pgardn-question

Originally Posted by dr.fager

Hey, I know it's a stretch but you're our resident scientist. I'm trying to find the guts of one of those garden solar LED's that come on when there's no light on the solar panel. Unfortunately, the only thing I can find are the lights themselves...they come in 4-6 packs, I could buy one and gut it but I'd rather not. I can find the solar panels...and AA chargers which is all they are but not sure how to make it only come on at night.

Spent way too much time on google...moderate electronics knowledge...

anyway thought it was worth a shot.

It probably works the same way that street lights do. They come on at night because the have a Cadmium-sulfide photo resistor. What this resistor does is resist the flow of current. So when the battery in your yard light is fully charged, like during the end of the day, no current from the battery will be allowed to flow to the light because the resistance of the photoresistor is very high as long as light shines on it. Now interestingly enough, when night comes, this resistors resistance changes drastically. It becomes a very good conductor so current can flow right thru this resistor to the light.

The next question of course would be how exactly does the Cadmium sulfide combination lose practically all its resistance when it is dark. For this I have a guess. Post back if you want my guess. It based on the electron configuration that is probably present in the Cadmium Sulfide combination.

Anyways do this experiment. Take your solar yard light during the day and put your finger over different components in the circuitry. If the diode(light) comes on, you have found your photo resistor. Oh yes. Do this late afternoon so you know the solar panel has already fully charged the battery.

I will try and find a picture of what one should look like. I have found them in my broken solar yard lights. My daughter and I have broken the stems on some of them so they just lay around so we decided to open them up.

An LED is a light emitting diode. I just wrote diode above. But the bottom line is that they require a very small current and put out a lot of light, that is why they are so wonderful.