Re: IR detector system, biasing of photo diode

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.
 

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.