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What photodiode to use for daylight sensing? Spectrum?

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Full Member level 3
Sep 19, 2008
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I have a problem with the standalone daylight sensor (Renesas ISL29102) I use in that it is way to slow to decay.
The reason is that I need to measure the ambient light in between PWM cycles of some LEDS.
Old sensor:

I am considering re-designing the circuit to use a photodiode and amplifier instead.
The original sensor has a spectral range of ~450-660nm with approximately 550nm peak.
It is claimed to close to human eye response.

The photodiodes with reasonable lead times do not have this specrtral range.
Do I need such a wide spectral range to sense ambient light?


This is to dim down some leds when the ambient light level is lowered.


Photodiodes are rather fast. If your circuit is slow, then most probably because of it's external circuit.
Using a different photodiode won't solve the problem.

You say you want to dim LEDs according ambient light. Usually you need no fast detector for this.
To operate such a system properly you just need to avoid that LED light hits the ambient sensor. A mechanical shield should solve this or just a suitable orientation of the sensor.

Please show us a photo of your application including LEDs and sensor.


I think you may have missed the fine points.
I need a fast sensor because I have to take the readings in the PWM led cycle I have 3ms where the LEDS are off.
In that time, the sensor must decay / settle and I need to take a sample (ADC reading).
The problem is that the LEDs and sensors are all on the same PCB and the LEDS bleed light into the sensor.

For example, the sensor I am currently using, takes over 20ms to fully decay and so I cannot get lower light readings out of it within the time I have available.
Hence I am thinking of re-designing this to use discrete parts ie. a photodiode + amplifier.


I think I already did understand.
The problem is that the LEDs and sensors are all on the same PCB and the LEDS bleed light into the sensor.
* For you may do a sophisticated solution with hardware effort and software effort.
* or do the simple solution and use an optical shield (black tube, or sth else) to avoid the LEDs bleed into the sensor.
Many other applications have done this before.

But to answer your question:
I'd use a sensor suited for the needs:
* if you are focussed on visible light, then use a sensor for this range.
* if you are used on IR...
* UV...
* red color only ....
You know best which spectum you want to detect.

Schematics for fast photodiode signaling are available in the internet and have been discussed here in this forum.

If you really want to go this way:
I recommend to use a TIA.
And use an LPF with a tau in the range of 300us ... 600us to suppress noise.
You may do sampling synchronized to your PWM, but you may also do random sampling and choose the lowest (darkest) value, it should have the lowest impact of LED light.


I presume you know that LED lamps don't necessarily use PWM, your ambient light detection method is thus limited to a specific application.

Unfiltered silicon photodiodes have a spectral response with maximum in the near IR. Photodiodes with eye sensitivity filter are available on the market, e.g. Vishay BPW21R or TEMD6200FITX01. They differ in the suppresion of the near IR light, important if you want true weighted light intensity of incandescent lamps. Unlike ambient light sensors with amplifier, a photodiode should be always able to achieve fast response with respective amplifier.

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