How many voltages is needed to light an LED?

for science fair i am lighting an LED bulb from potatoes, lemons, limes, apples, and onions. 1 fruit doesn't have enough voltage to light it up, but 3 seems to do it. How many voltages are needed to light up the LED. And what makes some of these fruits have more voltage then others. i found the apple created... show more for science fair i am lighting an LED bulb from potatoes, lemons, limes, apples, and onions. 1 fruit doesn't have enough voltage to light it up, but 3 seems to do it. How many voltages are needed to light up the LED. And what makes some of these fruits have more voltage then others. i found the apple created the most.
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