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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: IR detector system, biasing of photo diode
Date: Mon, 28 Oct 2024 11:08:50 +0100
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On 27-10-2024 21:39, Joe Gwinn wrote:
> On Sun, 27 Oct 2024 10:49:27 -0700, john larkin <JL@gct.com> wrote:
> 
>> On Sun, 27 Oct 2024 13:42:27 +0100, Klaus Vestergaard Kragelund
>> <klauskvik@hotmail.com> wrote:
>>
>>> On 27-10-2024 13:40, Klaus Vestergaard Kragelund wrote:
>>>> On 27-10-2024 13:26, 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.
>>
>> A photodiode won't saturate as long as it has a few volts of DC across
>> it. It might melt if there's no current limiting.
>>
>>>>>
>>>> Actually, wont a simple high pass filter work equally well?
>>>>
>>>> Photo diode with bias -> capacitor to gain block....
>>>
>>> Like this:
>>>
>>> https://electronics.stackexchange.com/questions/416184/how-does-this-op-amp-photodiode-circuit-behave
>>
>> The LC tank combines background light rejection and bandpass filtering
>> and has high signal gain, with two parts.
>>
>> I think there are photodiodes with colored plastic, essentially a
>> cheap optical bandpass filter. Used in TV remote receivers.
>>
>> The windows in TVs may be optical bandpass filters too. They work with
>> very little signal from the remote, in high room light.
> 
> To this I would add a trick.  We know something very useful about the
> 10 KHz modulation, its exact frequency, given that it is (or can be)
> generated electronically, and thus its frequency is ultimately
> controlled by a logic-clock crystal oscillator.
> 
> So feed the amplified signal from the 10 KHz LC tank to a I+Q homodyne
> circuit, filter to pass signals from DC to a 10 Hz and compute the
> magnitude of the received signal - this is used for figuring out the
> direction to the docking station.

Very nice, sort of like for lock-in detection, I have done that before, 
works great. Would take the tolerances out of the passive components.

> 
> The phase of the received signal is discarded, as it is effectively
> random because the TX oscillator phase with respect to the RX
> oscillator phases is uncontrolled and unknown.
> 
> The advantage over a high-Q LC tank is that the resonant frequency of
> the tank need not be that precise.
>