Low-cost sensing and communication with an LED

Posted Feb 21st 2006 11:00AM by Eliot Phillips
Filed under: misc hacks

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]

Reader Comments

(Page 3)

41. 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 3:09PM on Sep 3rd 2006 by grendelt

42. 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 3:09PM on Sep 3rd 2006 by Straus

43. this hack is awsome

Posted at 3:09PM on Sep 3rd 2006 by mike

44. 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 3:09PM on Sep 3rd 2006 by Phobia

45. 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 3:09PM on Sep 3rd 2006 by Vdawg

46. 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 3:09PM on Sep 3rd 2006 by Vdawg

47. #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 3:09PM on Sep 3rd 2006 by enterrupt

48. 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 3:09PM on Sep 3rd 2006 by Joshua Green

49. 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 3:09PM on Sep 3rd 2006 by justdiy

50. 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 3:09PM on Sep 3rd 2006 by AL

51. 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 3:09PM on Sep 3rd 2006 by deadmoo

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

Posted at 3:09PM on Sep 3rd 2006 by legend

53. ^^ 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 3:09PM on Sep 3rd 2006 by Alex

54. 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 3:09PM on Sep 3rd 2006 by martin

55. 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 3:09PM on Sep 3rd 2006 by Joe

56. 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 3:09PM on Sep 3rd 2006 by William

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

Posted at 3:09PM on Sep 3rd 2006 by int8088

58. 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 3:09PM on Sep 3rd 2006 by Ludw

59. 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 3:09PM on Sep 3rd 2006 by hawkeyeaz1

60. 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 3:09PM on Sep 3rd 2006 by IMWeasel

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