Re: IR detector system, biasing of photo diode

john larkin <JL@gct.com> 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/Datashee
t/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%20absor
ption.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.
 
Just don't fry the photodiode in high light.
 
An optical bandpass filter would hugely improve things, take out most
of the sunlight.
 
 
 
 

 
Yes, the classic solution was to operate PD into an inductor so DC didn’t
overload.