Path: ...!eternal-september.org!feeder2.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: john larkin Newsgroups: sci.electronics.design Subject: Re: IR detector system, biasing of photo diode Date: Tue, 29 Oct 2024 10:26:05 -0700 Organization: A noiseless patient Spider Lines: 185 Message-ID: References: <8d921b57-5097-d474-879e-01215a5809b5@electrooptical.net> <58bec831-d3a5-199a-a586-f358a22e9e7f@electrooptical.net> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Injection-Date: Tue, 29 Oct 2024 18:24:11 +0100 (CET) Injection-Info: dont-email.me; posting-host="60616ff21b1db1d007d312301e34b4d2"; logging-data="1743938"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/qFnWMF5h7GN2HwLrXWaAA" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:BH058LLRdVU7f4iY9CE2B0KWxhE= Bytes: 9126 On Tue, 29 Oct 2024 17:02:02 -0000 (UTC), Phil Hobbs wrote: >john larkin wrote: >> On Mon, 28 Oct 2024 20:31:14 -0400, Phil Hobbs >> wrote: >> >>> On 2024-10-28 17:10, john larkin wrote: >>>> On Mon, 28 Oct 2024 15:49:30 -0400, Phil Hobbs >>>> wrote: >>>> >>>>> On 2024-10-27 08:26, Klaus Vestergaard Kragelund wrote: >>>>>> On 27-10-2024 03:26, john larkin wrote: >>>>>>> On Sun, 27 Oct 2024 02:19:14 +0200, Klaus Vestergaard Kragelund >>>>>>> wrote: >>>>>>> >>>>>>>> Hi >>>>>>>> >>>>>>>> I am working on an IR detector that will guide a robot into a docking >>>>>>>> station. >>>>>>>> >>>>>>>> A IR transmitter on the docking station transmits a beam, and 2 IR >>>>>>>> detectors on the robot detects the beam and lets the robot navigate >>>>>>>> towards the target. The working distance is a couple of meters. >>>>>>>> >>>>>>>> I need it to be insensitive to ambient light/sunlight. >>>>>>>> >>>>>>>> The IR detectors are placed in a tube, to narrow in the beam angle and >>>>>>>> to avoid sunlight (since it is seldom the sun is actually that low in >>>>>>>> the horizon) >>>>>>>> >>>>>>>> The IR transmitter will be modulated with 10kHz (TBD) frequency, low >>>>>>>> duty cycle. Low duty cycle to be able to drive the LED with high >>>>>>>> current, frequency modulated so that the receiver can ignore the effect >>>>>>>> of daylight (DC) >>>>>>>> >>>>>>>> If the LED on the docking station has higher radiant intensity at the >>>>>>>> point of the robot (2 meters away) than possible IR from sunlight, then >>>>>>>> that would be perfect. >>>>>>>> >>>>>>>> Example of transmitter: >>>>>>>> >>>>>>>> https://www.vishay.com/docs/83398/vsmy2850.pdf >>>>>>>> >>>>>>>> Has up to 1000mW/sr. Seems my basic calculation for a 15 degree beam, >>>>>>>> shows less than 10nW/m2, while sunlight has 1W/m2. So driving a beam >>>>>>>> that has higher output than sunlight seems unlikely. >>>>>>>> >>>>>>>> I would use a IR phototransistor at 850nm, something like this: >>>>>>>> >>>>>>>> https://www.ttelectronics.com/TTElectronics/media/ProductFiles/Datasheet/OP505-506-535-705.pdf >>>>>>>> >>>>>>>> >>>>>>>> Or a photo diode: >>>>>>>> >>>>>>>> https://docs.rs-online.com/9f58/0900766b816d8a09.pdf >>>>>>>> >>>>>>>> Fed from reverse 3.3V and into a transimpedance amplifier to boost the >>>>>>>> signal with bandpass filter. >>>>>>>> >>>>>>>> One can get digital IR detector used in a remote control systems: >>>>>>>> >>>>>>>> https://www.vishay.com/docs/82491/tsop382.pdf >>>>>>>> >>>>>>>> It has AGC, but digital output. I need analog output to be able to zero >>>>>>>> in on the transmitter beam. >>>>>>>> >>>>>>>> I have been looking for IR detectors that has the analog output, not >>>>>>>> just the digital, but have not found any. >>>>>>>> >>>>>>>> If the photodiode detector is subjected to sunlight, I am guessing I >>>>>>>> would need very high gain on the 10kHz modulation frequency to pick up >>>>>>>> the burried signal in the DC from sunlight. >>>>>>>> >>>>>>>> How do I best bias the photo diode for optimum detection of the 10kHz >>>>>>>> signal while being immune to the ambient sunlight? >>>>>>>> >>>>>>>> I have chosen 850nm which seems to be a good wavelength. The spectrum at >>>>>>>> sea level has some dips due to water