Low-cost sensing and communication with an LED

posted Feb 21st 2006 11:00am by Eliot Phillips
filed under: misc hacks

ledtouch_photo

LEDs are extremely common in electronic devices. They are used as light emitters, but can also be used as light detectors since they are photodiodes. By quickly switching between light emitting and detecting, you can use the LEDs to determine the ambient lighting and even do bidirectional communication. MERL has a good paper covering the basics of how this system works and how they used it as a “last-centimeter” communication device. The system can be implemented using one LED, a resistor, and two I/O pins. So, it could be used cheaply in almost any device. The microprocessor quickly switches the LED between emitting light, detecting light (LED acts as a charging capacitor), and measuring the discharge rate of the LED to determine light level. Jeff Han has a neat video demo of how this system can be used as a proximity sensor.

UPDATE: [hawkeyeaz1] pointed to a blog covering one person’s investigations into LED sensors.

[thanks branen]

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Comments [67]

Reader Comments

Lame, HackADay ain’t what it used to be. Before they’d publish hacks that an average guy could do with just a minimal understanding of electronics, and some extra stuff as well for guys who were far more advanced. That was great, and half of the hacks were fun things like how to get around this or that. Now its just all stupid stuff like mousepad coolers and technical reports on LED’s with no actual hack.

Posted at 11:06 am on Feb 21st, 2006 by Straus

I agree that’s not a hack that’s just a lame report on somebody passing time with LEDs and the ball and plate thing from the other day is the same.

Posted at 11:27 am on Feb 21st, 2006 by Dacidboome

i’m sorry straus that you’re stuck in ignorance, read up a little – this is near ground-breaking stuff.

I had no idea an LED could hold a charge. Simply by reading the RC time from it, you can use it to sense light. Wondering if the BASIC Stamp RCTIME is quick enough to handle this.

Posted at 11:33 am on Feb 21st, 2006 by grendelt

Oh no, its so HARD to wire up an LED a resistor and a uC. I think this is a good hack, I had never thought of using an LED as a sensor instead of an output. I may be able to use this system in a robotics project I am working on.

Just because you don’t have a use for this “hack” doesn’t mean everyone else on HAD doesn’t.

Posted at 11:34 am on Feb 21st, 2006 by Dax

Get over yourselves, people. This hack is great, it’s one of the coolest things I saw lately, and I saw a lot of them. It’s a LIGHTING device that is used to SENSE movement. Now, if this isn’t great, I don’t know what is. Plus, it uses a LED, one of the most common devices found out there, in ways that others didn’t even dreamt of. For me, it clearly beats downgrading the PSP firmware. No offense.

Posted at 11:50 am on Feb 21st, 2006 by bogdanm

Not really groundbreaking (as the NYC experimenters note, this is mentioned in the 1977 Forrest Mims book), but a good primer on an often forgotten topic. At least Hackaday isn’t posting daily about how to crochet your own pirate robot ipod cozy like some other publication (cough, Make)

Posted at 11:57 am on Feb 21st, 2006 by Oog

This has been featured on other sites similar to hack-a-day, but it is no less cool. Since I saw it not long ago (the multi touch display mention the other day is by the same group), I’ve been looking for resources on it. I have found one guy who did this after originally seeing the article, and he has blogged about his experiences, and has schematics, etc. His blog about this is here: http://projects.dimension-x.net/technology-and-projects/ledsensors/

I am personally wondering if LEDs as sensors can be used as sensors for glass. The Multi touch display works by frustrated total internal reflection, but requires a camera on the back. Why not use LEDs on the edge (like the MT-FTIR suggests – http://mrl.nyu.edu/%7Ejhan/ftirsense/index.html) but a mirror on two edges so the sensors can sense when something touches the screen (causing a dark spot where the light is reflected down, not back at the LED)…. Kinda hard to explain without a pic. :P

Posted at 12:00 pm on Feb 21st, 2006 by hawkeyeaz1

hmm, its sensing multiple things at once… could this be a breakthrough in multi-point sensing tech?

Posted at 12:02 pm on Feb 21st, 2006 by drumr

“I had no idea an LED could hold a charge.”

Ditto, and I’ve got an electrical engineering degree. You’d think they would have mentioned it.

Anyway, I’m sorry the first two commenters need their “hacks” springing fully formed from this website like Athena from Zeus’ forehead, but some of us don’t mind learning new things that we can apply to our own projects with a little work.

