There is a device that is essencially a like a lightbulb, with vacuum inside, Instead of the lamp filament, there are four vanes or leafs that are coated black in one side and metalic silver on the other. 

When light strucks this aparattus, due to the pressure of radiation in the black and silver parts it begns to spin. This is a Crookes' radiometer. It you don't get the shivers for a something that spins out of light, you are in the wrong trade.

The idea is attach small and light magnets into the shaft so that when the Crookes' radiometer spins, it generates a small electromagnetic field. Add thousands of this units, and perhaps you've got a beacon that does not require circuitry or solar pannels to function.  

It is meant to be used in space.