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96 lines
2.9 KiB
Plaintext
96 lines
2.9 KiB
Plaintext
design specs:
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- text readable at up to 50 m
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- automatic centering of image
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- non-aggressive design
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- suitable for dim ambient light, such as an illuminated street
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at night
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- powered by off-the shelf batteries
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- content programmable via wireless interface
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- use medium-strength authentication for wireless communication
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opportunistic component selection:
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- reuse ben-wpan (IEEE 802.15.4) circuit, and ben+atben or
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laptop+atusb for (remote) editing
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- use ATmega 48/88/168 series controller
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- use ADXL321 series acceleration sensor
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- use APTD1608SEC LEDs
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- use unregulated battery power
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visible field:
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- height: ~25 cm, with 16 pairs of LEDs, at 8 mm spacing
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- horizontal movement (arm waving) is about 120 cm
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- this yields up to 80 horizontal square "pixels"
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- text capacity
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height width gap character (per line)
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16 px 10 px 2 px 6.8
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7 px 5 px 2 px 11.7
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7 px 5 px 1 px 13.5
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- text height
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pixels cm
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16 24
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7 11.2
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physical structure:
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- mount LEDs on stick
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- batteries and main circuit in handle or extension (see below)
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- make stick strong enough to resist excessive bending/breaking
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under wind load
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- make stick as weak as possible to avoid perception or use as
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weapon
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stick length:
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- LEDs occupy (16*2-1)*8 mm plus component width, in total
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roughly 249 mm
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- add 2 cm for termination at top
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- add extension for increased visibility
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extension:
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- assume small spectators are 10 cm smaller than nearby
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obstacles (e.g., shoulders of tall people)
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- assume distance between spectators is 80 cm
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- to be visible at 50 m, the bottom edge must therefore be
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50 m / 80 cm * (10 cm / 80 cm) * 10 cm = 78 cm
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above eye level of small spectators
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- assume waver to be about 20 cm taller than small spectator
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- hand above head position yields distance of about 30 cm
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above eye level
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- therefore, extension should be 78 cm - 50 cm = 28 cm
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handle design:
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- about 10-12 cm in length
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- include button for control functions. button can be located
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on extension
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- acceleration sensor should be at a high position, possibly
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on extension
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overall length:
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- 25 cm for LEDs
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- 2 cm for termination (head)
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- 28 cm for extension
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- 12 cm for handle
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- 67 cm total length
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electrical, LEDs:
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- no multiplexing
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- max. 30 mA per LED pair (series)
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-> max. 16 * 30 mA = 480 mA for LEDs
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- series resistor is 56 Ohm. Allow up to additional 10% voltage drop
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in trace.
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-> Rmax per LED pair is ~ 5 Ohm
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- assume 30 cm trace
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- variant A: switching transistors at LED
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- average length is 15 cm
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- Rmax is 5 Ohm / 16 = 300 mOhm
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- http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
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yields for 500 mA at 2 oz/ft2:
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- minimum width 2.3 mil / 0.06 mm
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- 111 mOhm
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=> 8 mil traces are more than sufficient, even for power rails
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- variant B: switching transistors in handle
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even less demanding
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authentication:
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- shared secret (128 bit, MD5-hashed text with 128 bit salt)
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- salt (128 bit)
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- MD5
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