from microbit import *

def Drive(lft,rgt):
    ld, rd = 0, 0
    if lft<0:
        ld = 0xff
        lft += 255
    if rgt<0:
        rd = 0xff
        rgt += 255
    i2c.write(0x28,bytes([1,lft,ld,rgt,rd]))

def Follow_line():
    i2c.write(0x27,b'\x02')
    ln = i2c.read(0x27,1)[0]
    drv = [(0,0),(255,255),(255,255),
    (63,255),(0,255),(255,63),(255,0)]
    lt,rt = drv[ln]
    Drive(lt,rt)
    sleep(20)

def Colour():
    i2c.write(0x27,b'\x01')
    return i2c.read(0x27,4)[0]
    
while True:
    c = Colour()
    # check for red(2), green(3), blue(4)
    if c>=2 and c<=4:
        # colour event
        if c==2:
        # stop for 2 seconds (red)
            Drive(0,0)
            sleep(2000)
            Drive(255,255)
            sleep(1000)
        elif c==3:
        # turn left (green)
            Drive(255,255)
            sleep(750)
            Drive(0,255)
            sleep(1800)
            Drive(255,255)
            sleep(500)
        elif c==4:
        # turn right (blue)
            Drive(255,255)
            sleep(750)
            Drive(255,0)
            sleep(1800)
            Drive(255,255)
            sleep(500)
    else:
        Follow_line()

The key to this is the Colour function. To get a colour reading, we write a 1 to the colour sensor. We then read back 4 bytes and look at the value of the first byte. The number returned tells us which colour was sensed,

  1. Black
  2. Red
  3. Green
  4. Blue
  5. Another Colour

This feature of the micro car is pretty cool. Notice that the values you can get from a reading correspond to the 3 colour channels plus black. This is easiest to work out with a colour sensor. Other colours are a little more tricky to read reliably - but not impossible...