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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 00:31:41 +0100
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On 28-10-2024 20:16, Phil Hobbs wrote:
> On 2024-10-27 08:20, Klaus Vestergaard Kragelund wrote:
>> On 27-10-2024 03:05, Phil Hobbs wrote:
>>> 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?
>>>>
>>>
>>> Phototransistors are horrible for that sort of job— too small, too 
>>> noisy,
>>> not repeatable, for a start.
>>>
>>> It’s not signal/background you care about, it’s signal/noise, 
>>> specifically
>>> the shot noise of the sunlight.
>>>
>>> An optical filter will help reject sunlight, and a bigger detector will
>>> help more. The real win is reducing the FOV with lenses as well as 
>>> baffles,
>>> tubes, and so on.
>>>
>>
>> I have tried to search for optical filters. Where would one get those?
> 
> 700-nm plastic longpass filter material comes in sheets.  You can get 
> smallish chunks of it on the jungle website and various other emporia.
> 
> Fancier things, such as narrowish bandpasses and custom wavelengths, 
> tend to be glass and quite a lot more expensive.  I usually get those 
> from Omega Optical, but there are European suppliers as well.
> 
> For this use, a regular 700-nmm plastic longpass will get rid of most of 
> the daylight, which is what you want.  You can also get photodiodes with 
> the filter material included, e.g. the ever-popular BPW34F.
> 

Great recommendations, thanks. I have purchased a bunch of the BPW34F.

>>
>> Baffles and tubes, we can do ourselves, any guideline on the best 
>> surface of the inner tube?
> 
> Regular old flat black paint.  But a lens on the transmitter will be the 
> biggest win.
> 
>>
>>> Check out the Hamamatsu S6968–super good medicine.
>>>
>>
>> Looks like a very big die. Is that the main reason to use that one, to 
>> get better sensitivity?
> 
> It has very low capacitance for its area, it has a lens to increase the 
> detection area, and (crucially for my uses, which are generally at 
> higher frequency) it has very very low series resistance.
> 
> The series resistance of the diode contributes Johnson noise that can't 
> be removed by bootstrapping.  You don't care too much at 10 kHz, but at 
> higher frequency the 50-300 ohms' worth of Rseries in most diodes will 
> trash the SNR--there's not much use in a 300-pV/sqrt(Hz) TIA if the 
> diode itself contributes way over a nanovolt.
> 
So in fact the devil is in the detail. I have not done higher 
performance optics before, so riding on the learning curve.