LEDs as photosensors? Awesome!

Posted at 12:09 pm on Feb 21st, 2006 by furtim

I was thinking that you could use this as a type of biometric password device. If you could interface this with usb (or serial, easier), build a driver that detects each led as a part of a matrix, then by simply doing a getsture on the pad while detecting it, waving a certain way or giving it the bird, it would detect that as the proper password. Pretty cool if you ask me.

Posted at 12:11 pm on Feb 21st, 2006 by grayskies

Also, the merl link still has a dead pdf link. So I’d refer to ‘justDIY’s’ blog I mention above

Posted at 12:21 pm on Feb 21st, 2006 by hawkeyeaz1

nvm on the dead link :P

*examines konqueror*

Posted at 12:34 pm on Feb 21st, 2006 by hawkeyeaz1

Actually, for those of us who are very attentive readers, we already know that LEDs can be used as light sensors to track the sun

http://www.hackaday.com/entry/1234000557070102/

Extremely cool! I like LEDs :D

Posted at 12:41 pm on Feb 21st, 2006 by jared

pretty cool with what I imagine to have millions of uses… Get some tiny leds on a thin circuit board with plastice over it and you’ve got a ultra slim keyboard…

Posted at 1:04 pm on Feb 21st, 2006 by LogOff

Very interesting, lots of uses.

Posted at 1:17 pm on Feb 21st, 2006 by matt

Well, i agree, it is a good hack (and it is a hack), however generally speaking these hacks all have some feasable albiet unconventional uses. This lacks that. It isn’t even partially economical to use LED’s for an input device, unless total stealth is required (secret agents needs keyboards too i suppose). I guess it would be a cool way to replace your pressure sensative tracking surface on your mouse, or it could make a neat tablet, but from a cost standpoint it just plain isn’t economical to mass-produce. Sure, it’s cheaper then BUILDING your own pressure sensative tablet, but it isn’t cheaper then BUYING one. And is this even all that accurate? Sure, it has loads of pressure layers to be abused, but you would need alot of little LED’s arranged in a dense manner just to get what you want, and even then, you have a bright red (or whatever color you choose) light.

I guess if you manipulated it in such a way that you could use this to add real hand gestures to your computer it would be cool and cost effective since there is no real product out on the market for that, but in comparason to all of the current interface devices it isn’t all that great.

Don’t get me wrong, it’s cool, it’s neat, and if i had the time i sure a hell would do it myself, but it isn’t practical, atleast for the time bieng.

Posted at 1:40 pm on Feb 21st, 2006 by Paragraph

Straus, Dacidboome, maybe you two have forgotten that this site is driven by reader submissions. Instead of getting pissy, maybe you should find or make something cool.

Just a thought…

Posted at 1:43 pm on Feb 21st, 2006 by TNS

One of the coolest uses of LEDs I’ve seen in a while.

I love the stuff that gets posted here but most of the time is just “oh that’s kind of cool… completely useless to me but kind of cool” This is actually something I can use.

Interesting enough I think I saw something just like this only a few days ago. I happened to be in Redmond visiting a friend and stopped by the MS visitors center and they had a display with 2 large LED arrays pointing at a piece of glass with a map of the campus projected on it by a separate projector. It responded to hand gestures on or near the glass that allowed you to move, rotate, and zoom the map. considering I didn’t see anything in the apparatus outside of the Projector and the LED arrays (both plugged into a PC) I wouldn’t be surprised if the LEDs were being used in a similar fashion to collect the hand gestures.

Posted at 1:48 pm on Feb 21st, 2006 by Twistedsymphony

logoff, i was thinking exactly the same thing as you.

Posted at 1:56 pm on Feb 21st, 2006 by myst

It seems to be a learning curve is a lost hope among some readers.

Posted at 1:58 pm on Feb 21st, 2006 by Ryan

hey, LEDs, photodiodes, and solar cells are the same thing, as any engineer ought to know. They’re just optimized for different properties.

I knew this was basically possible — I think I probably read the Forrest Mims thing as a kid, come to think of it — but what makes this cool (and it is cool IMHO) is the clever way of hooking it up to the micro.

Also, that LED-array touch sensor demo is just slick.

