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

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1. #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 3:09PM on Sep 3rd 2006 by Ben

2. 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 3:09PM on Sep 3rd 2006 by Victor

3. 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 3:09PM on Sep 3rd 2006 by bc

4. 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 3:09PM on Sep 3rd 2006 by *mobile*

5. I'm just thinking out loud on this, but could this make things like the screens from "1984"?

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

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

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

7. 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 3:09PM on Sep 3rd 2006 by tuckie

8. 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 3:09PM on Sep 3rd 2006 by Ty

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

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

10. "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 3:09PM on Sep 3rd 2006 by furtim

11. ....star trek keyboards?

Posted at 3:09PM on Sep 3rd 2006 by alex k.

12. Some other uses of led as a sensor ,for sun tracking! http://www.redrok.com/electron.htm

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

13. 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 3:09PM on Sep 3rd 2006 by justdiy

14. 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 3:09PM on Sep 3rd 2006 by hawkeyeaz1

15. 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 3:09PM on Sep 3rd 2006 by hawse

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

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

17. 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 3:09PM on Sep 3rd 2006 by Alex CD

18. 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 3:09PM on Sep 3rd 2006 by bugloaf

19. 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 3:09PM on Sep 3rd 2006 by hawkeyeaz1

20. 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 3:09PM on Sep 3rd 2006 by Ben

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