General

your independent variable is the lemons and potatoes since that's the thing you're altering to see the effects. the dependent variable is the luminosity of the lightbulb, like how bright it is, or if it even lights up. that's the dependent variable because your luminosity depends on how good the lemon or potato is at lighting stuff up. i believe the control is the light of the lightbulb when you plug it into an outlet or something. you have to compare your results to something, and that something is your control. if you're testing lemons vs potatoes, you have to know what a good battery does, and to do that, you use an actual battery and see the results of that.

edit: your control variables are the things that stay constant. in your experiment, you should be using the same type of lightbulbs, wires, and all that stuff. you're changing the battery source from lemons to potatoes, so those are your independent variables. if you change the lightbulb you're using, then that's another independent variable. you need controls so you make sure you're comparing only the things you want to compare, like lemons vs potatoes

We're missing a lot of information about your procedure, so I'll have to make a few assumptions for my answers below. Keep in mind that there are always many variables of each type, but only several that you will actually be interested in. There is not a single control variable, but many control variables. We often specify which variables we will change and state that "all else is held constant."

DEPENDENT VARIABLE: (I assume you only have one) This is what you measure. In your case it is the brightness of the light bulb.
INDEPENDENT VARIABLES: This is what you are changing. It is your source of electrolyte, i.e. lemon or potato juice.
CONTROL VARIABLES: Everything that you must keep constant in order to make sure that the differences in the brightness of the bulb is a result of the independent variables, and nothing else. These would be the electrodes you use (the pieces of metal you stick into the food electrolyte sources, propbably zinc and copper), the wire used, the bulb used, and any other constants. Many things we will take for granted, such as constant temperature and pressure, and you likely will not need to mention them.

Source: I'm a (freshman) chemistry major and I love it woooooooooo

[quote=rydog77]basically an independent variable is a variable that is not dependent on anything else in the experiment. So changing anything in the experiment would not change this variable. Dependent variables are sort of the opposite. A change in the test would cause a change in this variable. A control group is a group in which you change nothing, so you have something to compare your results to.[/quote]
omg so cute! How do you do that?

OT: independent variable: thing that isn't influenced by other factors, but in turn influences all other factors. Use a ceiling light as an example; the switch on the wall is the indepedent variable which affects the whole lightbulb system
dependent variable: the lightbulb turning on or off if you will
control variables: the surroundings and such, such as the wall, the cover for the ceiling light. Things that don't change when you change IV

basically an independent variable is a variable that is not dependent on anything else in the experiment. So changing anything in the experiment would not change this variable. Dependent variables are sort of the opposite. A change in the test would cause a change in this variable. A control group is a group in which you change nothing, so you have something to compare your results to.

The dependent variable is your outcome measure, that is "dependent" on the influence of the independent variables. For example, "the height of a plant after 30 days (dependent variable), as affected by the (independent) variables: hours of sunshine per day, quantity of water per day, and amount of fertilizer per day".

A control is necessary as a reference against which you compare your experimental group. So, for the same example, your hypothesis is that increasing the number of hours of sunshine will affect plant growth, and you will measure this by the height of the plant after 30 days. The control group would get a standard light exposure, and your experimental group would get more light exposure. After 30 days, you would compare the plant height in the control group against the plant height in the experimental group.

Source: Yahoo answers http://answers.yahoo.com/question/index?qid=20090317125554AAX1V9y

Edit: in your case the dependent variable is the light bulb, the independent variable is the lemon or potato (size,ripeness, etc.)
the control would be a lemon light bulb and you have to determine whether using a potato is better than a lemon (i.e the light shines brighter)

.........i think, idk