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i know that diodes won't pass current if the voltage over them is below a certain level
you might be able to use a diode that won't pass current if the voltage is less than say 0.2V (not that i know if such a diode even exists)
and the signal coming out of the diode will go through a transistor to drive a LED or whatever
and plz do correct me if i'm wrong since i got a pretty basic understanding in electronics