About
I designed and built a pair of poi with build-in accelerometers. LED brightness varies based on how fast the poi are being spun.
This project involved designing double-sided PCBs with an unusual form factor, then molding and casting to create a slip-on protective cover.
Images
When the poi are spun slowly, the acceleration profile is visible.
When spun quickly, they stay consistently bright.
Circuitry
The system is powered by two AA batteries and a 5V boost regulator. A double-throw switch activates the system when turned on and connects power to ground through a resistor when turned off (to dissipate stored charge).
To make spin-dependant brightness, I designed a simple op-amp circuit to boost the output of the accelerometer:
The value of R1 controls the current coming out of the accelerometer, and the ratio of R2/R1 sets the gain of the system.
P1 is a trimmer potentiometer used to adjust the baseline brightness, since the accelerometers come from the factory with ±0.1V variability. In the fourth image, it has been adjusted so that the LEDs are at full brightness with no accelerometer output.
R3 is a current-limiting resistor; each diode has one.
The NMOS transistor T1 (in conjunction with RC) slows down the start-up of the system, ensuring that large current demands only kick in after the boost regulator has had time to settle.
Mold Design
The protective covers are cast in a three-part mold. Two parts can be cut on a CNC mill, but the third part of that mold has to be cast in a separate two-part mold. Below is a height-map image of the molds parts (with lighter colors representing higher parts of the model).
Casting a flexible material into the blue mold will create a two-part mold for the yellow model. The yellow model can then be inserted into the red mold, which is used to cast the final cover.
Hardware
All of the components are available from DigiKey:
| LEDs | Op-amp |
| Accelerometer | Switch |
| Battery clips | Trimmer potentiometer |
| NMOS transistor | Boost regulator |
| Inductor | (plus assorted 1206 passives) |
Materials
Molds were cut in machinable wax with a Roland Modela.
Negatives were cast in of Dragon Skin; OOMOO could work too.
The intermediate mold piece was cast with Smooth-Cast 300.
The final protective covers were cast in PMC 121/30 Dry.
West System epoxy resin was used to seal the boards. This prevents corrosion of the copper traces and makes components (especially the battery holders) more secure.
Files
Mold design file (kokopelli file, 4 KB)
PCB design file (zipped Eagle 6 files, 557 KB)
This is a fairly challenging project, and this page doesn't offer detailed instruction. Only take it on if you're feeling bold and independent.