Path: ...!news.misty.com!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: Klaus Vestergaard Kragelund Newsgroups: sci.electronics.design Subject: Re: IR detector system, biasing of photo diode Date: Sun, 27 Oct 2024 13:21:48 +0100 Organization: A noiseless patient Spider Lines: 114 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Injection-Date: Sun, 27 Oct 2024 13:21:48 +0100 (CET) Injection-Info: dont-email.me; posting-host="8041e8f1e0a9f63d6cfa54aa0653b17a"; logging-data="474093"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19szNnNjfRf9czsIIgGgLzF/mgjfD2kRaE=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:P9epShfL399w4ePn3t6J7f9dsoc= In-Reply-To: Content-Language: en-US Bytes: 5662 On 27-10-2024 04:50, Bill Sloman wrote: > On 27/10/2024 1:05 pm, Phil Hobbs wrote: >> 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 buried signal in the DC from sunlight. > > If the sunlight signal doesn't just saturate the signal chain, which > wrecks the gain. > >>> How do I best bias the photo diode for optimum detection of the 10kHz >>> signal while being immune to the ambient sunlight? > > Bias doesn't really matter unless you are looking for avalanche > multiplication of the charge carriers - if there there's enough bias to > let you collect all of them. Single photon avalanche diodes are a > different can of worms, but you should have plenty of photons, so why > would you bother? > > More bias does reduce the capacitance across the detection diode, making > it a bit faster, but the reduction is a hyperbolic function of bias, so > each extra volt makes progressively less difference. > Ok, noted :-) >>> 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? >>> >> >> Phototransistors are horrible for that sort of job— too small, too noisy, >> not repeatable, for a start. >> >> It’s not signal/background you care about, it’s signal/noise, >> specifically >> the shot noise of the sunlight. >> >> An optical filter will help reject sunlight, and a bigger detector will >> help more. The real win is reducing the FOV with lenses as well as >> baffles, >> tubes, and so on. > > In a vaguely similar sort of situation I got my mechanical colleagues to > put a graphite liner inside the tube, and cut a screw thread into the > liner. They thought I was nuts, until they skipped cutting the screw > thread and I promptly complained about the loss of performance. > >> Check out the Hamamatsu S6968–super good medicine. > > Hamamatsu do seem to have good products. >