Posted at 2:11 pm on Feb 21st, 2006 by Wim L

Ya I agree that this hack is even more interesting than the FTIR display because I think i have a chance of doing this one myself. I was about to order some LEDs for an xbox I’m modding so I guess I’ll be ordering some extras. If you check out the parent site of this and the FTIR post: http://mrl.nyu.edu/%7Ejhan/ you’ll see they’re working on some really freakin cool stuff.

Off-topic:
Twistedsymphony – are you the Twistedsymphony on the xbox-scene forums?

Posted at 2:50 pm on Feb 21st, 2006 by Ben

Anybody else wondering if this can also run in infrared ? :) By the way, #10, you pointed out one of the first thoughts I had while reading this. It’s definetely doable.

Posted at 2:51 pm on Feb 21st, 2006 by Bogdan marinescu

From everything I’ve read about it, I don’t see why not, as LEDs can detect the freq they put out and lower (red detects red, yellow, green… yellow detects yellow, green but not red),.. ROYGBIV, where IR is to the left or R.

http://www.sensorsynergy.com/helpfulhints.htm
UV likely would work as well, but it would be awefully hot…

Posted at 3:22 pm on Feb 21st, 2006 by hawkeyeaz1

This is sweet. I’m going to build myself an interactive multipoint touchpad.

I suppose this works for OLEDs, too? Imagine a flexible transparent display with multipoint touch recognition. The future is now.

Posted at 3:41 pm on Feb 21st, 2006 by bugloaf

Not a hack – i learnt this when i was 9, wat happened to the old hackaday, stop linking outside and some real hacking

Posted at 4:20 pm on Feb 21st, 2006 by Alex CD

Awsome, down with the haters. Projects like this is why i come to hackaday. Who cares about xbox and psp mods.

Posted at 4:29 pm on Feb 21st, 2006 by koft

This is cool! I have a boatload of those matrix led things and never realized I could use them as an input. I’m thinking of putting like 5 by 5 of them together now to make a little drawing table. WOuld love to have seen the source code for the pics on that though to save some time duplicating the effort.

Thanks!

Posted at 4:58 pm on Feb 21st, 2006 by hawse

http://forums.linear1.org/index.php/topic,445.0.html

11th post the guy gives the PIC source (at least for an initial version).

Posted at 5:23 pm on Feb 21st, 2006 by hawkeyeaz1

….star trek keyboards?

Posted at 5:30 pm on Feb 21st, 2006 by alex k.

Some other uses of led as a sensor ,for sun tracking!

http://www.redrok.com/electron.htm

Posted at 5:33 pm on Feb 21st, 2006 by dem

hi all – thanks for the linkage!

it has been a while since I’ve worked with this project – but if anyone sees anything they’d like on my blog (hires pics, etc), incl source code, just drop me a note (g-mail gordonthree) … the code is nothing fancy, and its written in Proton Plus Basic for PIC microcontrollers.

Posted at 6:03 pm on Feb 21st, 2006 by justdiy

“I suppose this works for OLEDs, too? Imagine a flexible transparent display with multipoint touch recognition. The future is now.”

I did imagine it… And now I have to change my pants.

Since OLEDs operate on the same basic physical property — a p-n junction created by a boundary between two different semiconductor materials — and just are built and applied in a different way, I assume you probably could implement a multitouch interface with them. Which would be crazygonuts awesome.

Posted at 6:11 pm on Feb 21st, 2006 by furtim

H.A.D. rocks……this is a great article.

Posted at 6:35 pm on Feb 21st, 2006 by Wurx

what was up there awhile ago? ”
Microsoft OLE DB Provider for ODBC Drivers error ‘80004005′

[Microsoft][ODBC SQL Server Driver][TCP/IP Sockets]SQL Server does not exist or access denied. Line 530 ”
hmm?
Hackaday — hacked?? :)

Posted at 11:49 pm on Feb 21st, 2006 by Ty

That oled version would be slick! Right now a big limitation with touchscreens is the touchscreen overlay. This reduces the visibility of the screen greatly.

Posted at 12:04 am on Feb 22nd, 2006 by tuckie

Looks like this could have many neat applications. Strange how some things were discovered long ago but not many know about it…

Posted at 12:08 am on Feb 22nd, 2006 by Alan

I’m just thinking out loud on this, but could this make things like the screens from “1984″?

