Need help building a science project with LEDs and solar cells

[Octoman]

I am helping my younger brother with his science project and I needed some help my self.

What I wanted to do was build a circuit that uses a solar cell(s) to light up a LED, also at the same time I wanted to charge a capacitor to store some current so that when the solar cell(s) is removed the LED will stay on for a while (at least 30 seconds). We are trying to show how solar energy can light your world and also how stored solar power can provide electricity when the Sun is not available. I had planned on buying the parts from Radio Shack (www.radioshack.com). I just a little trouble with the circuit.
Should the capacitor be hooked up in parallel with the LED (using the solar cell as the voltage source)? Also using the info provided by the manufactures for capacitors and LEDs how do I calculate how long the LED while stay on? Any visual diagrams would be helpful no matter how rough they maybe. You can post here to share with everyone or if you like you can e-mail them to me:

Octoman@houston.rr.com

Thanks for the help guys and gals:)

[Shep]

The parallel or series I don't know I think series but not for sure.

To calculate how long the LED will stay on you need to figure the RC time constant. Resistance X Capacitance = Time. So using this formula we can determine any value as long as you know the other two. you want at least 30 sec after voltage is removed. You need to determine the internal resistance of the LED. A LED will drop .7 volts across it so a resistor in series to ground must be used to keep from burning out the LED. I assumed a resistor vale of 330 OHM's and a voltage of 1.5 volts. since the LED will drop .7 Volts that leaves .8 across the resistor in a series circuit current is constant. By using OHM's Law we can determine current through the resistor by voltage/resistance .8/330=2.4 milli amps. Then take the .7 volts that is across the LED and divide by the current .7/2.4m= 291 ohms. Now we find total resistance by adding the LED resistance to the resistor value 330+291= 641

We now know the resistance and we know the time we want the light to stay on. SO with a little algebra we come up with the formula Capacitance= Time(in Seconds)/Resistance,
30/641= 46801micro farads. a 47,000 micro farad cap will work but I don't think you will find one that big. You can substitute your values in the formulas to get the exact Cap for your needs. If you need too you can add some more resistance to the circuit and use a smaller cap Radio shack has a 10,000 micro farad cap in the 10V section by my calcification you will need around 3000 ohms of resistance to use it.
*edit*
I forgot to mention you can make a larger cap by putting smaller ones in parallel.