How is it done?

[SurJector]

cool - thanks for the reply ... I'd been worried that this stuff was just scaring everyone / or giving them headaches!

You're not going to scare me with micro-electronics !

I did read some theory about using a resistor, to measure the current stored in the led... according to what I read you place a resistor as a short, across the led, and then measure the voltage across the resistor, then using I=V/R it will tell you the current being generated by the led

the problem with this is, if you want to also use the led as an indicator, your resistor needs to be high impedence so it won't short out the led... and then you are back to square one.

That's what I was trying to say as my second proposition. I don't have any PIC at home and much less protoboards so I can't try for myself, but out of thin air I'd try 10k, then 1k, then 100k to see if it gives anything.

Picked up a quad channel op-amp last night, of course they only had one.   I'm hoping the op-amp will give me more sensitivity since the leds are not very sensitive to their own light reflected from my fingers ... when I use a white piece of paper I get a lot better results.  I've also noticed (when I was more awake) that a 1ms sample time is causing a slight flicker of the leds... but reducing the sample time is causing terrible results in the sensitivity department

I'm sorta confused as to how I'll shoe-horn this op-amp in, putting it directly between the led and the pic will prevent the pic from being able to bias the led forward or reverse, that is unless I use a third control line from the pic... for each led

line 1 = adin ... connected to the output of the op-amp
line 2 = digital ... connected to the cathode on the led
line 3 = digital ... connected to the anode of the led

led cathode connected to op-amp in+ (yes, backward, i think)
led anode connected to op-amp in-

then I forward bias, reverse bias and then go high-z on both digital lines, leaving the led to discharge into the op-amp, sample the adc real fast, and then resume forward bias

whatcha think?

This sounds correct (as for the op-amp: you want to put the + where the 5V is when reverse biased). It's probably difficult to do without the third line; you can try to use only one line as a clock a reverse bias all the leds together, but that would be a terrible mess; as an alternative, you may be able to keep the anode at a constant potential (around 4V) with a simple resistor ladder and free a line (that's a little bit better than before).

Code:

     +5V
      |
     ---
 1R  | |    CL is current limiting resistor. R is an arbitrary value (not too high)
     | |
     ---
      |-CL---|>|------- line 1 (0V when on, 5V when charging, high-z when sampling)
     ---   |-   +|
 4R  | |  ----------
     | |   \      /
     ---    \    /
      |      \  /
      |       \/
     GND       -------- line 2 (always sampling)

Looks fishy !

[justDIY]

I tried a 10k and a 100k resistor, both were too much and completely drained the led before the adc had a chance to sample it

I did get a reading using the 100k and much faster sampling time (50 uS) however, the dynamic range was severely compressed, it seemed all the resistor was doing was attenuating the signal from the led, but not exagerating it.

----

I have upgraded to four leds in the array now, acting as four separate inputs.

a 750 uS sampling rate provides an acceptable range, and no visible flicker...  the increased light output of having two more leds has helped the array 'see' my fingertips a lot better, even with the faster sampling rate (down from 1000 us).

I need to make up some more cables to go from my pic protoboard to my breadboard, so I can run the full eight leds

Here's the active sensor array:



This is me moving my finger around the array:


The numbers are tenths of a volt ... so 32 = 3.2v, 8 = 0.8 ... Each color represents one of the sensors.

So, I need to figure out the analysis side; how do those differing values relate to where the object is in the array and how far it is above the array - sort of like a 3D touch-pad!

Also need to work on scaling the project to support an enormous sized array... My thoughts on that subject:

I need to be able to reverse the bias for individual leds, keeping all the others lit.

Maxim has a 16 to 1 'selector' ... but I wonder if it is latching (if that is the correct term)... that is, will the switch hold its state (high or low), even when it is not actively being driven.

the easiest (hah!) way I can think of is using an army of pics, something cheap like the 16f684 has 12 adc channels ... use it for adc and reverse bias ... and use something beefier like an 18f2xxx as the 'master control', each of the slave pics would also be accompanied with a 16 bit i2c port-expander, for driving the anode sides of the leds

well enough for now, time to fool around a bit more!

[sanjed]

then I forward bias, reverse bias and then go high-z on both digital lines, leaving the led to discharge into the op-amp, sample the adc real fast, and then resume forward bias

whatcha think?

That bit melted my brain.  Have you been watching a lot of Star Trek lately ? 

