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I'm looking for a few hints in designing a light detector circuit.

Priorities are that the circuit is simple (both as in few parts and as in relatively easy to understand), rather cheap, and can give a rough estimate of the light level.
It should be able to clearly differ between night, early morning light, a cloudy day, direct sunlight etc. Much more isn't strictly necessary, but would be a very nice bonus. (This is for a simple greenhouse temp/light monitor/logger circuit.)
Analog or digital doesn't really matter (the board has a microcontroller with an ADC), but I suppose anything except off-the-shelf solutions are analog.

I'm not quite sure where to begin, or even what class of photodetector to use. Since bandwidth is completely irrelevant (I need a reading every few minutes), I would suppose that the more important spec is how wide a lux range they can handle.

I'd also appreciate some input on noise sensitivity. Given the (lack of) accuracy required, do I need to be careful with separate analog/digital grounds etc., so that the lower-end measurements are still OK?
External power will come from a switching supply, but there's a LDO first thing on the board. However the rest of the board is digital (microcontroller, temperature sensors, SPI Ethernet chip).

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You can hardly think of anything simpler than a photodiode or phototransistor: place a series resistor and you have a voltage proportional to light level. The resistor's value sets the sensitivity. In the past I've used the SFH3410 for this several times.

You want a phototransistor with a eye-matched sensitivity curve. Most phototransistors are rated for a limited light range, like the SFH3410, which is only specified between 10 lux and 1000 lux. I've used it for levels down to 1 lux as well, and it was still linear at that level.

For higher levels (direct sunlight in summer can go beyond 100 000 lux) I would suggest to use a second phototransistor where you place an ND (neutral density) filter in front of it. A 99 % filter will reduce the 100 000 lux to 1000 lux, so you can measure the higher light levels with the second phototransistor.

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  • \$\begingroup\$ Interesting idea with the ND filter. Where would one buy such a tiny one (for cheap), though? I was thinking about about logarithmic output solutions, but I'm not sure how I'd build one. \$\endgroup\$ – exscape Aug 28 '12 at 15:10
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    \$\begingroup\$ @exscape - uh-oh, he wants them for cheap :-(. Well, you can get photographic ND filters for 15 dollar, but a 1.8 ND (that's 98.5 % reduction) will be rather expensive, even from lower end brands (say 50 dollar). The reason is that the filters have to be even to a high degree, but for your application that's not necessary. Also you only need a square cm of it. I would try to get a small sample from a sheet, or a reject. DIY from a piece of fabric may work as well. \$\endgroup\$ – stevenvh Aug 28 '12 at 15:55

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