LEDs as sensors!

[66Ralphy99]

Thanks for that - wanna make a touch remote controller for Winamp, for when I am in the shower (will be sealed in a box, with windows for the leds to make it water resistant). Gonna use a picaxe, then hack a cheap usb numeric keypad (unsolder keys & replace dem with relays) to give an input for next or last track or volume up or down. Coool!

[100roberthenry]

will this be affected by daylight...and darkness?...

[100roberthenry]

hi im looking to just fade 1 led up and down by this type of sensor...any ideas..i want to be able to fade a led up and down with my foot?... :)

[CARLHALEY1964]

can you do the same with Flexi Force Sensors 

[kiriakoskakos]

Type PRIZE before youtube and hit enter

[GustavoFire]

00:54 OVNIS!

[herecomesurman]

can you please give us the schematic diagram of this project and it's components? thank you in advance!

[eduardo17cova]

my friend could you tell me how to do this proyect please

[msplace08]

very nice, good job!

[extremisto47]

ш can't get how it is working. Is there a light sensor and you using ADC to sense a light???

[McGuywer]

When one LED is on, the other one is used as input sensor, then switch places. Read the description.

[extremisto47]

what sevsitivity of LEDs in mV??? I never heard leds can be used like sensors... hm

[McGuywer]

Don't know numbers, but certainly less sensitive than a true photodiode. Any semiconductor can manifest some degree of photovoltaic effect. I've removed the metal casing of small transistor and under light some voltage did buid up. The solar cells are also made of silicone junctions.

[extremisto47]

I have few Soviet transistrors in metal. If remove a part of its body they became a fototransistors. Sensitivity depends of its amplification characteristics.

[atpinternational]

can i get instruction on how you made this

[Lexakron]

Your pumping waaay to much juice threw those two LEDS

[DjSprinks890]

Wow thats pretty cool mate! i never new that L.E.D's acted as capacitors?? are they photo L.E.D's tho? ie IR L.E.D's?

Might have to go maplins and get some high bright ones and try! plus gives me an excuse to see the fit girl that works in there on wednesday! Lol thanks! :)

[jollino]

No, you don't even need to use a LED in forward bias configuration at all. The point is that any LED _generates_ a voltage when it _receives_ light (just like an oscillation generates a sound and a sound generates an oscialltion, as in crystal glasses being broken by a sound). You can try this by connecting a voltmeter across the pins of any *disconnected* LED, and putting the LED under a bright light.

[jollino]

What we are doing is essentially disconnecting the led from the circuit, quickly connecting it to the ADC and measuring whatever voltage is across its terminals. You will inevitably lose something, because as fast as your MCU may be, it will still need to set the pins and start the ADC reading.

So, in short: the 'LED on' part is entirely unnecessary: you can keep another LED always on, and use the 'input' LED to continuously start ADC readings.

[jollino]

That's actually what Roland does on its VA-7 and VA-76 keyboards (I have a VA-7 :)). It was two infrared LEDs that allow you to change midi parameters in real time by waving your hand above it. The whole ordeal is marketed as D-Beam, but it's the same thing I was describing. Pointing a digital camera to the thing reveals that one LED is always on while the other is off (presumably because it's reading).

[jollino]

Note that LEDs are most sensitive to the same wavelength that they emit, so you should use LEDs of the same kind!

[skierplaterandy]

i think the two leds are on a seperate cycle

[skierplaterandy]

i figured, cuz its hard to switch them on and off at the speed of light XD

[skierplaterandy]

but then would you have it on a on/off cycle so you can take a reading without interference? or are you trying to figure out range from the light that bounces back?

its not really that hard to prevent flicker is it? why would it be flickering?

[skierplaterandy]

use pwm (pulse width modulation)

change it so the on pulse is longer than the off one.

and yes make it high frequencey

[Fuckyeahimover18]

yea, when your using leds as inputs it will be highly sensitive. because it has a very little difference. if your worried about interfearence you should use photo resistors and just hide them.

if its near an appliance then definitely it will get interference too.

what exactly are you trying to do with it though?

[skierplaterandy]

wait what?

[jollino]

That's correct. I didn't even use a specific reference voltage; I found that ADC readings at the top and the bottom of the scale were not as reliable, as if it lost some accuracy.

I don't have the schematics or the code anymore, so I can't tell for sure what frequency the LEDs are working at. It's probably in the order of a few dozen milliseconds.

If I were to re-do this, I would use a few transistor to control the LEDs, to have higher current in shorter pulses (= higher granularity).

[rdoetjes]

Wow that is rather cunning I never knew that fact about LED's functioning as a capacitor when flipping the current around briefly. NICE STUFF 5 STARS

[melsaidy]

it is amazing ciruit could i get the schematic of this project and the SW you used please.

is it ciruit can used to detect motion for long distance

thanks for advance

[dtfbandit]

don't believe it is for long distance since the light must be reflected from one led to another....

1. you must have proper magnification lences for the receiver

2.and the light must be coherent like that of a laser

[Berserk879]

im assuming you used C while making this?

what did you do to detect the voltage of the leds?

would you post your code?