absorption. >>>>>>>> >>>>>>>> https://sciencetech-inc.com/web/image/49169/Spectrum%20with_out%20absorption.png >>>>>>>> >>>>>>>> >>>>>>>> Seems like 750nm would be better, since then the IR from the sun is >>>>>>>> lower, but does reduced the effective range of the system during >>>>>>>> fog/rain. Probably that's why these system do not use 750nm >>>>>>>> >>>>>>>> Other considerations? >>>>>>> >>>>>>> You could drive the LED with a square wave, 10 KHz or whatever. The >>>>>>> photodiode could have +DC on one end and the other end can hit a >>>>>>> parallel LC to ground, resonant at 10K. >>>>>>> >>>>>>> That takes out the sunlight DC component and adds bandpass filtering. >>>>>>> >>>>>> >>>>>> That's a very nice idea. The Q should not matter much, just as long as >>>>>> DC is removed. >>>>>> >>>>>> The photodiode will still be subjected to the high ambient light, but >>>>>> the gain would be close to zero for the stage after. I would then still >>>>>> need to be sure the photodiode is never saturated by ambient light. >>>>>> >>>>>>> Just don't fry the photodiode in high light. >>>>>>> >>>>>> >>>>>> So adding a resistance in series with the diode? >>>>> >>>>> Nah, the Johnson noise kills you. It's easier to just calculate or >>>>> measure the photocurrent from direct sunlight and design around that. >>>>> You only need enough bias to ensure linear operation at high current, >>>>> maybe a volt or so. >>>>> >>>>> You will want to put a filter in the second stage to get rid of the >>>>> nasty high-frequency noise peak. I usually use a two-pole Sallen-Key >>>>> with equal resistor values, which has predictable gain (1.00) and low >>>>> component-value sensitivity, and is super simple. >>>>> >>>>> Resist the temptation to do anything floral with the TIA stage, such as >>>>> LC or *especially* gyrator filtering. A large inductor is a disaster in >>>>> a TIA, because if it doesn't cause instability, it'll still pick up crap >>>>> from every VF motor drive on the block, and deposit it right into the >>>>> summing junction, where you really really don't want it. >>>> >>>> Small shielded inductors are cheap, and 10 KHz is not a common >>>> switching frequency. >>> >>> VFDs put out large amounts of magnetic crap from the hundreds of hertz >>> on up. I saw your VFD EMI filters at your Otis St shop. ;) >> >> That was conducted EMI. 20 volt spikes everywhere on the top floor. >> Mag fields drop rapidly with distance, 3rd power or something. >> >>> >>>> >>>> Put the two inductors close together. They will see mostly the same >>>> mag fields, so a couple of resistors added somewhere will cancel the >>>> pickup. >>>> >>>> Or add a third, between them, to drive their bottom ends, again >>>> canceling mag field pickup. >>>> >>>> Or make each L from a pair, arranged so the pickups cancel. >>> >>> Or just do three lines of algebra to pick the right resistor value, AC >>> couple, and be done. >>> >>>> >>>> TV remotes work if you bounce the light off the ceiling in a well-lit >>>> room. >>> >>> "Well-lit", as in probably 1000 lumens of LED or fluorescent light, >>> which has very little output in the >700 nm region. >>> >>>> But the acoustic approach would be better. Omni MEMS microphones have >>>> built-in amps and cost 20 cents. >>> >>> There are lots of imponderables there, though. For instance, on account >>> of the slow speed of sound in air, a 1 m/s breeze (2.2 mph) will make >>> the apparent direction of the acoustic source move by 3 mrad. >> >> It's homing into the mother ship so a breeze will very slightly curve >> the path. >> >> You're an optics guy, so maybe don't like the sound thing. >> > >Hidebound prejudice is the only possible explanation. ;) > >Cheers > >Phil Hobbs > >(I don’t necessarily dislike the sound idea, but it’s more of a science >project than the LED approach. ) Think so? A quick experiment would be easy. Two MEMS mikes would feed an oscilloscope directly. No inductors or TIAs or filters needed. One could trigger the scope from the source and signal average for extra fun. ========== REMAINDER OF ARTICLE TRUNCATED ==========