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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Wed, 30 Oct 2024 08:09:58 -0700
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On Wed, 30 Oct 2024 09:45:42 +0100, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

>On 30-10-2024 00:31, john larkin wrote:
>> 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
>>>>>
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