help for interactive LED display

[mr roboto]

hi I am somewhat new to robotics and I need some help. does anybody know how to make an interactive LED display? I have an idea for a flat array of LED lights that responds to changes in light like your hand passing over the array. I know that I will need some sensors but I am not sure what sort of micro chip I will need. I would like if the lights changed in a patern but random is okay.

Thank you anybody who would want to help.
 

[Fr0stAngel]

hi,
You can use LDR(light dependent resistors) or photo-transistors to detect the change in light( i.e a cup placed on the table) and then use any 8 bit micro controller to to decide the patterns and the locations of LEDs to be turned ON or blink etc.

[want2learn]

Do a google search on charlieplexing, you should get most of the information you need that way.

Jim

[Soeren]

Hi,

Do a google search on charlieplexing, you should get most of the information you need that way.

"charlie-plexing" is only good for having a few LED's lit a time, not for large arrays.

[want2learn]

"charlie-plexing" is only good for having a few LED's lit a time, not for large arrays.

Yes but

I would like if the lights changed in a patern but random is okay.

I thought going for a random pattern would be the simplest solution and have more readily available information.

Not the first time I've been wrong though.

Jim

[mr roboto]

thank you fr0st angel how much do you think this solution would cost? (I dont have a very large buget)

[Soeren]

Hi,

I thought going for a random pattern would be the simplest solution and have more readily available information.

I think you misunderstood what I was saying.

If you want to "charlie-plex" a lot of LED's, there simply isn't enough time for each to make a suitable amount of light.
Consider a microcontroller that'll handle 25mA max per pin (not on all pins concurrently though, but let's forget that for now).
Say you want to use eg 8 pins for the c-'plex, enabling 56 LED's to be handled. Only one LED can be on at any time (there are ways to "cheat" but that complicates the control immensely).
That means that each LED gets 25mA/56 = 446µA average... Not all that bright (...x2  )

There's an issue with this in any form of 'plexing, but with "charlie-plexing" the problem is so pronounced, since you the entire thing is directed to use weak controller I/O. With regular plexing, you can use drivers for a higher current and have a much faster repeat rate, thus getting the average up to normal levels.
Just be aware that there are limits to the current that you can feed an LED, no matter the M/S, mainly due to the very thin wires bonding to the crystal - they'd evaporate with a too high current and while the LED crystal might still be usable, there's no way of getting the current through it then.

[madsci1016]

This is an Arduino Shield of 126 charlie-plexed LEDS

LoL Shield - Basic Test

So is this:

Arduino + lol shield

using 12 IO pins I believe.  (@20mA max)

Not that I completely disagree with Soeren, but large displays with many LEDS 'on' is possible. But it is tricky, has risks, and not the most feature rich way to do it.

[Soeren]

Hi,

This is an Arduino Shield of 126 charlie-plexed LEDS
[...]
using 12 IO pins I believe.  (@20mA max)

Not that I completely disagree with Soeren, but large displays with many LEDS 'on' is possible. But it is tricky, has risks, and not the most feature rich way to do it.

Looks OK in the dimmed setting it's shot, but how does it look in daylight?

You can get very efficient LED's (if you're willing to pay), but if it's true c-plex, the average LED current is 126µA (20mA/126)... That won't throw much light with regular LED's in a properly lit scenario.

[madsci1016]

Looks OK in the dimmed setting it's shot, but how does it look in daylight?

From what I hear it's ok in light.

Here's a 'better lit' video

John Park in the Maker Shed: LoL Shield

You can get very efficient LED's (if you're willing to pay),

I've bought one of these, but haven't taken the time to assemble it. It was only $25 for the whole kit. And that's after the standard Sparkfun/equiv markup of roughly 15-25% in parts price from major depos like Digi-key.

[galannthegreat]

Why not just drive the outputs with FETs?

[Fr0stAngel]

Why not just drive the outputs with FETs?

well because using FETs if you put a 0 to the base of the FET, that would mean an 'open' circuit. In charlie-plexing you need this 'open' circuit AND ground on the same pins.
@topic, heres a nice overview of charlie-plexing, if you are not planning to use many LEDs  (budget cuts  )
http://www.instructables.com/id/Charlieplexing-LEDs--The-theory/
well i cannot really estimate the cost for your project unless i know the dimensions and requirements. and since i'm really lucky enough that i get most of my stuff from a very big local electronics market (one of the biggest in ASIA) , i do get it cheap....so my estimates could be slightly off ..

[galannthegreat]

Hmm, never thought of that. I should look into Charlie-plexing more, it sounds interesting and might be of some use for a future project I want to do.

[Soeren]

Hi,

From what I hear it's ok in light.
[...]
Here's a 'better lit' video
[...]
I've bought one of these, but haven't taken the time to assemble it. It was only $25 for the whole kit. And that's after the standard Sparkfun/equiv markup of roughly 15-25% in parts price from major depos like Digi-key.

Viewing it without sound for now (since GF's sleeping right next to where I sit). Looks OK, but I doubt it's with 126µA (average).

Please post when you've built and reviewed the one you have, as I'm a bit curious about what can be done. Perhaps try one of the LED on DC 126µA (and 10..20mA) for reference before it goes into the board.

[Soeren]

Hi,

Why not just drive the outputs with FETs?

well because using FETs if you put a 0 to the base of the FET, that would mean an 'open' circuit. In charlie-plexing you need this 'open' circuit AND ground on the same pins.

So far no problem, but you forgot to mention that it also needs to be able to be positive - which could be done adding a P-ch device - most digital outputs are MOSFET push-pull, just copy that to a more powerfull device and Bob's your uncle.
However, the main goal of c-plex, controlling many LED's from a few pins for saving $$ would be lost in the process.

well i cannot really estimate the cost for your project unless i know the dimensions and requirements. and since i'm really lucky enough that i get most of my stuff from a very big local electronics market (one of the biggest in ASIA) , i do get it cheap....so my estimates could be slightly off ..

You lucky devil... Do they still sell transistors by the weight like groceries?

[Fr0stAngel]

You lucky devil... Do they still sell transistors by the weight like groceries?

well i wouln't know when they used to do that, but no ,its more like in 'dozens' ...
like i got bright white leds similar to
 http://www.sparkfun.com/products/533 for like 0.35 $(US) for 25 leds the other day  
or the the RGB ones for 0.9$ a dozen  etc  
and ofc save on shipment charges for many products