Infrared LED / detector design
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From: miso on 21 Feb 2010 18:23 On Feb 21, 12:44 am, Fred Bartoli <myname_with_a_dot_inbetw...(a)free.fr> wrote: > m...(a)sushi.com a écrit : > > > > > On Feb 19, 5:05 pm, Matt <hamplif...(a)gmail.com> wrote: > >> 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. > > > I don't think they make the old XR-2211, which was a classic tone > > decode chip. > > > I think you should consider my AM demod scheme. It sounds complicated, > > but you can get a linear estimate of the reflected signal, not a go/no- > > go detection of the carrier. Locking on a signal sounds good, but all > > that means is you have a means to detect when the light of known (a > > priori) modulation is being reflected, but not how much light is being > > reflected, which is related to the distance to the object somewhat. > > [Reflected light also depends on the color of the wall, how shiny it > > is, etc.] A PLL scheme can lock on a signal in the mud, but you > > probably don't want that kind of sensitivity. Rather, you just want to > > know if you are near an object, so the more linear approach like AM > > demod seems more appropriate to me. If you were building a > > communications systems, then the PLL would make more sense. > > Then don't lock on, but lock-in :-) > > Synchronous detection is really simple to setup and a simple on/off > modulation scheme will provide the OP the distance information he needs, > with excellent distance detection range. > > Oh, I just read your post and see it's what you've said. > > And if the uC has an ADC and it's not too heavily loaded, then you can > just choose a low enough chopping frequency and have almost all that > done in SW. > > -- > Thanks, > Fred. I don't think he stands a chance of measuring distance that the rates he is using. I am suggesting measuring reflectivity and implying distance. This is a situation where the transmitter and receiver are side by side, so why bother with sync since you have a priori knowledge of the modulation source since it is sitting right next to you. ;-) It is not like a communications scheme where the transmitter and receiver are separated. He could generate the clock in quardrature and do a Hilbert transformer with a bandpass filter. Add the mixers and you have one serious demod. Still, I think the bandpass filter and envelope detector are the way to go since it is very simple.
From: Matt on 21 Feb 2010 22:30 Wow, I had no idea I'd receive this much help here. Looks like I found the right forum! I really don't want to seem like the typical forum dullard and have y'all hold my hand all the way through this. Anyways: I built an emitter driver circuit from an 555 I had in the parts drawer. It works very well and seems to happily source upwards of 150mA.... so I'm planning to skip the transistor and just use that to drive the emitter. The detector circuit is being a little annoying however...... or maybe I'm just being perfectionist again. I'm using a phototransistor, collector to ground, emitter to +5VDC through a 1k resistor. I pick the output off the emitter to my scope. http://s250.photobucket.com/albums/gg256/spiral_72/?action=view¤t=02212201.jpg So the leading edge has a ramp to it, the trailing edge is just what I expect. Larger values of R-sub-L increase the leading edge angle, while an R-sub-L or 0ohms will make a nice square wave while pulling 30mA....... unsuitable for a battery powered application especially considering this is only a sensor. ARG! Anyways, I believe the advice is correct. The emitter and detector are right beside each other so I don't need to make this any more complicated than I need to.... I've got the timing circuit right there for use on the detector circuit.... I'll probably do that, run a HI/ LOW pass filters and be done with it..... Unfortunately I've been fighting with the detector square wave thing so I haven't gotten to that point...... I considered cleaning up the square wave with a one- shot, but that will trim the leading edge.... I dunno. I'll bookmark all the Ir modules you listed. Thank you. I haven't given up on components just yet. I'm still having fun :) Oh, I pointed a TV remote at my circuit and could see the data packet. COOL!
