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'[ee] What causes this LED effect?'
2016\09\17@223514 by Gordon Williams

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Hi,

I have an arduino nano where I am using a red LED for a light sensor.  It seems to be working well and has good sensitivity, but there is some effect that I don't understand.  I hope someone can provide an explanation.

The setup consists of an LED with a 220 Ohm resistor soldered to one of the legs.  The light/resistor than gets connected to pins 7 and 8 on the board through a pair of twisted wires.  I reverse bias the LED to charge it up then change the positive pin to an input and read it in a loop while counting how long it takes to go from high to low as the charge bleeds off the LED.  The more light shining on the LED the faster the pin goes from high to low and the lower the resulting count.  All this works as expected.

The thing that I don't understand is when I touch the twisted wires the count decreases indicating that the charge is bleeding off faster.  I understand that there is likely a capacitance effect from having my hand close to the wires, but I would have though that this would have increased the capacitance and therefore this should have taken longer for charge to bleed off and not shorter.  Does the fact that they are twisted have anything to do with it?  If I wet my fingers before I touch the wires it seems to become a lot more sensitive to touch as you almost don't have to touch the wires before you see the effect.

Thanks,

Gordon Williams




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2016\09\17@225351 by James Cameron

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Environmental RF induced into your body will be passed through your finger, changing the voltage unpredictably.

You may have a different result if you repeat the test inside a faraday cage, or change seating position or hip angle; thus your wavelength.

On Sat, Sep 17, 2016 at 10:35:12PM -0400, Gordon Williams wrote:
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2016\09\17@233019 by Gordon Williams

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Sitting down, standing up, etc. doesn't change it. It is very repeatable and steady.  But having one hand on the wires and the other touching my desktop case does!

Gordon


On 16-09-17 10:53 PM, James Cameron wrote:
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2016\09\18@003105 by James Cameron

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Try not to be so conductive?  ;-)

You might also look at the signal at the point you are touching, using an oscilloscope, to see if there is anything interesting happening.

On Sat, Sep 17, 2016 at 11:30:16PM -0400, Gordon Williams wrote:
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2016\09\18@062208 by Isaac M. Bavaresco

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Gordon,

Q = C * V    <=>    V = Q  / C

You have a fixed amount of charge (that is bleeding off quickly), so if
you increase the capacitance then the voltage will drop.

Isaac



Em 17/09/2016 23:35, Gordon Williams escreveu:
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2016\09\18@091706 by Gordon Williams

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Hmm, because the wires are tightly twisted wouldn't any noise be cancelled out?

When I touch the case and wires the count goes back to what it is without me touching the wires.

Gordon


On 16-09-17 11:30 PM, Gordon Williams wrote:
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2016\09\18@092745 by Gordon Williams

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I'm a beginner at this.  If my hand adds capacitance and I charge up to the same voltage then I would have more charge.  The rate of the charge bleed of is dependant on the light shining on the LED and a bit on the high impedance of the input pin.  My thought was if it was a capacitance effect then the count should increase with my hand on the wires, which is opposite of what I am seeing.

Gordon


On 16-09-18 06:22 AM, Isaac M. Bavaresco wrote:
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2016\09\18@113526 by Allen Mulvey

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If you apply a capacitor to a live circuit the effect
varies. A discharged capacitor would have a very different
effect than one fully charged.

I suspect it is nearly impossible to determine the state of
charge of your body at any given time.

Allen

> {Original Message removed}

2016\09\18@143306 by Isaac M. Bavaresco

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Em 18/09/2016 12:45, Peter Onion escreveu:
> On Sun, 2016-09-18 at 07:22 -0300, Isaac M. Bavaresco wrote:
>> Gordon,
>>
>> Q = C * V    <=>    V = Q  / C
>>
>> You have a fixed amount of charge (that is bleeding off quickly), so if
>> you increase the capacitance then the voltage will drop.
>>
>> Isaac
>>
>>
> I think this only the case if you touch (and add capacitance) during a
> discharge cycle ?
>
> At the start of the next charge cycle the voltage will be restored to
> maximum and the discharge should be slower due to the extra hand
> capacitance.
>
> PeterO

That seems correct. In the next cycle the extra capacitance will get
charged too. Need to think further.

