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| Source at commit bf2e2c30463d22b5e34fa795eb98b35a855d90ae created 10 years 11 months ago. By Werner Almesberger, tornado/led/led.cmp: update for new components and changed references | |
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| 1 | design specs: |
| 2 | - text readable at up to 50 m |
| 3 | - automatic centering of image |
| 4 | - non-aggressive design |
| 5 | - suitable for dim ambient light, such as an illuminated street |
| 6 | at night |
| 7 | - powered by off-the shelf batteries |
| 8 | - content programmable via wireless interface |
| 9 | - use medium-strength authentication for wireless communication |
| 10 | |
| 11 | opportunistic component selection: |
| 12 | - reuse ben-wpan (IEEE 802.15.4) circuit, and ben+atben or |
| 13 | laptop+atusb for (remote) editing |
| 14 | - use ATmega 48/88/168 series controller |
| 15 | - use ADXL321 series acceleration sensor |
| 16 | - use APTD1608SEC LEDs |
| 17 | - use unregulated battery power |
| 18 | |
| 19 | visible field: |
| 20 | - height: ~25 cm, with 16 pairs of LEDs, at 8 mm spacing |
| 21 | - horizontal movement (arm waving) is about 120 cm |
| 22 | - this yields up to 80 horizontal square "pixels" |
| 23 | - text capacity |
| 24 | height width gap character (per line) |
| 25 | 16 px 10 px 2 px 6.8 |
| 26 | 7 px 5 px 2 px 11.7 |
| 27 | 7 px 5 px 1 px 13.5 |
| 28 | - text height |
| 29 | pixels cm |
| 30 | 16 24 |
| 31 | 7 11.2 |
| 32 | |
| 33 | physical structure: |
| 34 | - mount LEDs on stick |
| 35 | - batteries and main circuit in handle or extension (see below) |
| 36 | - make stick strong enough to resist excessive bending/breaking |
| 37 | under wind load |
| 38 | - make stick as weak as possible to avoid perception or use as |
| 39 | weapon |
| 40 | |
| 41 | stick length: |
| 42 | - LEDs occupy (16*2-1)*8 mm plus component width, in total |
| 43 | roughly 249 mm |
| 44 | - add 2 cm for termination at top |
| 45 | - add extension for increased visibility |
| 46 | |
| 47 | extension: |
| 48 | - assume small spectators are 10 cm smaller than nearby |
| 49 | obstacles (e.g., shoulders of tall people) |
| 50 | - assume distance between spectators is 80 cm |
| 51 | - to be visible at 50 m, the bottom edge must therefore be |
| 52 | 50 m / 80 cm * (10 cm / 80 cm) * 10 cm = 78 cm |
| 53 | above eye level of small spectators |
| 54 | - assume waver to be about 20 cm taller than small spectator |
| 55 | - hand above head position yields distance of about 30 cm |
| 56 | above eye level |
| 57 | - therefore, extension should be 78 cm - 50 cm = 28 cm |
| 58 | |
| 59 | handle design: |
| 60 | - about 10-12 cm in length |
| 61 | - include button for control functions. button can be located |
| 62 | on extension |
| 63 | - acceleration sensor should be at a high position, possibly |
| 64 | on extension |
| 65 | |
| 66 | overall length: |
| 67 | - 25 cm for LEDs |
| 68 | - 2 cm for termination (head) |
| 69 | - 28 cm for extension |
| 70 | - 12 cm for handle |
| 71 | - 67 cm total length |
| 72 | |
| 73 | electrical, LEDs: |
| 74 | - no multiplexing |
| 75 | - max. 30 mA per LED pair (series) |
| 76 | -> max. 16 * 30 mA = 480 mA for LEDs |
| 77 | - series resistor is 56 Ohm. Allow up to additional 10% voltage drop |
| 78 | in trace. |
| 79 | -> Rmax per LED pair is ~ 5 Ohm |
| 80 | - assume 30 cm trace |
| 81 | - variant A: switching transistors at LED |
| 82 | - average length is 15 cm |
| 83 | - Rmax is 5 Ohm / 16 = 300 mOhm |
| 84 | - http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/ |
| 85 | yields for 500 mA at 2 oz/ft2: |
| 86 | - minimum width 2.3 mil / 0.06 mm |
| 87 | - 111 mOhm |
| 88 | => 8 mil traces are more than sufficient, even for power rails |
| 89 | - variant B: switching transistors in handle |
| 90 | even less demanding |
| 91 | |
| 92 | authentication: |
| 93 | - shared secret (128 bit, SHA1-hashed text with 128 bit salt) |
| 94 | - salt (128 bit) |
| 95 | - SHA1 from avrcryptolib |
| 96 | - due to chip limitations, the secret key for firmware updates is sent |
| 97 | in the clear |
| 98 | |
Branches:
master
tornado-v1
