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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Mon, 28 Oct 2024 12:14:52 -0000 (UTC)
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john larkin <JL@gct.com> wrote:
> On Sun, 27 Oct 2024 22:59:59 -0000 (UTC), piglet
> <erichpwagner@hotmail.com> wrote:
> 
>> Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:
>>> 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....
>>> 
>> 
>> If you make it part of the bias network yes, a gyrator either a single
>> transistor or op amp could take the role of inductor.
> 
> When you need an inductor, an inductor makes an excellent inductor.
> 
> But the dual microphone thing, electret or mems, sounds like a much
> better way to go.
> 
> 

Some folk are scared of inductors. 10kHz does mean quite a few milli
henries.

Of course they might be able to reverse the process and have one receiver
on the robot and two emitters on the docking station, a bit like aircraft
VOR


-- 
piglet