Science Buddies: "Ask an Expert"

Hi,

I am doing a project for my school's science fair. My project is on fruit and vegetable battery. The question I am trying to answer is, "What fruits or vegetables will power an LED light the longest".
I went to a hardware store and they did not have any LED lights that could be powered by 2 volts. Instead, they gave me a 1 cell AAA light which should be powered by 1.5 volts. I am producing more than 1.5 volts, though.

I have tested lemons, potatoes, limes, and tomatoes, none of them have powered the bulb at all. The thing is, I have tested the bulb and it does work. The batteries are producing ample amount of voltage. One lemon battery produces about 1 volt while a lemon battery connected in a series produces almost 2 volts. I have also measured the amount of voltage an AA battery is producing and it produces the same amount as one lemon battery. I have been using alligator leads and a multimeter. I know I have connected it correctly as well. I have also watched videos and followed those.

I have tried testing it and trying many different ways. Like changing the connections, the copper wire, and the light bulb, nothing is lighting the bulb up.

I do not seem to know what the problem is. I have consulted some people at the hardware store also but they did not have an answer. They do not know why it is not lighting the bulb up.

Do you have any suggestions on what to do? Why is the Alkaline AA battery powering it but the fruit/vegetable battery not powering it? Do I need more batteries connected in series? Is there not enough current being created, if so how do I produce more current?

My rough draft for the final report is due 10/18/12.

Thanks for your help,
Rpedneka

Hello rpedneka,

Here are a few ideas about what may explain your LED not lighting.

1) Be sure the battery is connected to the LED with the correct polarity. No light will be produced if the polarity is reversed. Since you have succeeded in lighting it with the AAA cell, you probably have this right.

2) The voltage produced by your lemon battery depends on how much current is being drawn by the LED. Be sure to measure the voltage being produced while it is connected to the LED. You may be able to increase the current the lemon can supply if you use bigger electrodes.

3) The light output of a LED depends strongly on the voltage applied to it. It is quite possible that it produces no visible light at 1 V. but lights up brightly at 1.5 V.

4) As you note, you can get higher voltage with two or more batteries in series.

Good luck, WW

LED (Light Emitting Diodes) typically have a forward voltage drop of 0.9v to 1.2v. If the voltage in your circuit is less than the forward voltage drop of your diode, no measurable current will flow, and certainly not enough to cause light emissions. You can look up the typical forward voltage drop of your particular LED on the manufacturer's data sheet. If you have access to a DC volt meter (or DVM), you should be able to measure the open circuit (nothing connected except the meter) voltage that your battery is generating.