Driving Motors (from Microbit)

One thing I've always thought was pretty dumb was all the Raspberry Pi crowd building robot buggies using the Raspberry Pi - not because its a good thing to do, but because they think a Pi is the answer to everything! The Pi is a terrible boar to base a buggy around. It's expensive (comparatively), difficult to power, unreliable, and its actually just not very good at the task (lack of HW PWM, and timing reliability). The microbit however is ideal.

Kitronik sell a buggy kit for a £25 (plus £5 postage), but you can get the same motor, wheels, and a much nicer chasis for about about £7 on eBay. To make the cheap one work all you need is a motor driver board. They also sell one of those for £11.50 (its included in the buggy kit). It's a nice looking board. but you can get exactly the same functionality using a generic board which costs £1.50. The only catch is you will need a breakout board so you can hook stuff up to the microbit, (kitronik £4.50), but you'll need one of those anyway for other projects. It's also going to get a bit messy, so the Kitronik motor board is probably a good choice if you want an all in one solution, but if you've already got some motors, and a breakout then the £1.50 eBay motor driver is a great addition to your parts box.

This is the sort of board you want (eBay: Arduino motor driver). They can look a bit intimidating to hook up as theres lots of wires but they're pretty obvious.

Starting at the end, the screw terminals on the left and right are where the motors go. Just connect the two wires from the motor into a terminal pair and tighten them up. Don't worry about connecting them the right way - get it wrong and your motor runs backwards. You can either fix this in software, or just switch the wires around (in workshops the first thing I get kids to do once they've hooked up a motor is check its direction, and reverse wires if  necessary so everyone's idea of forwards is the same!).

The terminal block with three wires is marked (from left to right) 12v, Gnd, 5V. Don't worry that we won't be operating at these voltages... 12v is a maximum(ish). If you're using the "yellow" motors, then they can run up to about 6v, so connect a power source/batteries of 5-6v between the left input, and to the Ground pin. The left pin provides power to the motors, but it also provides a lower voltage supply to the rest of the electronics on the board - that's what the 5V is for. If you're using a microbit DO NOT CONNECT ANYTHING ELSE TO 5V!

That's the heavyweight side hooked up - now to look at the controller side:

There are four pins on the board labeled in1-in4. (There are also a couple of pins labeled en-A and en-B butthey should have jumpers on them, and we can ignore them) - these need to be connected to four output pins on the Microbit. If you just want to control one motor you could connect them with croc-clips to D0 and D1, but i'm using the MB^5 (MicroBit Breakout Board Breakout Board) I build in the last post. It exposes pins 13-16 as a row of headers which is perfect for this. I just connected them straight across. You could connect them straight to a regular microbit breakout board - you'll just spend more time counting/checking pins. You'll also note there's also a ground connection from the boards GND pin to the 0V pin on the Microbit. DO NOT ATTEMPT TO CONNECT POWER BETWEEN THE BOARDS... you'll destroy your Microbit.

With that in place we're read to do some software:

make m1fw digital output D13

make m1bw digital output D14

when start

.forever

..set m1fw to yes

..set m1bw to no

..wait 1 secs

..set m1fw to no

..set m1bw to yes

..wait 1 secs

Here I've set up a single motor. Microbit pins 13/14 are connected to in1,in2 of the motor board, which control out1/out2 which are connected to what we'll call motor 1.

Driving a motor using two pins can be a bit confusing, but a little creative variable naming goes a long way. I've called the two output m1fw and m1bw: motor 1 forwards and backwards. So to go forwards, set m1fw to yes, and m1bw to no. To go backwards m1fw is no, and m1bw is yes. To stop you set them both to no (or set them  both to yes if you want, but no make more sense).

We could do the same for motor 2, but we can be a bit more clever.

make m2fw analog output D15

make m2bw analog output D16

make speed number

when start

.forever

..set speed to sin of (timer*100)

..if speed>0

...set m2bw to 0

...set m2fw to speed

..if speed<0

...set m2fw to 0

...set m2bw to -speed

..wait 0.1 secs

If you remember when we flashed an LED we used an analog output to flash the LED very fast, and it looked like it was dimming. We can use the same trick to control the speed of our motor. Pins 15/16 are driving in3/4 on the board. I've created a variable called "speed", which I set to something between -1 and +1, which are going to represent full reverse and full forwards. I use a Sin wave just to create something interesting.

If speed is positive we want to go forwards, so set m2bw to 0, and m2fw to how fast we want to go forwards. If we're going backwards, then M2fw is 0, and m2bw to -speed (because speed is negative, so -speed is how fast we go backwards).

The "turning on and off fast" can break if we try and change the speed to often, so we add a 0.1 second delay in there just to keep things running smoothly. If I was using this in a bigger program I might write an update motor script to handle all of this and make everything a bit cleaner:

make speed number

when updateMotor

.if speed>0

..set m2bw to 0

..set m2fw to speed

.if speed<0

..set m2fw to 0

..set m2bw to -speed

when start

.forever

..set speed to sin of (timer*100)

..broadcast updateMotor

...wait 0.1 secs

If you like you can even write it as:

make m2fw analog output D15

make m2bw analog output D16

make speed number

when start

.forever

..if speed>0

...set m2bw to 0

...set m2fw to speed

..if speed<0

...set m2fw to 0

...set m2bw to -speed

..wait 0.1 secs

when start

.forever

..set speed to sin of (timer*100)

While this isn't the most efficient system it is the most "scratch-like". We've got one script updating the motor speed every 1/10th of a second, so the "main" program at the bottom can just do its thing, and set the speed it would like the motor to be running at, without worrying about how that happens.

Hopefully as the microbit ecosystem matures we'll see a whole range of cheap boards but there's a whole load out of bits on ebay already that are easy to hook up once you've got some kind of breakout.