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Solution #1

use a TLC5940NT that can control about 16 LED each

Naturally leads to a PWM version

Solution #2

use the snootlab's i2c led driver solution that can control 16 LED each

  • 64*i2c led driver = 9.95*64 = 636.80
  • 1*i2c shield = 17.95
  • 1 ardunio = 20
  • O(price) = 674.75 euros

Solution #3

  • use shift registers... ?

Solution #4

use two Peggy 2 board :

  • 2* http://evilmadscience.com/tinykitlist/157
  • O(price) = 95*2 = $180 --or far less as you won't need the giant PCB. They have a "parts kit" that includes the 328 and the IC's for $15. you'd still need LED's and transistors and other support components. --cw

looking at the peggyLE schematics, they are not using several of the outputs, it looks like a 32x30 grid is possible with the IC's included, and by adding another IC in place of the buttons 32x45 is possible. --cw => they use two 4 to 16 demux for highside driving. They therefore *have* to keep one output not connected, to allow an "off" state for each of these chips. One solution would be to use two such chips, plus two generic I/O, leading to a (quite ugly) 10 to 32 demux functionality. -- cq => that's only for the 74HC154's, the STP16DP05's can use all 16, so expanding that way there can 48x30 which gives 1440 LED's --cw

Solution #5

  • Check with Electrolab, where another similar project is ongoing :-)

=> actually, the (current) project there is about a 8x8x8 led cube (and some pov, too). Which are a bit different. But hey, somehow these are still led based project, so why not.

Solution #6

So, the goal is to achieve a 1024 led screen, with, if possible, PWM on each led, for a total price <150€ (connectors, leds not included) ?

My proposal would be to mix #1 (eg use TLC5940 chips, for their ability to PWM their outputs) and #4 (actually, only reuse the 4 to 16 demuxes (74HC154) idea, with two additionnal GPIO from the uC to achieve a 5 to 32 demux). That is :

  • two TLC5940
    • drive each row in PWM mode
    • can be daisy chained, and they take 8 I/O (or possibly less than that)
    • cost about 3€ each at www.mouser.fr
  • two 74HC154
    • drive each column (one after each other)
    • cost less than 1€ each
  • some power stage:
    • worst case is all 32 rows ON with 100mA each. So each column power driver should be able to sustain a 3.2 A continuous current.
    • the evilmadscientist schematic proposes 1.5A pnp transistors ;)
    • it'd be a better idea to get some (any) tougher pnp transistors (maybe few € total)
  • This requires about 18 I/O for control
    • it is out of reach of an Arduino (true only when all the features of the TLC5940 are used. There are solutions to use an Arduino anyway. Note that I consider UNO and similar boards, not mega ones - which are too expensive)
    • it is quite doable with a Teensy (http://www.pjrc.com/teensy/ about $30, similar in functionalities to an arduino otherwise)
    • or we could just use an naked Atmega chip, as I got a USBtinyISP Kit to program it (compatible with avrdude)
  • some power input is required
    • I suggest a computer PSU (reuse a standard connector to get 5v on the circuit)
  • some connectors are required
    • I suggest using standard RJ45 connectors (which stand 1.5A if I remember well. Meh...)
  • some passives are required. As usual.

Total cost: O{100€ ?). Sounds good to me :)

issues/open questions

  • will the TLC be well adapted to drive each column only 1/16th of the time ? That is, still work well when asked to somehow refresh data at (total refresh rate)*(number of columns)
    • according to datasheet, the max required clock speed is 4096*(refresh frequency). Even with a *16 factor, it isn't out of reach of a simple uC
    • will the leds give enough light when pulsed at max 1/16th of the time at 100mA ?
      • my guess would be: yes
  • how to make the board ?
    • it can be done on a standard protoboard (quite small, I would even say)
      • Check if standard PC power connectors fit on these. If not, use a wire + standard connector.

=> it will => do we have some at hand for the first prototype build ? --cq

      • Check if there are thru hole RJ45 connectors available. Looks like there are some (quite cheap, O{10€} for a 8 port version) but pins won't fit on a protoboard.

=> otherwise, we can use female connectors from a patch bay (I got a dozen of them). => yup, that was the main idea ; the problem is that the pins of these things doesnt fit well on breadboards, from what I've seen. TBC... --cq

      • Solid core cat5 wire will fit in a breadboard, so some keystone jacks and short lengths of cat5 can be connected
  • if one want to really use 12 bit resolution for each of the 1024 leds, using a 8bit uC with 8k of RAM is... probably not the best choice
    • I guess it'd be okay to downgrade the resolution to few bits per led, eg never drive LSBs for each led and use only 4bits resolution.

=> I cannot see any big flaw in this solution. Does anyone have a better view here ? --cq

  • "it is really an issue to be unable to buffer data"
    • wouldnt some usb to serial chips have some additionnal RAM ?
  • "it wouldnt hurt to have some more processing power onboard"
  • "you're such a p... to use monochrom leds"

=> we already have a stock of 1000 red leds.

Solution #7

Similar to #6, but instead of TL+demux, use a uC which has 64 I/O that can be dedicated to the led matrix.

  • I cannot think of any chip that would be as cheap and as "simple"
  • software layers would be easier
  • additionnal power/interfaces would be required to drive the rows

One possible solution:

Solution #8

  • 42 ?

Solution #9

for 9€ you have two mcu: a stm32bits uC to upload the stm8its. The stm32b can be hacked ?

  • I've 2 stm8s-discovery if you like to test. st-IDE run only on windows can be hacked to run on linux, i don't know ?