Posted at 1:31 am on Feb 22nd, 2006 by sully

Read about LED keyboard a year ago. It looked nice but the probable cost wah increadeble. I don’t think that this guy will be well-accepted by customers due to its cost.

to AM ET by sully: do you really what to have such a thing at your apartment? Can’t believe this :))

Posted at 6:03 am on Feb 22nd, 2006 by *mobile*

had – Please ignore my ignorant peers. Hacks are not “getting around this or that”. This particular post has a lot of applications (for your _creative_ readers) and the ball and plate one had a few interesting “hacks”. It had object recognition, real time feedback, etc.

Posted at 7:36 am on Feb 22nd, 2006 by bc

LED’s photo voltaic properties can be used to design a simple robot that would find the brightest local spot. Connect two LEDs to a comparator and use the comparator output with an inverter to drive two cheap DC pager motors.

Posted at 8:20 am on Feb 22nd, 2006 by Victor

#22 – Thats funny that you posted that right after me because the first thing I thought of when I started shopping for LEDs was to use IR! The one downside to using this in a practical application that I can see is that it will light the whole room up red.

Also does anyone know if there are even any oled panels (no matter how small and what inputs) available at all to do any testing? I know its a fairly new tech so I don’t know how available it is. Just thinking about a 1/2mm thick display/touch screen makes me want to start building right now.

Posted at 8:42 am on Feb 22nd, 2006 by Ben

Now to the important stuff: is a Basic Stamp 2 fast enough to handle this?

Posted at 9:03 am on Feb 22nd, 2006 by Mikael Mannberg

i dont see how this is worthy of hackadays attention, and the other multitouch display han is working on gets dissed a few posts back.

as far as the oleds, im pretty sure you can buy them at mouser now.

Posted at 9:08 am on Feb 22nd, 2006 by JSlice

If you could make it in blue, and set up an algorythm for security entry – it would be a very cool door lock

Posted at 12:32 pm on Feb 22nd, 2006 by Alex CD

is there a schematic or block diagram available for this? I would like to build it as a science fair project.

Posted at 1:38 pm on Feb 22nd, 2006 by rick marshall

I don’t see why people think that this (and other electronics construction projects) are not worthy of being on hack-a-day.

But I also don’t see why three of the top commented on posts are “HOW-TO: PSP 2.00 to 1.50 downgrade (146),” “How-to: PSP 2.00-2.60 homebrew with eLoader (76),” and “Xbox 360 ATX power supply (73).” I am usually rather dissapointed when I see things like this posted. I’m not saying they don’t qualify as hacks, they just aren’t very interesting.

Posted at 3:33 pm on Feb 22nd, 2006 by IMWeasel

Rick Marchall: Look at justdiy’s blog mentioned in the update. Also, justdiy mentions in comment #32 if anyone wants schematics, source code, etc to email him.

Posted at 5:04 pm on Feb 22nd, 2006 by hawkeyeaz1

This is indeed a very cool hack!

Would it be possible to just read the leds at a certain interval and display something the rest of the time? That would make a very cool keyboard, that could displat information as well.

And just imagine, a keyboard wich could be transformed into a gigant touchpad with the press of a button… That sure would be something :)

Posted at 5:58 pm on Feb 22nd, 2006 by Ludw

wow thats friggin awesome never new you could use a led as a sensor to. learn something new everyday :-D

Posted at 8:09 pm on Feb 22nd, 2006 by int8088

wow, this is awesome! there is so many uses for this and the possibilites are endless. I didn’t know that LEDs could recieve too. That’s truly awesome, i agree with imweasel, i see what you mean!

Posted at 8:38 pm on Feb 22nd, 2006 by William

It’s all a matter of opinion people. The techno-nerds want their complex hacks anc to learn something new, the wannabe lazy hackers just want something cool, quick, simple, and useful.

Both get what they want eventually, and both types of submissions (easy and fun/ complex and educational) are perfectly valad hacks.
I’m a lazy wannabe hacker yet I enjoy learning a thing here and there. So for the love of god,stop whining and deal with it.

Posted at 12:38 am on Feb 23rd, 2006 by Joe

firstly: i don’t care if it’s not economically viable as a replacement for tablets, keyboards or whatever, this thing is awesome. i want one in blue that does mathmos-style pulsing effects while im not using it, as a secondary input device that looks cool. at least while i’m too poor to afford an eluminex keyboard, that is :)

again, to all the people pointlessly dissing this hack because it’s supposedly too complex or has no market value at the moment- how economically viable is the psp downgrade, the Xbox beowulf cluster or the geodesic cardboard tent? and how likely is the average hacker to hand build something as involvedas the tiny mp3 player or an r/c tank made from a printer? the purpose of this site isn’t to showcase the next big thing in computing/electronics, or to give you how-tos on simple projects (though those are nice), its to let people share what they are doing and give readers some ideas that they might want to use/recreate/modify.

