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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Sun, 27 Oct 2024 22:59:59 -0000 (UTC)
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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.

-- 
piglet