Re: IR detector system, biasing of photo diode

On Mon, 28 Oct 2024 15:49:30 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2024-10-27 08:26, 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.
 

Just don't fry the photodiode in high light.
>

 
So adding a resistance in series with the diode?

>
Nah, the Johnson noise kills you.  It's easier to just calculate or
measure the photocurrent from direct sunlight and design around that.
You only need enough bias to ensure linear operation at high current,
maybe a volt or so.
>
You will want to put a filter in the second stage to get rid of the
nasty high-frequency noise peak.  I usually use a two-pole Sallen-Key
with equal resistor values, which has predictable gain (1.00) and low
component-value sensitivity, and is super simple.
>
Resist the temptation to do anything floral with the TIA stage, such as
LC or *especially* gyrator filtering.  A large inductor is a disaster in
a TIA, because if it doesn't cause instability, it'll still pick up crap
from every VF motor drive on the block, and deposit it right into the
summing junction, where you really really don't want it.

Small shielded inductors are cheap, and 10 KHz is not a common
switching frequency.

Put the two inductors close together. They will see mostly the same
mag fields, so a couple of resistors added somewhere will cancel the
pickup.

Or add a third, between them, to drive their bottom ends, again
canceling mag field pickup.

Or make each L from a pair, arranged so the pickups cancel.

TV remotes work if you bounce the light off the ceiling in a well-lit
room.

But the acoustic approach would be better. Omni MEMS microphones have
built-in amps and cost 20 cents.