[sdiy] What are these buttons?

[Speth, John]

> I just saw the Tenori-On and am wondering what those buttons are?
> I'd like to
> use some lighted buttons in a project but the ones I have seen are
> very
> expensive.

I wondered about the Tenori-on too.  It's been discussed here before.  At that time I collected two very relevant PDFs about the technology.  You can find them temporarily at this dropbox:

http://dl.dropbox.com/u/23053666/nime2006_172.pdf
http://dl.dropbox.com/u/23053666/nime2006_342.pdf

LEDs have a lesser known function as crude photo detectors when they are reverse biased.  Capacitance is a function of light energy.  I'm sure there are many ways to measure capacitance but a really simple way is to do it as described in this EDN article:

http://www.edn.com/article/462289-LED_senses_and_displays_ambient_light_intensity.php

The beauty of using an LED as a photo sensor is that you can multiplex the detect function with the illumination function and emit light with it when you're not measuring its capacitance.  I've experimented with the method described in the EDN article.  It works.  I didn't attempt the illumination multiplexing idea but I think it's feasible.

It sure looks like the Tenori-on uses those methods to do its tricks.  It's a cool use of dirt cheap LEDs.  I suppose the cost challenge becomes obvious when you consider how to drive 256 LEDs faster than human eye persistence and detect touches rapidly enough using those same LEDs.

JJS


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