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LED coaster

Discussion in 'Electronic Design' started by [email protected], Jun 30, 2007.

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  1. Guest

    RGB LEDs are now cheap enough (<40 cents each) to be just about
    suitable for display projects. Check out the video drinks coaster I
    built for my wife.

    Lessons learned from this one

    - The pull-down current from 74HC is enough to drive superbright LEDs
    nicely even at a 10:1 multiplex ratio. I could have eliminated 60% of
    the SMT on the board :(
    - eBay is a great source of cheap LEDs, but expect a few early-life
    failures due to inadequate factory testing.

    I have four more PCBs. Time for a 20x20 display....

  2. Nice...
    You could add a touchscreen to the top of the thing to give it
    responsive feedback. You can get replacement touch screens for phones
    through eBay for a few quid.
  3. Guest

    You could add a touchscreen to the top of the thing to give it
    I read a paper somewhere describing the use of the photodiode
    behaviour of LEDs to build a touch sensor. The idea was to

    - turn the LED on and then off
    - monitor the voltage across the LED as the parasitic capacitance
    discharges through the photodiode

    If there is a finger over the LED, the voltage will drop off more
    slowly. I'll try and dig it out. I'd need to redesign the board
    layout, but providing the timescale on which discharging takes place
    is short enough, it might well be workable, and certainly would make
    for a nice effect.

  4. On Mon, 02 Jul 2007 00:20:13 -0700, in
    was this it?

  5. Guest

    was this it? Yes, I believe so. Thanks for tracking that down. As I said, it would
    be fiddly to integrate exactly their technique with the current
    scanning circuitry. It should be possible with a clean-slate design,
    which I probably need to do anyway.

    Fortunately my wife really likes her coaster, so I can get away with
    another 100 quid of PCB fabrication.

  6. Tam/WB2TT

    Tam/WB2TT Guest

    20+ years ago somebody i knew made his wife a pair of LED ear rings to wear
    to a Christmas party.

  7. Donald

    Donald Guest

    I have checked ebay for LEDs like the surface mount ones on your coaster.

    None to be found.

    Please post a link to those you seem to be using.

    thank you

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