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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 13:40:18 +0100
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On 27-10-2024 13:26, 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.
> 
Actually, wont a simple high pass filter work equally well?

Photo diode with bias -> capacitor to gain block....