From: Tim Williams on 22 Feb 2010 00:14 "Matt" <hamplifier(a)gmail.com> wrote in message news:87d1112a-0dd5-419f-80b7-dec955275d7e(a)u9g2000yqb.googlegroups.com... > The detector circuit is being a little annoying however...... or maybe > I'm just being perfectionist again. I'm using a phototransistor, > collector to ground, emitter to +5VDC through a 1k resistor. I pick > the output off the emitter to my scope. > > http://s250.photobucket.com/albums/gg256/spiral_72/?action=view¤t=02212201.jpg > > So the leading edge has a ramp to it, the trailing edge is just what I > expect. Larger values of R-sub-L increase the leading edge angle, > while an R-sub-L or 0ohms will make a nice square wave while pulling > 30mA....... unsuitable for a battery powered application especially > considering this is only a sensor. ARG! Try using it as a photodiode. Leave the emitter or collector open circuit and use two remaining pins as a photodiode. I forget which junction works better; it seems to me B-E is supposed to absorb most of the photons, but B-C has better electrical characteristics (i.e. will stand more than 5V reverse bias). The downside to photodiodes is the low current, which makes voltages move slowly. To solve this, you use a transimpedance amplifier, so the diode drags a few microamperes out of a "virtual ground", not really needing to produce any voltage gain, while on the other side of the op-amp, real volts appear, along with as much current as you'd like. By running the photocurrent into a base-emitter junction (as on a phototransistor), you increase the current by hFE times, but hFE varies considerably, so you get poor results. If you can figure out how to process your signal primarily in the current domain (current sources, sinks, mirrors, etc.), you might get acceptable performance without drawing too much total. It's also potentially easier to handle wide range signals, since you only have to worry about leakage currents (which can be arbitrarily small, given a good transistor choice) instead of voltage offsets. Of course, this is something chip designers are better at, not the least reason for which is because discrete transistors don't match as well. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: JosephKK on 23 Feb 2010 00:59 On Sat, 20 Feb 2010 20:48:36 -0800 (PST), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >On Feb 19, 5:05 pm, Matt <hamplif...(a)gmail.com> wrote: >> 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. > >I don't think they make the old XR-2211, which was a classic tone >decode chip. Don't know if it is of any use, but MT8870 clones are still available. > >I think you should consider my AM demod scheme. It sounds complicated, >but you can get a linear estimate of the reflected signal, not a go/no- >go detection of the carrier. Locking on a signal sounds good, but all >that means is you have a means to detect when the light of known (a >priori) modulation is being reflected, but not how much light is being >reflected, which is related to the distance to the object somewhat. >[Reflected light also depends on the color of the wall, how shiny it >is, etc.] A PLL scheme can lock on a signal in the mud, but you >probably don't want that kind of sensitivity. Rather, you just want to >know if you are near an object, so the more linear approach like AM >demod seems more appropriate to me. If you were building a >communications systems, then the PLL would make more sense. You could make 40 kHz primary modulation and DTMF on that as secondary.
From: miso on 23 Feb 2010 05:04
On Feb 21, 9:14 pm, "Tim Williams" <tmoran...(a)charter.net> wrote: > "Matt" <hamplif...(a)gmail.com> wrote in message > > news:87d1112a-0dd5-419f-80b7-dec955275d7e(a)u9g2000yqb.googlegroups.com... > > > The detector circuit is being a little annoying however...... or maybe > > I'm just being perfectionist again. I'm using a phototransistor, > > collector to ground, emitter to +5VDC through a 1k resistor. I pick > > the output off the emitter to my scope. > > >http://s250.photobucket.com/albums/gg256/spiral_72/?action=view&curre.... > > > So the leading edge has a ramp to it, the trailing edge is just what I > > expect. Larger values of R-sub-L increase the leading edge angle, > > while an R-sub-L or 0ohms will make a nice square wave while pulling > > 30mA....... unsuitable for a battery powered application especially > > considering this is only a sensor. ARG! > > Try using it as a photodiode. Leave the emitter or collector open circuit > and use two remaining pins as a photodiode. I forget which junction works > better; it seems to me B-E is supposed to absorb most of the photons, but > B-C has better electrical characteristics (i.e. will stand more than 5V > reverse bias). > > The downside to photodiodes is the low current, which makes voltages move > slowly. To solve this, you use a transimpedance amplifier, so the diode > drags a few microamperes out of a "virtual ground", not really needing to > produce any voltage gain, while on the other side of the op-amp, real volts > appear, along with as much current as you'd like. By running the > photocurrent into a base-emitter junction (as on a phototransistor), you > increase the current by hFE times, but hFE varies considerably, so you get > poor results. > > If you can figure out how to process your signal primarily in the current > domain (current sources, sinks, mirrors, etc.), you might get acceptable > performance without drawing too much total. It's also potentially easier to > handle wide range signals, since you only have to worry about leakage > currents (which can be arbitrarily small, given a good transistor choice) > instead of voltage offsets. Of course, this is something chip designers are > better at, not the least reason for which is because discrete transistors > don't match as well. > > Tim > > -- > Deep Friar: a very philosophical monk. > Website:http://webpages.charter.net/dawill/tmoranwms The Texas Instruments website has lots of tips on TIA for photodiodes, mostly from their Burr Brown acquisition. Even if it takes actual money (ack!), I suggest using parts with specifications. Digikey or Mouser will do the job. Usually for a project that you release into the wild, you never depend on one safety mechanism. For instance, if the thing is going to run into a wall, you would have your photo sensor scheme and maybe a simple bump detector, even if it is just a plain old switch. Put a big red button on the top of the robot for a master power kill. Nuances like this separate the skilled engineers from the Phil Allison's of the world. ;-) |