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Wheelchair fairy lights

A set of smart fairy lights for my boy's night out on the town

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Today my wife asked "Why don't we go to the 2KidsFest lantern parade? - we just need something that glows!". "Sure! lets get some glow-sticks, but can I also put some fairy lights on the boy's wheelchair? - please"

This project details an emergency hack that involved reverse engineering of some damaged LED lights, building a new controller with the interface required to drive them.

This was all finished off in an afternoon - ready just in time to party!

This simple project embodies the maker ethic. It isn't about doing a full design and careful component selection - it is about rules of thumb, flying by the seat of your pants and building something fun with the junk you have to hand, for your family and others to enjoy!

About the only technical part of the design was the decision not to use a H-bridge but to use four logic-level outputs in parallel to drive each end of the LED string, and maybe the selection and placement the current limiting resistors.

The high logic level voltages are about 3.3V, and the LED string requires about 2.2V, leaving 0.55V to be dropped in either the output buffers or in one or more current limiting resistors. A single 10 ohm resistor at one end could work, but using a 27 ohm resistor on each output ensures proper current sharing and offers a little bit of overload protection.

Perhaps a true hardware hacker would rely on the I/O pin's 50mA current limiting, but that would be just too crass!

  • 1 × Scrap of strip-board
  • 1 × LED lights A string of LED lights that had been attached by a lawnmower.
  • 2 × A USB Power bank
  • 8 × 27 Ohm resistors
  • 2 × A Papilio One FPGA board.

View all 10 components

  • Built in a day

    Mike "Hamster" Field07/05/2014 at 10:14 0 comments

    The whole project was completed today in an hour or so. 

    I first tested the LED string with a few flat AA batteries and a multi-meter - they need about 100mA @ 2.2V to light the LEDs. With the batteries one way round half the LEDs light, the other way round the other half lights. That works out to about 2mA per LED - sounds about right. 

    To be able to switch the LED string's voltages from positive-negative to negative-positive would normally need a H-bridge driver, much like what is used drive a motor, but time was short and 100mA isn't that much current to supply.

    I decided to use an FPGA to sequence the LED patterns, as I can write the required FPGA code quicker than I could write the software for a small micro - and I like FPGAs, The device used can source or sink about 25mA on a pin, so to drive each wire of the LED string four of these were wired in paralleled along with 27 ohm resistors to ensure that current was balanced.

    Some strip-board, pin headers and a screw terminal were used to build a small daughter board, and the LED string was cable-tied to the wheelchair's frame.

    A little bit of HDL was hacked up to sequence the light. It consists of a clock divider to give a 3,600Hz clock enable pulse, and then two four-bit PWM drivers, with a PWM frequency of 60Hz in each direction to avoid excessive flicker. This was then tested and programmed into the FPGA board's configuration flash. 

    The FPGA board was then put in an anti-static bag and duct-taped to the wheelchair's handle, along with a 3.5Ahr USB power bank, which will give a day or so of run time at full brightness - more than enough for a lantern parade!

View project log

  • 1
    Step 1

    Cut a suitable size of stripboard.

  • 2
    Step 2

    Cable-tie the LED string to the wheel chair's frame, and duct-tape the controller and power bank somewhere handy.

  • 3
    Step 3

    Tweak the design to give whatever patterns you like!

View all 11 instructions

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Adam Fabio wrote 07/24/2014 at 03:32 point
Thanks for submitting your lighted wheelchair to The Hackaday Prize! The final wheelchair looks great! I know you've completed the project, but try to give us video before time runs out.

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