IR detector again

   Klaus, you might mount the tubes at a slight outward angle. When one is
saturated steer toward the other.
   R V Jones' "Most Secret War", which Phil mentioned, describes a German
navigational aide that quided aircraft on landing. Two beams were lined
up with the runway but diverging a few degrees. The pilot approached
keeping the reception of the beams equal.
   Your approach has a higher frequency with reciever/transmiter
altered some. All in all though, suprisingly similar.

Hul

  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:
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:
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:
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?