Isaac

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2016\09\19@023733 by rubenjonsson

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Try a shielded wire where the shield is connected to the case.

/Ruben

On Sun, 18 Sep 2016 09:17:05 -0400, Gordon Williams <spam_OUTgwilliamsTakeThisOuTspamncf.ca>
wrote:
> Hmm, because the wires are tightly twisted wouldn't any noise be
> cancelled out?
>
> When I touch the case and wires the count goes back to what it is
> without me touching the wires.
>
> Gordon
>
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2016\09\19@025224 by James Burkart

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Curious, why are you using an LED as a light sensor?

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On Mon, Sep 19, 2016 at 12:37 AM, <.....rubenjonssonKILLspamspam@spam@bredband.net> wrote:

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2016\09\19@025646 by James Burkart

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Did you consider that your body temperature may be playing a bigger role in
your findings than the capacitance?

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On Mon, Sep 19, 2016 at 12:52 AM, James Burkart <.....jamesKILLspamspam.....burkartstudios.com>
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2016\09\19@032515 by James Cameron

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Don't know about Gordon, but for us we needed a sensor so we could
turn off a display backlight on a reflective LCD panel when in
sunlight, but we didn't need it enough to pay for a real sensor, LEDs
were cheap enough for the job; and already on the assembly
bill-of-materials.

http://wiki.laptop.org/go/Light_Sensor

On Mon, Sep 19, 2016 at 12:52:22AM -0600, James Burkart wrote:
> Curious, why are you using an LED as a light sensor?

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2016\09\19@093513 by Gordon Williams

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The LED is already there to indicate whether a mister is on or off.  It can also serve to tell me whether it is sunny, cloudy or night.  Zero cost with a few lines of code.  Simple and works really well.

Gordon Williams



On 16-09-19 02:52 AM, James Burkart wrote:
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2016\09\19@093846 by Gordon Williams

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Happens really fast with almost no, or no touching of the wires. Also far away from a junction.  Temperature is very unlikely.

Gordon Williams


On 16-09-19 02:56 AM, James Burkart wrote:
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2016\09\19@094846 by Byron Jeff

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On Mon, Sep 19, 2016 at 12:52:22AM -0600, James Burkart wrote:
> Curious, why are you using an LED as a light sensor?

I've done this a time or two, so I'll offer some input.

First the seminal paper on th subject by Dietz et. al.:

http://math.hws.edu/vaughn/cpsc/336/docs/led-sensor.pdf

A search for iDropper LED will pull it up.

The basic idea is to use an ultra cheap, already connected sensor for low
speed bi-directional communication. I started investigating it when I had a
timer based controller that needed to be irregularly reconfigured. The
previous iteration had worn out its config button. It already had an
indicator LED. With this setup, it could be configured by shining a bright
light onto that indicator LED. A $1 LED flashlight worked great.

The advantages are that it's cheap, it's touchless, and it offers a modicum
of tamper prevention because there is no obvious buttons to mess with.

The iDropper concept in the paper offers modulated communication for
options such as LED smart keys.

The point is that it uses what is likely going to be available as an
indicator anyway. It's interesting that you can still use the incoming
sensor even if you are using the LED as an output indicator. And the cost
is exactly one extra I/O pin for reverse biasing the LED.

Hope this gives you some insight...

BAJ

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2016\09\19@132631 by Dwayne Reid

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Is there any chance that your body is picking up an AC electric field and that voltage is being coupled to the wires?  An AC voltage coupled to the wire feeding the input will cause the apparent threshold voltage to shift.