[wooferkiIIer]

Hey i am also investigating the subject. 

i've also read http://www.merl.com/publications/TR2003-035/ and it looks to
me you can use digital inputs for the sensing. The digital inputs of PIC's have a treshold value of about 0.8V. So you could measure the time needed to discharge the LED/CAP, as a value for amount of light.

with some clever programming, the digital inputs can even be used to make it an analog measurement.

I wil go experiment some this weekend myself. quite busy at the moment

Have fun and keep on freakin'

[Cabwood]

I believe you could use half the LED array to illuminate the other half, uniformly over the entire surface of the array. If you imagine a chess board where all the white squares are iluminated while all the black squares are being sampled, and then swap them so that now the black squares iluminate while the white squares are sampled.

Then another idea - since the sample procedure is so short, you could perform the above two cycles, creating a very short, imperceptible burst of light, leaving you with eons of time before the next sampling to do whatever you want with all the LEDs. Three cycles, then:

1) 2ms - Light whites, measure blacks
2) 2ms - Light blacks, measure whites
3) 20ms - make a picture with all the LEDs

Step 3 could be to switch on the LEDs which the previous measurements indicated detected light. so that the LEDs glow following the movement of anything in proximity, as in the video.

Another observation is that since the ilumination of one set of LEDs is only as long as the sampling of the other set, you can light them up really bright - perhaps with twice as much current as you would normally use. As long as the average power they dissipate is within limits, they should be fine, and this would surely improve performance in high ambient light.

Another ambient light fix: would it be possible to perform two measurements for each LED - one charging the LED's capacitance without any illumination from the other LEDs, and then another with illumination from the others LEDs? That way, I imagine the derived values are determined by difference in discharge times, rather than some static threshold.

I love this - I'm going to wrestle with my mind for weeks with this, I just know it. I simply have to figure out some multiplexing system to make all this easy for the PIC.

[Rob]

Welcome to the boards, Cabwood. I think you'll find some multiplexing discussion on the boards here.

http://forums.linear1.org/index.php/topic,748.msg4285.html#msg4285 for starters.

[jfmateos]

Dear justdiy:

I would like to study the source code of the PIC that you used in this 2x2 led sensor matrix. Where could I fing it?

thanks

I tried a 10k and a 100k resistor, both were too much and completely drained the led before the adc had a chance to sample it

I did get a reading using the 100k and much faster sampling time (50 uS) however, the dynamic range was severely compressed, it seemed all the resistor was doing was attenuating the signal from the led, but not exagerating it.

----

I have upgraded to four leds in the array now, acting as four separate inputs.

a 750 uS sampling rate provides an acceptable range, and no visible flicker...  the increased light output of having two more leds has helped the array 'see' my fingertips a lot better, even with the faster sampling rate (down from 1000 us).

I need to make up some more cables to go from my pic protoboard to my breadboard, so I can run the full eight leds

Here's the active sensor array:



This is me moving my finger around the array:


The numbers are tenths of a volt ... so 32 = 3.2v, 8 = 0.8 ... Each color represents one of the sensors.

So, I need to figure out the analysis side; how do those differing values relate to where the object is in the array and how far it is above the array - sort of like a 3D touch-pad!

Also need to work on scaling the project to support an enormous sized array... My thoughts on that subject:

I need to be able to reverse the bias for individual leds, keeping all the others lit.

Maxim has a 16 to 1 'selector' ... but I wonder if it is latching (if that is the correct term)... that is, will the switch hold its state (high or low), even when it is not actively being driven.

the easiest (hah!) way I can think of is using an army of pics, something cheap like the 16f684 has 12 adc channels ... use it for adc and reverse bias ... and use something beefier like an 18f2xxx as the 'master control', each of the slave pics would also be accompanied with a 16 bit i2c port-expander, for driving the anode sides of the leds

well enough for now, time to fool around a bit more!

[jfmateos]

Dear justdiy:

This is a video of the results I have got till now, but i would like to study your PIC source code to impove it.

Thanks

http://www.youtube.com/watch?v=l0joutRSC1M

[thebman80]

then I forward bias, reverse bias and then go high-z on both digital lines, leaving the led to discharge into the op-amp, sample the adc real fast, and then resume forward bias

whatcha think?

That bit melted my brain.  Have you been watching a lot of Star Trek lately ? 

LOL that went through my mind as well this looks like the begging technology of the Star Trek Holodeck. Light sensors interacting with your movement neat stuff even though I haven’t really understood half of what is going on in the processes technologically speaking.

