Built in a day

A project log for Wheelchair fairy lights

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

mike-hamster-fieldMike "Hamster" Field 07/05/2014 at 10:140 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!