[hahaiha]

Nice sensor. Can I get the circuit of this. Is this possible with 6 or more Leds. I need i it for atleast 6 Leds

[theEphimeralADub]

wow that was some crazy IR you captured running through the LEDs, are they hooked to an IR or Photo sensor?

[JRproducts]

This is great! Can you do this as effectively with only one sensor LED or does it work better with two?

Does pulsing forward bias effect the reading? If the LED was off, (reverse bias and high-impedance) would this still be possible?

Thanks,

Justin

[jollino]

I don't think that using only one LED will work, but I cannot say that for sure as I haven't tried that.

The idea is that the LED 'sees' the reflected light coming from another LED with the same wavelength, so one has to be forward biased and the other has to be reverse biased (and then disconnected) at the same time.

Using PWM for the 'transmitting' LED would work, as long as the 'receiving' LED used the same frequency. This is, in a way, similar to how remote controls work. :)

[JRproducts]

Thanks for the replay. I kinda figured that is why you need two.

What I meant was could you use the LED as just a sensor and never forward bias it? That would sense the ambient light right? Also as you said each LED is most sensitive to its color, so could you use a RGB LED to detect colors?

[jollino]

Yes, you could just alternate between reverse bias and high impedance; I am not so sure about sensing ambient light. The main problem is that LEDs tend to be sensitive about their own specific wavelength, so the possibility of detecting other shades would decrease. I haven't tried that too.

As for RGB leds... that might work, but you'd need to cycle through red, green and blue, and possibly calculate an average.

[ODFH]

Hello, My name is Oscar and from mexico city.

I have a two question, What kinds of sensor used? and What happens if a PIC16F84 it stopped sending signals to the pic if this already is running?

I would appreciate your prompt response

I have a project of a robot and wanted to know whether using a sensor that resivir signal to activate a relay and the making of crystal quartz Logoff

[programwithpassion]

I see. Thanks for your replies.

When you shine light on the reverse-biased LED, wouldn't the increased photocurrent discharge the charge on the junction (resulting in a lower voltage), as oppose to giving a bigger ADC reading?

I assume your bar graph displays the ADC range 0 - 255?

In my single-LED setup, I read lower ADC readings via serial port when I shone light on the sensor LED.

[jollino]

Oh, it seems youtube ate my reply. Yes, you are right -- I checked the code and the first thing I do with the ADC reading is subtract it from 255, effectively "reversing" it (I am using only 8 bits for the ADC as it's a proof of concept). The bar graph is handled by simply calculating how many leds need to be turned on (I have 8 leds, so each led is worth 32; I just divide my value by 32) and I turn them on accordingly with a for loop.

[P1Cpro]

Very nice!

Esp the bar graph.

Shouldn't the reading decrease when you hover your fingers over it, since more light is reflect back on the sensor LED?

How does this implementation work (in terms of code)? I tried something similar a couple of months ago

[jollino]

The code is fairly simple: you have to alternate the LEDs between being forward biased (Vcc on anode, GND on cathode), reverse biased (GND on anode, Vcc on cathode) and disconnected (tri-stated on both poles). At that point you start an ADC reading and measure the voltage you receive: the more light a LED sees when it's disconnected, the higher voltage it will shows across its poles. You can try this by using any led connected only to a voltmeter moving it under a table lamp. (continued)

[jollino]

You could use one led constantly on and the other alternating between reverse bias (charging) and high impedance (disconnected). The key here is letting it charge and quickly disconnect it to measure it. If you wanted to go all digital, you could measure the time it takes to discharge, but that would be tedious and imprecise, while ADC readings are usually quicker (and make the code much cleaner). However, digital readings could be used in an instantaneous fashion, for instance (continued)

[jollino]

if you wanted to use this system to have two led-equipped devices to communicate, in a kind of ultracheap serial wireless connection. The main issue would be making it synchronized, but using two leds (one for RX and one for TX) and USART outputs of any MCU would leave the burden of it to the hardware. In any case, even by reinventing the wheel, it would be a software wheel. Imagine being able to access the 'debug menu' of a television set by merely placing a terminal close to its standby led!

[P1Cpro]

I see. Thanks for your replies.

When you shine light on the reverse-biased LED, wouldn't the increased photocurrent discharge the charge on the junction (resulting in a lower voltage), as oppose to giving a bigger ADC reading?

In my single-LED setup, I read lower ADC readings via serial port when I shone light on the sensor LED.

Thats why I was thinking the bar graph

[antirealm]

cool man

[Anomalybilly]

Brilliant job, i would love something like this on my g15 keyboard :D. Perhaps send your idea to logitech?

[jollino]

Well, I'm sure Logitech knows about this. It's a little known but quite easy concept, in fact my Roland VA7 keyboard (music keyboard!) has implemented the same thing using a couple of infrared leds and gave it the fancy name of "D-Beam". Looking at the leds with a cheap camera (the cheaper the better, as they are more sensitive to infrared light), one can see how one is emitting light, and the other is not. :) In my case both appear as emitting light, but they're actually pulsing.

[hopeitshappy]

great work

[jollino]

Thank you :)