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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Tue, 29 Oct 2024 16:31:46 -0700
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On Wed, 30 Oct 2024 00:02:34 +0100, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

>On 29-10-2024 18:26, john larkin wrote:
>> On Tue, 29 Oct 2024 17:02:02 -0000 (UTC), Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>> 
>>> john larkin <jl@glen--canyon.com> wrote:
>>>> On Mon, 28 Oct 2024 20:31:14 -0400, Phil Hobbs
>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>
>>>>> On 2024-10-28 17:10, john larkin wrote:
>>>>>> On Mon, 28 Oct 2024 15:49:30 -0400, Phil Hobbs
>>>>>> <pcdhSpamMeSenseless@electrooptical.net> 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
>>>>>>>>> <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.
>>>>>>>>
>>>>>>>>> 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
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