Posted at 8:07 am on Feb 25th, 2006 by martin

^^
blue leds would not be practical, because- if I correctly read the links the LED can sense a shorter wavelength than its own- so a red LED can capture the most and a blue the least. http://www.sensorsynergy.com/helpfulhints.htm

Posted at 7:56 pm on Feb 25th, 2006 by Alex

this seems like some thing that would be great for us D.J out there

Posted at 5:27 pm on Mar 3rd, 2006 by legend

Just think about bidirectional OLED displays, Multi-touch tablet displays which can scan fingerprints, barcodes, and such that are placed on the screen. Maybe a bidirectional OLED display could even be a webcam too.

Posted at 7:27 pm on Mar 3rd, 2006 by deadmoo

which Forrest M Mims III book can i find this project in? a couple people mentioned seeing it in the 1977 publication, but i can seem to find it. i really like this project and i’m going to try using it in a tele-rehabilitation project for one of my classes.

also, if any one has written a program for this in Matlab, it would be really helpful if i could take a look at it.

Posted at 3:55 pm on Mar 8th, 2006 by AL

re #57 … on the ‘offical’ forrest mims website, there are no publications listed for 1977, but there are some related to the field of optoelectronics published between 1972 and 1982 – perhaps 1977 was given as some sort of median year of reference

official list of publications: http://www.forrestmims.org/pages/3/index.htm

Posted at 11:48 pm on Mar 12th, 2006 by justdiy

Hi,

I would really like to understand what the heck you guys are talking about. What kind of engineering would I need to major in to do stuff like this. I’m in 12th grade and I plan to major in electrical engineering, but the college I plan on going to has electrical engineering and electronic engineering. Which one would be better for me to major in, because I am really interested in this kind of stuff.

Posted at 9:30 am on Mar 24th, 2006 by Joshua Green

#59: I studied plenty of electronics in my electrical engineering courses. Unfortunately when i was in school the focus was on telecom careers. All the fun stuff (to me) is in the hardware and software development though, and not computer networking. My advice would be to study the curriculums and talk to your advisors early on when it’s still easy to change your major.

Posted at 7:59 pm on Mar 24th, 2006 by enterrupt

Here’s a thought for the ultra-geek. With this technology, imagine enclosing a box with each wall being covered in these sensors, and using it as a way of reading total human body movement… Thing about the Virtual Reality imput possibilities of this? I know it sounds silly, but as I was reading these posts and watching the short films, it seems very doable, albeit expensive. I’m sure we must have a reader with both the time and resources to take on such a task. I will watch with baited breath!

Posted at 6:55 pm on Apr 15th, 2006 by Vdawg

Posted at 7:10 pm on Apr 15th, 2006 by Vdawg

this hack is awsome

Posted at 2:30 am on Apr 17th, 2006 by mike

I’m not sure if backlights nowadays used LEDS, but I see no reason why a computer couldn’t be created that used such a backlight. Imagine a computer screen that checks the light levels and changes it’s output accordingly, no more getting blinded by a bright computer screen at night.

Posted at 11:34 pm on Apr 21st, 2006 by Phobia

@phobia That’s a really nice idea actually.

I think you could build a mod for your average laptop to use a LED as a sensor and then trigger the hotkeys on the keyboard for changing screen brightness to get a good enough effect.

Posted at 1:31 am on Nov 6th, 2008 by ebidk

OK using LED in an application where their light output isn’t the primary purpose is NOT a hack? Good Lord hackaday isn’t going even have a chance to please a few, much less most is it.

I’m sure are most like myself will never duplicate few of the hacks as exactly as presented here, but a lot learned goes in a note book. The notebook may be brain matter, paper or electronic. Unlike many publications hackaday costs nothing to view.

Posted at 2:18 pm on Nov 7th, 2008 by static

I can’t even solder but even I learnt in school that an LED can output light AND sense light.. Even I feel smart after reading these comments :x

Posted at 10:27 pm on Apr 14th, 2009 by steve