Electric Long-board

When this year’s lock-down started a few months ago, I finally bought myself a long-board so that I could keep up with the kids on their bikes or scooters when exercising.
It didn’t take long before I remembered that I always wanted to build an electric "toy" that could carry myself.
I contemplated a bike, a Go Kart and others, but it’s always been pretty obvious that the easiest to attempt would be a skateboard…

Some Calculations

From what I could see on the Internet, both for electric bikes and skateboards, one needs a few hundred Watts of power in order to be able to move around the 70-80Kgs of an adult human.
I decided to go for the lower bound and build something providing a max of 200W, mainly for safety (the more power the more speed and the more dangerous in general 🙂 ) but also for cost, the more beefy the components the more expensive they get.

The Chain and Sprockets

That’s how it all started, with a small chain and 2 sprockets that I bought online ages ago.

It was actually not obvious to screw the big sprocket onto one of the longboard’s wheels.

The Motor

After looking online it seemed that most builds, both custom and professional, used brushless motors. They are the most compact solutions for a given power rating, however they are also expensive, as well as the required controllers.
I therefore opted for a "good old" brushed motor, and could find a 150W one at around £25 on eBay.
As noted above, this is slightly below the minimum power, but that’s fine I really don’t need/want this board to go faster that jogging speed or be able to climb hills.
Also important the choice of voltage.
The higher the voltage the lower the necessary current (Power = Voltage * Current) which is important as high current needs thick wires and beefy motor controllers.
I noticed that a lot of setups online use 36V or 48V (50V is I think the limit beyond which it can start to be dangerous for humans, so sticking below that is recommended…), however I found that these cheap motors/controllers are usually for 12V or 24V, so went for the latter.

Motor controllers for high current / power are not cheap either, but it turns out there’s this IBT_2 board, based on 2 BTS7960 half bridges, that can be found for ~£10 and that is supposed to handle up to 30V and 43Amps.
So I decided to go ahead with this combination, which would cost ~£35, as opposed to £100-200 for a proper brushless motor and controller.
One seemingly small detail, but that took the better part of a day to solve, was removing the timed belt pulley that came with the motor and replace it with the smaller sprocket.
Designing a mount for the motor in FreeCAD and printing it was pretty straight forward.

The Power Source

Time to chose the battery now. I’ve thought quite a bit about this. The most energy dense are obviously LiPos, but not only they are more expensive than other choices but they can be quite dangerous if not handled properly. Especially the 6 cells that I would need to produce 24V.
Still, while weight wouldn’t be an issue (given my own 🙂 ) the space taken by a lead acid battery would be significant, and using NiMh or similar rechargeable would probably give a much lower autonomy.
So I did go with a 6 cells in series LiPo finally, but decided to add a 10Amps circuit breaker/fuse, to make sure it will never be pushed to its limits.
This is actually very low compared to it’s >70A nominal discharge rate, but my motor is rated for only ~9A, so that’s perfect, the further away from the battery limits we are the better !

The Magic Smoke

This was the 1st and main failure.
I used a voltage regulator that I had lying around, to drop the 24V from the battery to the 5V necessary for the Arduino.
Without paying attention that it was rated for max 15V input, which made it literally blow up after a few minutes of testing.
The annoying this was that during some of this time it connected everything that was on its output pin (remember that was supposed to provide 5V) to 24V directly…
This means that the motor controller, the Arduino and the ACS712 current sensor, all released the magic blue smoke.

The Box

As you can see in the video I did do some initial testing with all the electronics thrown into a plastic box and it ended up almost ripping the wires of the motor off.
This meant that it was time for a proper box !

The Remote Control

For this, I simply re-used the hacked Wii Nunchuck that I build for this RC car project (and then also re-used for the final version of self-balancing robot ).

Final Result

I personally like the "hacked" look due to the several colours and the blocky shape of the box 🙂
I actually hesitated for a while to install everything below the board (as it’s done on more professional looking conversions ) but I felt it would cause issues when going over bumps.

Code

As usual, all the 3D designs and the Arduino code used, are available on my Github account.

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