Connecting LEDs in parallel, but biased in opposite directions, allows you to visually see which direction (polarity) the power supply is attached to the circuit at any given time.
Brief circuit schematics with short video – List of pages
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Using different color LED can be used to color code which direction the voltage is being applied, and that current is flowing.
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Brief capacitor charging RC time constant demonstration circuit
Quick power supply polarity indicator LEDs circuit schematic to breadboard build
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