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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Tue, 29 Oct 2024 23:55:04 +0100
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On 29-10-2024 19:27, john larkin wrote:
> On Tue, 29 Oct 2024 17:31:03 +0000, John R Walliker
> <jrwalliker@gmail.com> wrote:
> 
>> On 29/10/2024 17:26, 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.
>>>>>
>>>>
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