mirror of
git://projects.qi-hardware.com/antorcha.git
synced 2024-11-22 20:33:09 +02:00
98 lines
3.0 KiB
Plaintext
98 lines
3.0 KiB
Plaintext
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
|