[cadstarsucks]

Found this on del.icio.us:

http://mrl.nyu.edu/~jhan/ledtouch/

Dude is using an led matrix display as some sort of interactive touch sensor

I've watched the video again and again, but I haven't figured out how he's using the led as a sensor and an emitter at the same time?

In a common anode matrix, if you put a pull up resistor on the low side and a I/V circuit on the high side you will get a negative voltage out that is related to the amount of light hitting the LED.  A threshold comparator will then give you a presence output.  If you put column switches on the pull ups you can scan for presence during the column off times.

Dan

[drain]

Hello people, this is my first post  I'm spanish.

I'm triying to test this touch-led by myself, but i don't get anything.

I'm using a pic16f84, using different delays (1 us, 100 us, 1 ms, 5 ms, ....), different resistors (447 ohm, 1k, 10k, ...)...

in the oscilloscope I can see a small "curve" (dont know how to say it in english, sorry) like the capacitor discharge "curve" (xD)... and, as I think, it should change while i'm moving my hand over the led... but nothing happens.



Hope i explained well. thanks

[BVnursery]

At least more work is being done.

[jfmateos]

Hello Drain
Hola Drain

I´m also spanish
Yo también soy español

and now I am trying to emulate Jeff Han´s multitouch led matrix (http://cs.nyu.edu/~jhan/ledtouch/index.html).
You can find my work at http://www.todopic.com.ar/foros/index.php?topic=20510.0.
The problem with the oscilloscope/multimeter is that it charges very much the LED, so the slope down curve you are seeing is not the real one. The Led will discharge slower without the oscilloscope.

If you are really interested in the development of this project I would like to collaborate with you.

[drain]

Hola Jfmateos. Gracias por contestar.

Ya conseguí que funcionara. Tenías razon con lo del osciloscopio. Pero yo lo hacía mal. Leyendo un poco más me dí cuenta de dónde estaba el fallo: Yo ponía el pin en entrada (alta impedancia) y tomaba el dato, cuando lo que tengo que hacer es medir el tiempo que pasa desde que lo pongo en alta impedancia hasta que el dato llega a 0.

Haciendo distintas pruebas ya he conseguido optimizar el funcionamiento... ahora bien, me pasa lo siguiente.

Cuando uso una resistencia alta (200k) el led tarda poco en descargarse (unos 30 o 40 ciclos del pic)... pero no se ilumina a penas, debido a la poca corriente.
Si uso una más pequeña... (1k) el led brilla bien, pero tengo unos tiempos de caida mucho mas lentos (~2500 ciclos) con lo que el led pasa mucho tiempo apagado y se observa un ligero parpadeo.

La solucion imagino que es la intermedia. Sabiendo que lo que quiero conseguir es algo como lo que te explico mas abajo... Qué me recomiendas?

Sobre tu proyecto... El panel que estas haciendo es interesante, pero para mí no le encuentro ninguna aplicación. Lo que yo estoy intentando desarrollar es un dimmer digital para luces y sonido. Como este -> http://www.youtube.com/watch?v=YampMtX5hn0

Muchas gracias.

(a ver como traduzco todo esto al inglés ... xDD no te rias mucho, que mi nivel es muy normalito)

---------------------------------

Hi JfMateos, thanks for the reply.

Finally it's working. You were right about the oscilloscope. I have searched and now I know how it works. You have to measure the time that high-Z pin takes falling to 0V.

Doing a lot of tests, finally I optimized the result. But now I've got an issue:

If I put a high value resistor (~200k) then it takes ~30 cycles of the pic... but the led won´t shine.
And if I put a ~1k resistor... it takes a longer time (~2500 cycles)... and the led is OFF all this time, so the result is the led blinking.

About your project... this led matrix is very cool, but i have no real application for it. What I'm trying to develope is something like this ->
 http://www.youtube.com/watch?v=YampMtX5hn0

Thanks a lot.

[jfmateos]

Where are you using the resistor, in the LED´s  anode or cathode?

In my project I use the PIC ADC to read the voltage at the LED´s cathode 10 times, and the resistor is in its anode. This way I don´t have to wait for the cathode to reach its low state; I jus interpolate the discharging slope through that 10 measures.

Now I am using a 220 ohm resistor, but I will try different values to investigate your problem... Could you post your source code?

What kind of LEDs are you using? I use intermediate luminosity red leds.

That dimmer is very cool... I am looking forward seeing it finished.