Introduction
There are two line sensors on the bottom of the Bit:Bot. They are
attached to the same pins as the micro:bit buttons. These are pins 5 and
11. You use a read_digital() on these pins to see if the sensor is
detecting a black line.
By making frequent adjustments to the speed of the left and right
motors, based on the readings from the line sensors, you can make your
robot follow a line or drive laps of a cricuit. If the left-hand sensor
senses a line, you need to drive the right hand motor more quickly, so
that the robot turns to the left. You do the same for the right hand
sensor.
The blog entry and the PCB silkscreen contain contradictory
information on the line sensors. I am deliberately avoiding being too
specific here. It will be useful for you to experiment with how which
sensor is on which pin and what values you get from a digital reading
for actions in the real world.
Programming
The line-following function in the code below works fine for a Lego
Mindstorms track. That has a pretty thick black line and this might need
to be tweaked to work more nicely on a different track.
from microbit import *
def Drive(lft,rgt): pin8.write_digital(0) pin12.write_digital(0) if lft<0: pin8.write_digital(1) lft = 1023 + lft if rgt<0: rgt = 1023 + rgt pin12.write_digital(1) pin0.write_analog(lft) pin1.write_analog(rgt) def FollowLine(): lft = pin11.read_digital() # read left line sensor rgt = pin5.read_digital() # read right line sensor fast=150 slow=50 if lft==0 and rgt==1: #turn right Drive(fast,slow) display.show(Image.ARROW_NE) elif lft==1 and rgt==0: # turn left display.show(Image.ARROW_NW) Drive(slow,fast) elif rgt==0 and lft==0: # straight on Drive(fast,fast) display.show(Image.ARROW_N) sleep(25)
# a small delay before starting sleep(1000)
# keep on following the line forever while True: FollowLine()
There are various things you might change in the function to get better performance. You could change the speeds that are used. There does need to be a pause between different calls to the Drive function, but it doesn't have to be the exact figure used here. You might also play around with the logic a little too.
Challenges
- Whatever you want to do later on with the line sensors, it's worth
doing a lot more experimentation with how you want to react to reading
the lines and how small adjustments make a difference to the overall
effect. Try different thicknesses of lines, different amounts of bend on
a corner and see what impact that has.
- Once you have something that can follow a line, you need a good
track. Go wild with black masking tape on white paper. Make a
complicated route and then work out how to make the best program to
navigate the route without deviating from the course. If you can, race
against a buddy's program or robot.
- Longest track EVER. Or at least the longest one you can make where you are.
- You can make a track that leads to something you want to bump into.
That might be something simple that you want to knock over, or more
complicated electronics that sense and react to the collision on a
second micro:bit.
- A little bit of Neopixel action might help you to work out what is
going on. The LightLeft() and LightRight() functions might help you
clarify how this program works. Some matrix work would add to the mix
nicely.
- You can use the line sensors to find something on an otherwise plain
track. Finding a line requires a different approach. You need to think
more carefully about how you react to sensor readings.
- You don't have to follow lines, you can avoid them, or change speed
when you cross them. You can do pretty much any movement when you detect
them.
- The truly brave thing to do is to create a maze of lines and write a
program that looks methodically for a way out. There are some difficult
concepts involved in mazes and how you can encode them for a computer
program to understand. You might want to start with writing programming
logic for some simple behaviours and a way to detect that escape has
taken place. Maze solving is a huge topic.
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