1-wire solar sensor?

I’ve been using Hobby Boards solar sensor modules for many years but from time to time they have failed (often due to moisture ingress into the jar in which I house it) and I have replaced them with new Hobby Boards sensors but they no longer appear on the website as Eric is closing Hobby Boards. My question is - does anybody know of an alternative supplier of 1-wire solar sensors or even instructions as to how to make one? My sensor is OK at the moment but I suspect it will fail sometime in the future and I would like to have a spare unit in stock.

Hello,

Rather sad about Hobby Boards. Erics efforts have been outstanding to the Weather Station hobbyist, and will be missed.
I wish him all the best for the future.

I have similar issue, and have found a company called www.sheepwalkelectronics.co.uk and spoken to Paul there.
At present he does not have a solar sensor available, but is going to create one in the future. You may wish to email him, add a bit more fuel to the fire :slight_smile:
His current humidity sensor could be adapted as he presents the unused pins of the 1wire ic to screw terminals, however you will need a separate 5v supply.

John

Thanks John - yes I’ve already been in touch with Paul and I bought a humidity sensor from him last year. I look forward to when he can offer a solar sensor & I’m interested in your comment about adapting the humidity sensor - presumably one would have to solder a photodiode where the humidity sensor was?

Hello,

No… it seems Paul has brought the other pins of the 1-wire chip to screw terminals on the board… I would suggest you give him a shout for full details.

John

Will do!

Have now been in touch with Paul at Sheepwalk Electronics and he sent me the following:

"Looking at the schematic for the HB DS2438Z based humidity/solar sensor it would seem that the solar sensor part requires no more than the photodiode and a resistor and four connections that are all available on the screw terminals of the v2 SWE3.

So, if I’ve got it right, you simply need to connect GND and VS- on the screw terminals, add a 390 ohm resistor between VS- and VS+ and then add a CLD240 photodiode between VS+ and +5V. This will only work if you’re using the SWE3 in external power mode as if you’re using parasitic power then +5V will not be present on that terminal"

The CLD240 photodiode is apparantly the one that Hobby Boards used to use but so far I’ve not found a UK supplier so I might contact the manufacturer (Clairex in the US) or else see if I can find an equivalent.

I think the SFH203P Photodiode was used more recently.

If you have 5 volts available then you can use an op amp and make a better (linear) solar sensor.

Thanks - plenty of those available. Yes, I will have 5v available - tell me more about using an op amp please

There are several users with home built solar sensors based on the classic photodiode + op-amp, current to voltage converter circuit (see attached). Use a single supply op-amp like the TLV2472 and pin photodiode like the SFH203P (the flat top clear package one). The feedback resistor will depend on the diode used, you want it to be sensitive enough that you can use a good diffuser which will give the correct “cosine” optical response. The ones I have built use milky sheet acrylic, 3-6 mm thick. Output is very linear vs. incident solar radiation.


photoamp2.GIF

Thanks Nico - I’ve already ordered a SWE3 from Sheepwalk Electronics plus a SFH203P so will try that first & then try the op-amp. Do I have to feed the output to an ADC like the DS2438 to interface with my 1-wire network?

I’m not familiar with the 1-wire parts. I was thinking that the diode in the original circuit must be connected to an analog input of some kind :dontknow:

Hello Everyone,

I’ve finally got round to having a proper play with this myself but given I don’t have an HB sensor to play with I’m struggling a little to get my head round what sort of values should be expected.

The Vsens+/- input on the DS2438Z can, according to the datasheet, read up to 250mV.
The HB schematic I have suggests a CLD240 photodiode and 390 ohm resistor.
The CLD240 datasheet suggests that at solar radiation levels of 1mW/cm2 the current through the diode will be 40uA.
A quick google suggests direct sunlight is around 100mW/cm2.

Now I’m not sure if the current to light ratio is a linear thing but there’s nothing to suggest otherwise so I’m assuming it is.
So on that basis in direct sunlight the current through the diode will be 100 x 40uA = 4mA.
Using V=IxR that would give a voltage over the 390ohm resistor of 1.56V.

However the DS2438Z can supposedly only read up to 0.25V on that input so I must be getting confused somewhere.

Separately to doing that maths I have got one of my modules assembled with a 390ohm resistor and SFH203P photodiode as per my earlier instructions.
Now the SFH203P has a photocurrent of 9.5uA at 1mW/cm2 which works out to 370mV at 100mW/cm2 over a 390ohm resistor. However even directly under a CF lamp I’m only seeing around 10mV maximum so either I’m way out with my maths or I’m way out assuming that being right next to a CF bulb is anything like direct sunlight.

Any thoughts?

Paul