Measuring Solar Radiation in Bolivia: Using LDR or Phototransistor?

[tinvicemp06]

I'm trying to measure solar radiation. Unfortunately I currently live in Bolivia, so I cannot get a solar cell. Instead I wanted to use a photodiode, but I wasn't able to find that either.

Do you think that it is possible to measure solar radiation using a LDR? What about a phototransistor?

An LDR's peak is around 550 nm, and the solar radiation comprehends a range from 500 to 2000nm. Plus bright sunlight implies around 30,000 lux while the cell's resistance for a LDR varies between 100 a 1000 lux.

 

[vk6kro]

Discarded calculators contain solar cells but these tend to give a fairly constant voltage out with differing solar brightness, which may not suit your measurements.

You could use a LDR or a photo transistor if you put an optical attenuator in front of it. Even a small piece of thin black cloth or some dark plastic sheeting would provide a lot of dimming when viewing through it.

A bigger problem would be to have your detector pointing towards the sun all the time. Do you intend to automate this, or just point it manually when you want to take a reading?

There are also big differences due to the time of day. In early morning, for example, the sunlight has to pass through a greater thickness of atmosphere than at noon.

 

[Jcouls29]

The best way to measure radiation is to use a pyranometer. Most don't need any external components or power sources. I had to purchase one for a job a while back. They're fairly accurate but you would have to have a way to read uV, or amplify this. Anyways, that's probably out of the price range and I don't know the application. But look into that technology. Photoresistors (LDR) and Phototransistors are sensitive to all light and don't really give you a "radiance" value. They are very inaccurate and depending on the time of year, will vary.

In your situation, though, the best bet would be to use the phototransistor. The cheapest and most accurate would be to use a solar cell and have it shorted with a resistor small enough to force the maximum current from the cell. Then measure with an op-amp circuit. Like vk6kro said, voltage is a constant in solar cells, so you have to rely on current.

More details on application may make it easier to help. I've worked extensively with solar technology and have built a few custom solar panels, so I can be helpful given the right situation. Don't hesitate to ask

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Justin Coulston
justin.coulston@gmail.com
TheModernEngineer.blogspot.com