Path: ...!weretis.net!feeder9.news.weretis.net!news.quux.org!eternal-september.org!feeder2.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: John R Walliker <jrwalliker@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Tue, 29 Oct 2024 17:31:03 +0000
Organization: A noiseless patient Spider
Lines: 190
Message-ID: <vfr64n$1fso2$1@dont-email.me>
References: <vfk0u0$3u9en$1@dont-email.me>
 <ro8rhjdlkrctc6cfv3jfjbuvi8v3r5hl5k@4ax.com> <vflbi0$eevd$3@dont-email.me>
 <8d921b57-5097-d474-879e-01215a5809b5@electrooptical.net>
 <bsuvhj5d738nk86nspb4u1vnuaibh40sgg@4ax.com>
 <58bec831-d3a5-199a-a586-f358a22e9e7f@electrooptical.net>
 <pt32ijt1soik4639dnge32plirb0iuvmgn@4ax.com> <vfr4e9$1kvcd$1@dont-email.me>
 <d162ij98ohhs4j245kcjl3gtimvqfiugr2@4ax.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Injection-Date: Tue, 29 Oct 2024 18:31:04 +0100 (CET)
Injection-Info: dont-email.me; posting-host="e8c3213792c25cfe257aefa16d22e0d1";
	logging-data="1569538"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/WXJRYPH7fN3YaZTtuvmFHiE4OwROhDSI="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:+UIlq4xXXhctxEvSgmqNE5wN8mA=
Content-Language: en-GB
In-Reply-To: <d162ij98ohhs4j245kcjl3gtimvqfiugr2@4ax.com>
Bytes: 9589

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.
>>>
>>
>> Hidebound prejudice is the only possible explanation. ;)
>>
>> Cheers
>>
>> Phil Hobbs
========== REMAINDER OF ARTICLE TRUNCATED ==========