Path: ...!weretis.net!feeder9.news.weretis.net!news.quux.org!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: Sat, 26 Oct 2024 19:26:32 -0700 Organization: A noiseless patient Spider Lines: 87 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Sun, 27 Oct 2024 03:26:35 +0100 (CET) Injection-Info: dont-email.me; posting-host="182e38949987c0037a44e541f2cdac84"; logging-data="4175267"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX190HtqAIlPipTMRHDpzrnq5" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:t3X1es6akc8mIai1QQIX8GziTqw= Bytes: 4167 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. Just don't fry the photodiode in high light. An optical bandpass filter would hugely improve things, take out most of the sunlight.