jerome arul

makes (almost) anything

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the sick stick

grow a tetrahedron

i was going to make a tetrahedral sculpture, something recursive and self-similar, that can can unfold into a tree.
but i didn't find a satisfying mechanism to both unfurl and refurl


source: wikimedia - ruggiero

playing with mechanisms

and i got bored
was uninspired
didn't have a good solution

here's the abandoned project site

now i got one week to make a final project
( see parkinson's law )
so let's move

move a human

always wanted to make a kickstick
( gondolier style / punting on a skateboard )
some cool people from rise robotics made the first kickstick
they're folks from mit (aaron and some other rise co-founders)
and kyle, parker and jeff from risd
but the project kinda fizzled out

i always wanted a wizard skate stick for myself

i want to dumb it down a bit

jen ordered me a BLDC
10s of dollars > about $90
got here thursday

50A flipsky 6354 190KV BLDC
• Max Power: 2450 Watts
• Max Current: 65 Amps
• Max Volts: 12S
• Max Torque: 7Nm
• M4 screws
Hole depth = 5mm
1/4in plate = 6.35mm
need M4 at 10-12mm length

these were stocked at archshops
• Max Cont Current: 40A
• Max Burst Current: 50A for 10 seconds
• Input Voltage: 2-6 cells li-XX or 6-16 Ni-MH/Ni-Cd battery
• BEC: 5.5V/5A Switching BEC
• PWM: 8~16 KHz

wiring diagram


got some motor dimensions

dxf file

picked up some 1/4" aluminum plate offcuts
thanks will

designed a motor mount
M4 bolt holes, holes for drywall screws
made a quick file to waterjet

file on the OMAX

thnks shah


triage: don't have time to design a new board
so gonna use the motor driver board from wk 10

only need VCC, GND
one pin for pot, one pin for pwm to ESC
pin 3 > PA02 - ANALOG
pin 5 > PA04 - ANALOG


motor driver traces png

that'll do

note to future jerome: just get a $3 servo tester, it'll do the trick


i wanted to use a lawn mower battery - 60V 5Ah
but it was too big
ESC voltage of "2-6S" means two to six 3.6V cells in Series
so that's 24V max
a dewalt 20V powertool battery is perfect!
B+ pin is hot, B- pin is GND

anthony is the saviour who bestowed his wisdom and patience upon me

first tested with a Leonardo > basic pot to servo sketch
then powered by dc power supply before trying battery
EECS has great drawers of lugs, terminals and junk

then tried a sketch from former fablabber who made a centrifuge
servo ESC sketch
the ESC has a start-up sequence to map the potentiometer values

ESC start-up

DC power supply (not cranked yet)

And Anthony thus wroteth: "Driving servos with SAMD chips. The typical arduino servo library doesn't (unless updated in the last year) work on SAMD devices. That being said creating your own PWM to set servo position is straightforward in software. It's perhaps a little clunkier in hardware though Neil has done most of that for you here and there is a nice library for the SAMD21 that uses hardware interrupts. Also a number of other choices that you can find on the good ol' google."
Library for my SAMD21

servo sketch (new library)

then tried my SAMD21 board

needed that new library
but the sample sketch was a bit hairy,
so anthony helped me with that too
combo'ed with the Loop from the first basic sketch

here's the sketch

and we're mobile

solder and heatshrink connections


gotta cast a tire
i found a vaccuum hose adapter thats the same size as the motor
and hot glued it onto a mug
i had some leftover urethane from molding/casting week

16 hrs later: FAIL
one edge didn't seem to set well
it was too soft / wrong durometer

makeshift mold with leftover reoflex


time to put it together

gonna use a wooden hand rail,
leftover from a home renovation
it has a nice flat face

1 1/4" exterior screws
yellow tape

sick stick... assemble!

countersink some holes


triage: gotta find a tire

kyle suggested 1/4in square o-ring's
i went to home depot to see if they had any o-rings
but instead found some rubber flex plumbing
this flex joint was nominal 2x2in
and fits so snug!

it cuts really easily with an olfa knife

flex joint for the wheel

cuts easily with a knife

then we ride

kyle on video


kyle helped me 3D print a file

found this dewalt battery holder STL online
edited it in Fusion: removed the rear for the power pins
• Import Mesh >
• Generate Face groups >
• Convert Mesh (parametric)/
• Now its editable as a Solid object. Export STL.

edited stl


i couldn't help but go skating this morning
and i took a tumble right before the final review

• busted the power switch
• 3D printed battery holder split
• the ESC seems to have error codes

was able to bypass the switch with alligator clips
it was rideable again
but then the tire started slipping off

twas a fun night! woohoo htmaa!




• bldc motor (50A) ~ $90
• ESC (40A) ~ $27
• SAMD21 mcu (VCC,GND, 2 pins w/ PWM)
• Dewalt battery 20V ~
• tire: 2" rubber flex joint (plumbing)
• 10K potentiometer ~
• rocker switch for power ~
• M4 x 12mm bolts for motor mount ~
• drywall screws to attach plate to stick ~
• 1/4" aluminum plate to mount motor ~
• tape, zipties
• stick ~


• check out VESC (avoid RC startup sequences)
• use a $3 servo tester to avoid milling a board
• try a finger throttle or slide potentiometer
• recess switches to survive impact
• some donut-like thing to protect hardware from falls
• glue tire to motor
• learn how to program "braking"