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Interesting fact of the day. While solar panels do generate some energy from visible light the overwhelming majority is in fact int he infrared part of the spectrum. In fact its peak is well within the infrared but has a black-body like curve.
Since solar panels are essentially just LEDs designed to have maximum surface area this works in both directions. If you apply a voltage to solar panels they will visibly glow in infrared light.
@freemo Interesting. I learned the solar panels can act like LEDs from a Steve Mould's video, but never thought LEDs can be used like a sensor that way.
@skyblond I'm familiar with the video. I knew about the idea long before that video but that video was the first time I saw it in action without actually frying the solar panel.
You may forget it but he actually also shows that leds are receptive in that video.
The fact you can use them both at the same time like that is something I thought was possible years ago when I heard about this and found people had done it once I started searching youtube. It makes sense.
That said its not a very practical way. You can actually detect the change in capacitance of the LED (which works even when it isnt powered) and get more accurate results and the same effect. So its not something I would ever expect to be used commercially.
@freemo, it reminded me for some reason how I and my father were building some toy in my childhood, and since photo sensor for activation was too expensive (and probably also ultra rare in Soviet Union), so poor man's sensor we made out of a transistor in aluminum casing by cutting top off, exposing chip to light, but can't recall was it generating small current or was changing resistance and we used it as a resistor
wow, thanks, that was nice trip by memory lane 
+bonifartius 𒂼𒄄
@skyblond Yup you sure can. In fact I've seen people use LED matrices as a poor mans touch screen. You light up the leds and basically measure the current draw. When you block it with your finger it reflects some of the light and changes the current draw slightly. So you can use this to detect where the finger is.
So in this way you can use it as both a light-emitting device AND a light-detecting device at the same time.