I did my science fair project on fruit batteries, & I thought ascorbic acid was the acid that reacted w/zinc and copper to make electricity, but the pears made more electricity than the lemons! (~0.85V compared to ~0.73V) I put together 2 pineapples and 3 lemons and generated 3.5 V, but that still didn't power a bulb which only required 1.4 V. What went wrong with my experiment? (The bulb works with a real battery, I tested it) I did have to use a galvanized nail and a tubular piece of copper in place of zinc and copper electrodes, but shouldn't that have worked just the same?

The problem with your light bulb is that your electrical source wasn't putting out enough current.

Electricity requires a potential difference ("volts") to do work, but there must be a quantity of electricity (current, measured in "amps") available to do the work! The power (in watts) provided by any electrical source is equal to volts ´ amps.

Your light bulb requires a potential difference of 1.4 V to light, but that assumes a minimum amperage as well (which may or may not be stamped on the bulb). It's calibrated to work with most standard power sources (like a battery), but a piece of fruit separating a piece of copper and a piece of zinc is not a standard power source...

Measure the amperage across your fruit battery, and compare to the amps required by your light bulb. You will have to experiment with ways to increase amperage (one way is simply to line up fruit batteries in series; the amps and volts add), but you might try increasing the size of your zinc and copper electrodes, or increasing the area of electrode exposed to the fruit.

"Potato battery" makes this answer searchable from the appropriate page of the MadSci Edible Experiments Archive.