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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Mon, 4 Nov 2024 18:57:55 -0000 (UTC)
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john larkin <jl@glen--canyon.com> wrote:
> On Tue, 29 Oct 2024 16:03:12 -0000 (UTC), Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> john larkin <JL@gct.com> wrote:
>>> On Mon, 28 Oct 2024 20:19:25 -0400, Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>> 
>>>> On 2024-10-28 08:14, piglet wrote:
>>>>> john larkin <JL@gct.com> wrote:
>>>>>> On Sun, 27 Oct 2024 22:59:59 -0000 (UTC), piglet
>>>>>> <erichpwagner@hotmail.com> wrote:
>>>>>> 
>>>>>>> Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:
>>>>>>>> On 27-10-2024 13:26, 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
>>>>>>>>>> <klauskvik@hotmail.com> 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.
>>>>>>>>> 
>>>>>>>> Actually, wont a simple high pass filter work equally well?
>>>>>>>> 
>>>>>>>> Photo diode with bias -> capacitor to gain block....
>>>>>>>> 
>>>>>>> 
>>>>>>> If you make it part of the bias network yes, a gyrator either a single
>>>>>>> transistor or op amp could take the role of inductor.
>>>>>> 
>>>>>> When you need an inductor, an inductor makes an excellent inductor.
>>>>>> 
>>>>>> But the dual microphone thing, electret or mems, sounds like a much
>>>>>> better way to go.
>>>>>> 
>>>>>> 
>>>>> 
>>>>> Some folk are scared of inductors. 10kHz does mean quite a few milli
>>>>> henries.
>>>>> 
>>>>> Of course they might be able to reverse the process and have one receiver
>>>>> on the robot and two emitters on the docking station, a bit like aircraft
>>>>> VOR
>>>> 
>>>> Or like certain radio navigation systems popular in Western Europe, 80 
>>>> or so years ago. ;)
>>>> 
>>>> That's a good example of Horace Darwin's maxim, "Always try it the other 
>>>> way round", quoted in "Instruments and Experiences" by R. V. Jones.
>>>> 
>>>> Jones is one of my technical heroes, and was instrumental in defeating a 
>>>> couple of the aforementioned navigation systems, specifically 
>>>> "Knickebein" and "X-Geraet", as explained in his book, "The Wizard War" 
>>>> (entitled "Most Secret War" in the UK).  Horace Darwin was a grandson of 
>>>> Charles Darwin, and a colleague of Jones's at Aberdeen after the war.
>>>> 
>>>> Cheers
>>>> 
>>>> Phil Hobbs
>>> 
>>> https://www.amazon.com/Instruments-Experiences-Measurement-Instrument-Technology/dp/047191763X
>>> 
>>> 
>>> 
>> 
>> Cheap at twice the price!
>> 
>> It’s a unique book—a collection of Jones’s groundbreaking papers on things
>> like measuring the ether drag on light in a moving dielectric, which would
>> be a niche interest except that each one is paired with an essay of about
>> equal length explaining how the measurement was done. 
>> 
>> Those pieces are of lasting value—they’re confidential in tone, including
>> the inventions required, the blind alleys, and very often, how somebody
>> made a big difference with a piece of hard-won advice, like that remark of
>> Darwin’s. 
>> 
>> Cheers 
>> 
>> Phil Hobbs 
> 
> One of my summer jobs in high school was working in the electronics
> shop of the physics depertment of a local university. They registered
> me as a fake student so they could pay me 70 cents per hour. I learned
> a lot.
> 
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