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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Tue, 29 Oct 2024 17:02:02 -0000 (UTC)
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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. )


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