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antorcha/NOTES
Werner Almesberger 56476539cf fw/: addition of boot loader (WIP) and assorted cleanup and improvements
The boot loader currently uses the protocol switch intended for the
application. This makes it too big to fit in the very limiting
constraints the ATmega168 poses - and there's not even cryptographic
authentication yet. We'll have to dumb it down quite a bit.
2012-06-18 10:56:43 -03:00

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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