LED "capacitance" won't get lower

[skimask]

Hi skimask,
I've read lots of threads and went already on your site (btw: thank you) and many others about this topic.
The problem is not to make it work - it does.
I want to understand how to modulate the LED's capacitance.
It's maybe more an electronics issue, not a PIC one at this time.
Any idea?

I tried a few different things, added resistors, added small cap's in parallel with the LED, couldn't really get the results to change to get a larger 'range'. The results would 'shift' up and down, but never really stretch out the way I wanted them to.
Are you trying to 'widen' the range of the number you get back from the function? In other words, instead of, say 0-10, you'd get 0-1000?
The key might be finding the right LED vs. adding components to the circuit, or just plain counting it all really fast.
As you can see from my page, basically, as soon as I got that to work, I called it good...something in the hobby toolbox for the future...

[rhino]

I've been doing some reading on this subject and had it in the back of my mind for sometime now, using an led as a switch instead of a mechanical switch. A few things that I've read that may influence the range your after are the color of the lens... I think a clear lens works better, and the amount of ambient light in the room. It would be an easy experiment to see how it reacts in total darkness compared to whatever your normal lighting conditions are. Here's a couple links for info.
http://www.edn.com/article/CA6387024.html
Check out some of the references at the bottom of the page. I especially like the reference to the Mitsubishi Electric Research lab paper.
THIS is originally where I saw a demonstration. And ofcourse Skimask has video proof that it works, not to mention nice enough to post some code. Nice video!

[skimask]

A few things that I've read that may influence the range your after are the color of the lens... I think a clear lens works better

I tried a couple of type of red lensed LEDs, barely got any results at all. Tried a clear-greenish lensed, got some results, but nothing useful. The LEDs I ended up using were IR LEDs. You can barely see them in that 'video' I made. They show up on the camera, but you can't really see them.

and the amount of ambient light in the room. It would be an easy experiment to see how it reacts in total darkness compared to whatever your normal lighting conditions are

I sorta wrote the code to compensate for ambient lighting. I'd keep track of the high and low values and the one that was the darkest was the one with my finger over it. And the code on the site isn't exactly the finished product. The 'real' code (before I shelved the project) had a couple of extra lines in it to 'reset' the high and low values over time so they wouldn't get 'stuck'.

http://www.edn.com/article/CA6387024.html

That same video is where I got the original idea from.

And of course Skimask has video proof that it works, not to mention nice enough to post some code. Nice video!

(Code posting) - First time for everything I guess

Someday, when I get my basement/LAB rework finished (major water damage, repainting walls in the works), I'll put it back together with a few more LEDs and redo the video. Bought the wife a new camera for Xmas last year (or was it me? I forget )...has a really decent movie mode on it.

[flotulopex]

Well,

I see we all have been reading and viewing the same documents and medias already.

I've tried this with lots of different LEDs (IR, simple colored red/ambre/green/yellow/blue/white, 7seg and even a bicolor one) and they ALL work. Of course, I get different results but it works always.

Since the way to "measure" the LED with a PIC is to calculate it's discharging time (state change), to make it work fast, I want to REDUCE the "RC time".

So, if now I'm counting around 4000 counts (see post #1) until the LED goes from state "1" to state "0", I want to speed-up things with lowering this value and make it become something around 100.

How can I reach this?

Depending on the surrounding luminosity, the scanning/counting process can vary from a few milliseconds to seconds.

As mentionned in lots of articles, the surrounding luminosity affects tremendously the discharge time of the LED and compensation is mandatory.

In my mind, the only way to compensate the longer discharging time due to surrounding darkness is to reduce the voltage applied to the LEDs.

Is there another way?

[Ioannis]

Since the discharge is performed by the PIC itself, I don't see a way to speed the process unless you connect an external Transistor in parallel with the LED. A mosfet perhaps. But that will complicate the things...

Ioannis

[flotulopex]

Okay; I probably ask the wrong question. Let me formulate another way.

If I recall well, the electrical law for capacitors is: Q = U x C = I x t

Where:
Q is the Load (Coulomb) - in french, la charge
U is the Voltage
C is the capacity (Farad)
I is the intensity (Amps)
t is the time (second)

A.- Is it correct/acceptable to apply this law to LEDs?

B.- Am I right to think that if I change the charging voltage of the LED, I'm going to modifiy its Load?

[skimask]

A.- Is it correct/acceptable to apply this law to LEDs?

B.- Am I right to think that if I change the charging voltage of the LED, I'm going to modifiy its Load?

1- I don't know if it so much applies to LEDs as it applies to the interjunction capacitance of a diode, so, yes, by definition, it does apply to LEDs.

2- If you change the charging voltage to the LED, I would think all you'd really change is the point at which the voltage dropping from the LED would trip the PICs pin input from logic high to logic low...(one setup charges to 5v, take awhile to drop to logic 0, other setup only charges to 3v, doesn't take near as long to drop to logic 0).

Why do you want to 'speed up' the discharge process?
What end result AREN'T you getting now that you really want?

Just trying to get a handle on what you're after...