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One of two symbols composed of a Zero (Circle) and One (vertical line) overlaid on top of each other.
Under the International Electrotechnical Commission (IEC) 60417 standard, Graphical symbols for use on equipment, 0 is the symbol for "off" and 1 is the symbol for "on." Combining these two symbols you get either the symbol for "on/off" (a circle with a vertical line inside of it) or "stand-by" (an incomplete circle with an opening at the top and a vertical line sticking out of the opening). For electronics it is very common to see the "Standby" symbol used nowadays, as the electronic device does not completely turn off but instead goes into a low power state.
The symbol in Purple Heart's eyes in images like Danbooru post #1151905 is an example of the "stand-by" symbol.