Great info thanks 
As I said it has evolved over time. I now have daughter boards on daughter boards. So it’s a bit of a dogs breakfast. But it works and I have no plans to upgrade it any time soon.
Love the idea, it’s a organized solution to supply low voltage power throughout the house. Would it be possible for you to post some schematics or a more detailed “how to”? It’s a great help for those wondering how to supply their projects?
I’m one of those people that just started putting stuff together myself: learning by doing…
Tom, I’m typing this from my phone, so you’re faster to respond to me than I am in finishing my post 
Thank you so much for sharing - works like charm !
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For anyone that has constructed this sensor I have a word of warning. Either the sensor or (more likely) the white plastic dome covering the sensor is aging badly, causing a reduction in output with time (years).
I use the sensor for lighting control and to get an estimate of my solar efficiency. However I noticed that my solar panel efficiency was getting better with time. Hooray!
Maybe not.
After a year or so I recalibrated my template sensor only to notice the increase in efficiency still slowly occurring.
I double checked that I had not done something stupid in the template calculation. I had not. Which meant that my measured light level must be decreasing with time. Looking at the BH1750 maximum over two years clearly shows this:
Next step will be to get up on the roof to replace the plastic dome to see if that improves things. I have a spare sensor and dome but that will suffer the same issue. So I’ll probably replace the dome with a small glass jar.
To be continued…
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Nice project, I went the lazy way with a Aqara as I needed something quick, but I still have some toys at home, so I might try that.
I was wondering if you had an idea about some “lux normalisation”. As the sensor is always in the same position (and not facing the sun), it gets a different reading based on azimuth and elevation. Maybe some sort of template sensor could help to calculate a theoritical value from a deviation from the south ?
A post was split to a new topic: Battery powered Lux sensor
Finally had time to check the ageing issue out.
The plastic domes that come with these sensors are not UV stabilised:
After 3 years mine was chalky white and could easily be chipped with a fingernail.
I replaced it with a 25mm diameter glass half dome bought from a bead & crystal shop.
LUX reading went from 14,000 to 48,000 
Word of warning about the glass domes. They are cheaply made and often have a thicker “drip” of glass at the top of the dome. This can act like a lens and could easily burn the PCB. I sorted through 4 or 5 before I found a nice even one.
Now al I have to do is recalibrate all my lighting automations and my solar efficiency sensor.
The other thing I noticed was the non outdoor cat5 cable is going to need replacing or protecting. I’ll probably just wrap it in self amalgamating waterproofing tape (Scotch 23 or similar).
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Would it be possible to install 6, or 8 of these sensors on one ESP32?
I would like to add them to my garage roof and measure lux levels in different areas to understand best location for solar panels. I can’t see the roof from the ground and it would be helpful to gauge best location by looking at all the data graphs together.
Could also try it in a few other locations around the garden
The BH1750 has only one address pin meaning you can only specify two separate I2C addresses (0x23 or 0x5C) for the sensor and thus you can only have two sensors on the bus, as each sensor must have a unique address on the bus.
The ESP32 has two separate I2C buses meaning you can have a maximum of four sensors.
However there is an i2C multiplexer component supported in ESPHome: TCA9548A I²C Multiplexer — ESPHome that has eight channels that each support a single device.
So with one of these on each bus then you could have up to sixteen BH1750 sensors.
How much do you expect the light level to change over the area of your roof?
Do you have a lot of tree shading?
Also keep in mind that the I2C bus was not meant to operate over distances greater than 30cm or so (you can push this to a meter or two). So considering you said:
I would suggest that using multiple ESP32s would be a more reliable and simpler option.