Re: IR detector system, biasing of photo diode

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:
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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:
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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.

 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.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com