- 4.5 V rail was intended to provide the ADXL32x with a supply voltage
  that's at a safe distance from its 2.4 V minimum. However, the current
  circuit wouldn't work with the acceleration sensor operating at 4.5 V.
  It turns out the system operates well enough with the 4.5 V rail joined
  to the 3 V rail.

- the 6 V rail drops to 5 V after relatively little use. Need to improve
  the power supply.

- connections between boards are sensitive to mechanical forces and break
  easily in the high-g environment we have here. Should consider moving
  to an FPC at least for the LEDs. A stiff support beam (e.g., an
  aluminium rail) helps to reduce mechanical stress.

- the vertical resolution is a bit coarse. We could improve it and make
  the whole system simpler by adding local demultiplexing with 74HC594
  or 75HC595. Such chips could drive the LEDs directly.

- the battery pack needs room for a plug. (The original design assumed
  the battery pack would be mounted on the stick, but this now seems
  undesirable, due to the large weight. Current stick weight is about
  120 g and each AA battery would add about 25 g.)

- the system really ought to have a button or even a joystick for user
  input.

- once a button is added, we could have local-only operation with images
  coming from Flash.