Infrared LED / detector design
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From: Jan Panteltje on 19 Feb 2010 17:28 On a sunny day (Fri, 19 Feb 2010 13:06:07 -0800 (PST)) it happened Matt <hamplifier(a)gmail.com> wrote in <461836ec-46b8-468d-b10a-63478ab26fda(a)w31g2000yqk.googlegroups.com>: >FWIW, I did search for quite some time for a data sheet. > >Radio Shack >Model: 276-142 OK, that is just an (any?) IR LED and photo transistor. You could use a 74HC4046 or even a 555 timer to pulse the LED at some frequency. To detect it, you need an amplifier of sorts, some transistors (or opamp) and capacitors and resistors can make a nice bandpass filter. Applictation notes of opamp circuits is a place to look. If you use any of the above chips, a simple opamp like the LM324 can work on 5V single supply, and be used to make a good filter, plus peak detector (so you get DC to switch something). Sort of like this: IR light 555 timer --- LED -----> [ ] Wall [ ] DC <-- peak detector -- bandpass -- amplifier <----- [ ] filter Use a different frequency for each side perhaps.
From: Bob Monsen on 19 Feb 2010 17:34 "Matt" <hamplifier(a)gmail.com> wrote in message news:9184f2dc-de55-4a2b-b50c-b68768a204aa(a)a18g2000yqc.googlegroups.com... > 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 You can use a PLL chip to do a much better detector than just trying to look at it yourself. The PLL will eliminate noise and concentrate on just the frequency you care about. Get an LM567, which is a PLL that is designed to do lock on a particular frequency (they call it a 'tone detector'). You use a resistor and capacitor to set the frequency, f = 1 / (1.1 * R * C). When it sees that frequency on its input, it pulls the output low. So, set up your receiver so its output is input through a cap into the LM567, and use the output signal to turn the follower (which must be pulled up to Vcc). Then, use a microcontroller or timer chip (555?) to make your transmitter output at the frequency you set for the lock frequency of the LM567. Calibrate the thing against a typical wall. Make sure the output of the transmitter is completely blocked from the input, and use a pot to adjust the amplitude of the receiver output so it just makes the LM567 pull down at the maximum distance you want it to detect at. The other way to do is to use a sharp IR sensor, which does all of this for you, at a price. See the IR sensor section at www.sparkfun.com for more info. Regards, Bob Monsen
From: Matt on 19 Feb 2010 20:05 First I had no idea there were PLL IC's available. Wow. I'll look into it. Second, the reason I'm building this from components is the Sharp sensors I found were $30 a piece, times probably four is WAY too much. Spark fun has em' for $14. Pretty tempting. Thank you all. I'd like the challenge of building a system from components so I may do that..... at least until I get discouraged :) I'll learn a lot though. I think I've got enough from your advice to research and build something. Thank you again. P.S. The brains of this will be the Parallax BS2. I'm having a ball with it. I'm made several kinds of bots with it. It sure is a lot more capable than the Atmel I used to use! I'll fire up the oscilliscope and play some more.
From: George Herold on 19 Feb 2010 22:41 On Feb 19, 4:02 pm, Matt <hamplif...(a)gmail.com> wrote: > > 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. Well if you don't mind spending a few bucks you can buy 'lockin' chips. The AD620 does the switching for you if you feed it the square wave and signal. There's probably more clever ways to do this. > Man, this got complicated fast! I had no idea. Really, sounds like fun, you've already got the robot moving around? George H.
From: miso on 20 Feb 2010 00:15
On Feb 19, 9:19 am, Matt <hamplif...(a)gmail.com> wrote: > Yea? Thank you, I should have mentioned it's not for data > transmission. Sorry about that. > > My idea is an array or Ir units (maybe 3 or 4) around the bot to be > used for navigation. Does the emitter have some built in resistor? If not, you should be driving it with current, not voltage. If you meditate on how a communications system works, the fundamentals are based on a priori knowledge of the signal. Thus if you send a chopped signal, you have a priori knowledge of that frequency. I'm more of a linear person, so I would AM the current going to the emitter. I suppose chopping is just as good since either method will give a strong signal at the fundamental frequency. Pick a frequency not likely to occur in real life. That is, you wouldn't want the detector to be triggered by a flashing computer display. Say you pick 50kHz. Your receiver, after some signal conditioning, could be feed to a switched cap filter bandpass. Low pass filter off the clock feed through, and AC couple to remove offset. Now from this point, you have two choices. You could drive an energy detector circuit, or do an envelope detector. That is rectify, low pass filter, and then the output is a signal relative to the strength of IR detected. Use a different frequency for left and right. |