Low-cost sensing and communication with an LED

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]

Comments

  1. William says:

    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!

  2. Joe says:

    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.

  3. martin says:

    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.

  4. Alex says:

    ^^
    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

  5. legend says:

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

  6. deadmoo says:

    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.

  7. AL says:

    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.

  8. justdiy says:

    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

  9. Joshua Green says:

    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.

  10. enterrupt says:

    #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.

  11. Vdawg says:

    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!

  12. Vdawg says:

    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!

  13. mike says:

    this hack is awsome

  14. Phobia says:

    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.

  15. ebidk says:

    @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.

  16. static says:

    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.

  17. steve says:

    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

  18. Dan says:

    Did anyone else catch the engadget, et al post about Apple’s patent to use individual pixels as touch sensors? That was for capacitive multitouch, but given this, couldn’t any oled panel provide feedback?
    It would have to be calibrated so that a touch is registered by the total occlusion of light to the diode. If not practical for touch interfaces (e.g. bad signal/noise) it could at least eliminate the need for additional ambient light sensors, or proximity (face) detectors on phones.

  19. Deyjavont says:

    If you think using LEDs for input it groundbreaking (although mentioned in the 70s) then look into Memristors (also hypothesized in the 70s, found a couple years ago by HP labs.) It’ll blow your mind.

  20. Nathan says:

    Interesting, interesting.
    Going to rebuild the iDropper thing with a SOIC Attiny13, an SMD LED and a tiny button cell.
    See if I can get the size down a bit, and maybe hack a monitor’s on/off LED and hook another Attiny13 to the monitor’s DDC channel for communication to it (DDC2 is just I2C)
    Voila, easy communication with cheap iDropper devices.
    Use it for auth, use it for doorlocks, use it for anything!

    –Nathan

  21. Led lights are great because they are long lasting and consumes less electricity.:';

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