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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: Sun, 27 Oct 2024 13:21:48 +0100
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On 27-10-2024 04:50, Bill Sloman wrote:
> On 27/10/2024 1:05 pm, 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 buried signal in the DC from sunlight.
> 
> If the sunlight signal doesn't just saturate the signal chain, which 
> wrecks the gain.
> 
>>> How do I best bias the photo diode for optimum detection of the 10kHz
>>> signal while being immune to the ambient sunlight?
> 
> Bias doesn't really matter unless you are looking for avalanche 
> multiplication of the charge carriers - if there there's enough bias to 
> let you collect all of them. Single photon avalanche diodes are a 
> different can of worms, but you should have plenty of photons, so why 
> would you bother?
> 
> More bias does reduce the capacitance across the detection diode, making 
> it a bit faster, but the reduction is a hyperbolic function of bias, so 
> each extra volt makes progressively less difference.
> 

Ok, noted :-)
>>> 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.
> 
> In a vaguely similar sort of situation I got my mechanical colleagues to 
> put a graphite liner inside the tube, and cut a screw thread into the 
> liner. They thought I was nuts, until they skipped cutting the screw 
> thread and I promptly complained about the loss of performance.
> 
>> Check out the Hamamatsu S6968–super good medicine.
> 
> Hamamatsu do seem to have good products.
>