Quick way to find out is either use a scope probe to look at the voltage on your hand **or** ground yourself to the circuit ground and see if that changes anything.

dwayne


At 08:35 PM 9/17/2016, Gordon Williams wrote:
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2016\09\19@164924 by Brooke Clarke

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Hi:

Could it be hum pickup from the AC mains?

-- Have Fun,

Brooke Clarke
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2016\09\19@172130 by Gordon Williams

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Wouldn't I see count number jump all over the place if it was this sort of interference?  When I grab the wires the count is very steady.

Gordon Williams



On 16-09-19 04:49 PM, Brooke Clarke wrote:
> Hi:
>
> Could it be hum pickup from the AC mains?
>

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2016\09\19@172542 by Gordon Williams

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Thanks for everyone's thoughts.  I thought there might have been a simple explanation for what is happening.

It is not really an issue to me because everything will be in a box so nobody will be touching the wires.

A scope would be required to look at this further, but for now I will have to leave it as a mystery.

Thanks again,

Gordon Williams



On 16-09-19 01:26 PM, Dwayne Reid wrote:
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2016\09\20@202411 by IVP

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> First the seminal paper on th subject by Dietz et. al.:
>
> http://math.hws.edu/vaughn/cpsc/336/docs/led-sensor.pdf

Thanks for that BAJ. I passed it on to some students who are already
plotting about secret data ports and keys on their devices !!

Joe


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2016\09\21@032925 by David C Brown

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So why has LEDCom not conquered the world?

On 21 September 2016 at 01:23, IVP <RemoveMEjoecolquittEraseMEspamEraseMEclear.net.nz> wrote:

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2016\09\21@041127 by IVP

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> So why has LEDCom not conquered the world?

Don't know about LEDCom but I have been told that some appliances
(eg refrigerators) have LED service comms. And there's Li-Fi

https://en.wikipedia.org/wiki/Li-Fi

http://www.computerworld.com/article/2983747/emerging-technology/that-theory-about-led-lightbulbs-transmitting-data-its-true-li-fi-is-here.html

Joe


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2016\09\21@123228 by John Ferrell

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Is this topic leading toward a led screen sensor array?


On 9/19/2016 9:35 AM, Gordon Williams wrote:
> The LED is already there to indicate whether a mister is on or off.  It
> can also serve to tell me whether it is sunny, cloudy or night.  Zero
> cost with a few lines of code.  Simple and works really well.

-- John Ferrell W8CCW
   Julian NC 27283
 It is better to walk alone,
than with a crowd going the wrong direction.
                  --Diane Grant


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2016\09\22@001118 by RussellMc

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On 22 September 2016 at 04:32, John Ferrell <RemoveMEjferrell13TakeThisOuTspamspamtriad.rr.com> wrote:

> Is this topic leading toward a led screen sensor array?
>

​Let's assist it :-)

Ken (offlist) said:

I forgot to mention that on YouTube there is a video from Jeff Han of a
multiplexed 8x8 LED array being used for both display and optical image
sensing.   As far as I can tell it worked by illuminating each of the LED's
in turn, and then sensing the reflected light on the adjacent LED's.  It's
quite a compelling demo when you see it.

The original video seems to have been removed from the web but the first
photo on this page looks like it's a still taken from it (IMMSMW):

hackaday.com/2006/02/21/low-cost-sensing-and-
communication-with-an-led/

Here is another similar (but less impressive) one:

https://www.youtube.com/watch?v=_CCndD10OSM

The idea has since been picked up by others who have made quite large
versions  - although I think some of those use separate proper optical
sensors interspersed in the LED array  - possibly to get around Jeff Han's
patent.

I had a good look for the video and all links seem dead.
There are TED talk ones of him and, presumably, that screen.

_________________________

​Ken also said:

Here is a paper on the subject  - which describes in detail how to solve
the drive-coupling problem that most who have tried to replicate Jeff Han's
approach have run into:

citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.
667.6422&rep=rep1&type=pdf



<citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.667.6422&rep=rep1&type=pdf>
     Russell
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