antorcha/doc/DESIGN

design specs:
- text readable at up to 50 m
- automatic centering of image
- non-aggressive design
- suitable for dim ambient light, such as an illuminated street
  at night
- powered by off-the shelf batteries
- content programmable via wireless interface
- use medium-strength authentication for wireless communication

opportunistic component selection:
- reuse ben-wpan (IEEE 802.15.4) circuit, and ben+atben or
  laptop+atusb for (remote) editing
- use ATmega 48/88/168 series controller
- use ADXL321 series acceleration sensor
- use APTD1608SEC LEDs
- use unregulated battery power

visible field:
- height: ~25 cm, with 16 pairs of LEDs, at 8 mm spacing
- horizontal movement (arm waving) is about 120 cm
- this yields up to 80 horizontal square "pixels"
- text capacity
  height width gap  character (per line)
  16 px  10 px 2 px  6.8
   7 px   5 px 2 px 11.7
   7 px   5 px 1 px 13.5
- text height
  pixels cm
   16    24
    7    11.2

physical structure:
- mount LEDs on stick
- batteries and main circuit in handle or extension (see below)
- make stick strong enough to resist excessive bending/breaking
  under wind load
- make stick as weak as possible to avoid perception or use as
  weapon

stick length:
- LEDs occupy (16*2-1)*8 mm plus component width, in total
  roughly 249 mm
- add 2 cm for termination at top
- add extension for increased visibility

extension:
- assume small spectators are 10 cm smaller than nearby
  obstacles (e.g., shoulders of tall people)
- assume distance between spectators is 80 cm
- to be visible at 50 m, the bottom edge must therefore be
  50 m / 80 cm * (10 cm / 80 cm) * 10 cm = 78 cm
  above eye level of small spectators
- assume waver to be about 20 cm taller than small spectator
- hand above head position yields distance of about 30 cm
  above eye level
- therefore, extension should be 78 cm - 50 cm = 28 cm

handle design:
- about 10-12 cm in length
- include button for control functions. button can be located
  on extension
- acceleration sensor should be at a high position, possibly
  on extension

overall length:
- 25 cm for LEDs
-  2 cm for termination (head)
- 28 cm for extension
- 12 cm for handle
- 67 cm total length

electrical, LEDs:
- no multiplexing
- max. 30 mA per LED pair (series)
  -> max. 16 * 30 mA = 480 mA for LEDs
- series resistor is 56 Ohm. Allow up to additional 10% voltage drop
  in trace.
  -> Rmax per LED pair is ~ 5 Ohm
- assume 30 cm trace
- variant A: switching transistors at LED
  - average length is 15 cm
  - Rmax is 5 Ohm / 16 = 300 mOhm
  - http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
    yields for 500 mA at 2 oz/ft2:
    - minimum width 2.3 mil / 0.06 mm
    - 111 mOhm
  => 8 mil traces are more than sufficient, even for power rails
- variant B: switching transistors in handle
  even less demanding

authentication:
- shared secret (128 bit, SHA1-hashed text with 128 bit salt)
- salt (128 bit)
- SHA1 from avrcryptolib
- due to chip limitations, the secret key for firmware updates is sent
  in the clear