Cor van de Water wrote:
Electronic circuits (The silicon wafer that carries the transistors
and other circuitry) is affected in its operation by light.

That's a good possibility. Note that *any* semiconductor on the board (the micro, other chips, transistors, diodes, and even LEDs) might be the light-sensitive culprit.

I once had a board that used a common zener diode as its voltage reference for the power supply. The zener was in a clear glass case, typical of inexpensive 1N52xx family zeners. When sunlight hit that zener, its voltage changed. Since it was the power supply's reference, that changed the voltage of the power supply, throwing everything else off.

Older products used to use EPROMs. You were supposed to cover the window on top (which was there to erase the part if it ever needed to be reprogrammed). But people often left it open. Sunlight hitting the chip inside could not only erase the memory; but would cause errors long before the exposure was enough to actually erase it.

As someone else observed, it's also possible that the product has a light sensor to adjust the backlight or display brightness to compensate for bright ambient light. If the power supply is weak, then sunlight could "command" more brightness than the wimpy power supply could supply. I've seen this happen with the Cruising Equipment/Heart Interface/Xantrex E-meter/Link10 family of meters.
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