Infrared LED / detector design
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From: Jan Panteltje on 19 Feb 2010 13:46 On a sunny day (Fri, 19 Feb 2010 09:57:53 -0800 (PST)) it happened Matt <hamplifier(a)gmail.com> wrote in <71adb032-1f37-4938-84f8-95050a639f5e(a)k41g2000yqm.googlegroups.com>: >The idea is pretty simple...... I think..... :) > >The robot runs around in the house...... If is gets close to the left >wall (like in a hallway) the left Ir emitter/detector sees the >reflection and the robot veers slightly right. Sound like that could work. >At the moment I'd like to only run two tx/rx pairs for left and right >to get me down a hallway and make some simple decisions to move about. >There are a lot of holes in this theory, but the Ir parts are cheap, >and I'm having a little fun in the process. > > > >Supposedly I can tune the detector by adjusting RL, but that's off a >datasheet from another part. I dunno, without datasheet it is hard to tell what is in there. > >Radio Shack sucks. No datasheet, How can they justify this!? Does it not have a part number, and did you google for it?
From: pimpom on 19 Feb 2010 14:40 Matt wrote: > Hello. It's been a while since I've posted. I'm working on an > Ir LED / > detector for a small robot platform I've made. I'm just having > fun > really, and learning! > > > I've read a considerable amount about this and I'm not getting > concrete facts (or at least what I want to hear) > > I purchased a Ir emitter and detector matched pair from Radio > Shack > Model: 276-142. I've not found a real datasheet for these, > which seems > to be a common problem. > > What I do know from bench testing is the detector triggers from > the > emitter pulses all the way from DC to 30kHz and probably higher > frequency than that. I just tested to 30kHz. > > I drove the emitter at 1.3V, 86mA 50% duty cycle squarewave > pulse. The > detected distance was about 6" > I also drove the emitter at 1.3V 100mA, the detected distance > was > about 36" > > From my reading, modulating the emitter at some frequency is > useful > for eliminating false triggers from ambient Ir sources. I > understand > that. > > What I need help on is tuning the detector to that frequency. > How is > this done??? The detector is a phototransistor. Having no > datasheet > isn't helping any. Currently the detector is seeing the emitter > and > pulses just fine, but it also sees any other sources as well > (halogen > recessed lighting in particular!). > > Thank you Readymade IR receiver modules like those used for TV remote controls are widely available. Digikey has a wide range to choose from at ~$1 US. They are tuned to standard frequencies like 38kHz and 56kHz (and others). To distinguish between left and right without having to code the pulses, use two Tx/Rx pairs operating at different frequencies.
From: George Herold on 19 Feb 2010 14:47 On Feb 19, 12:57 pm, Matt <hamplif...(a)gmail.com> wrote: > The idea is pretty simple...... I think..... :) > > The robot runs around in the house...... If is gets close to the left > wall (like in a hallway) the left Ir emitter/detector sees the > reflection and the robot veers slightly right. > > At the moment I'd like to only run two tx/rx pairs for left and right > to get me down a hallway and make some simple decisions to move about. > There are a lot of holes in this theory, but the Ir parts are cheap, > and I'm having a little fun in the process. > > Supposedly I can tune the detector by adjusting RL, but that's off a > datasheet from another part. > > Radio Shack sucks. No datasheet, How can they justify this!? It sounds like you'd like to do some synchronous detection of the IR. Chop the IR at some frequency and then pick out that same frequency in the detected IR. The phase shifts should be small and can probably be ignored. Which makes things easier. One way to do this would be to have the same square wave that turns the IR on and off also switch the gain of an amplifier looking at the detector signal. The gain should switch from +1 to -1. Then a bit of low pass filtering and you are done. For the background IR the signal first gets added and then subtracted. But your copped transmitter only adds to the signal.. Is this what you are trying to do? George H.
From: Matt on 19 Feb 2010 16:02 > To distinguish between left and right without having to code the pulses, use two Tx/Rx pairs operating at different frequencies. Actually that's a good idea. I hadn't thought of that, but I'm pretty sure it would have been an issue and reason for revision #1 !! > It sounds like you'd like to do some synchronous detection of the IR. > Chop the IR at some frequency and then pick out that same frequency in > the detected IR. The phase shifts should be small and can probably be > ignored. Which makes things easier. One way to do this would be to > have the same square wave that turns the IR on and off also switch the > gain of an amplifier looking at the detector signal. The gain should > switch from +1 to -1. Then a bit of low pass filtering and you are > done. For the background IR the signal first gets added and then > subtracted. But your copped transmitter only adds to the signal.. Yep, that's pretty much it. I was actually stuck on only picking the modulated signal up. The detector and emitter would be phase shifted and I thought it'd never work. Then I read up on Phase Lock Loop stuff and my brain started hurting...... I'm not sure I can handle PLL. It sounds like I have a lot of bench testing to do with chopper circuits..... either that or get an order together for Jameco with the tuned Ir detectors. Or make sure the robot only runs in the dark. HA! Man, this got complicated fast! I had no idea.
From: Matt on 19 Feb 2010 16:06
FWIW, I did search for quite some time for a data sheet. Radio Shack Model: 276-142 The only thing I found was people complaining there was no datasheet available and a TXT document with the specs I already have. 1.3V - 1.7V 150mA max continuous current, 930nm etc. etc. etc. And radio shack's web site only has the basic specs listed above. |