Re: LED with Solar Panel
You will have a
much cheaper circuit for long light times if you use a rechargeable battery (NiCD or NiMH). If you only want the light to run for a few minutes then a capacitor is fine. If you want it to run all night
The battery will hold a lot more charge and run your light a lot longer than an expensive capacitor, although the life of the capacitor circuit will be many, many years whilst the battery may only last a couple before needing replacement.
Another thing to remember is that a capacitor's voltage will decrease as it is discharged, needing an active regulator to the LED. A battery is much less troublesome untill it suddenly runs out.
As to the calculation...
a) Charge = Capacitance x Voltage
b) Current = Charge Flowing per Second
So, if you have a 1 farad capacitor and charge it to 5 volts, (a) says it will have a charge of 1x5 = 5 coulombs.
Now, if we draw a current of 30mA from it for the LED, (b) says that's a charge flowing of 0.03 coulombs every second. So, the capacitor will be emptied in 5/0.03 = 166 seconds.
See what I mean about it not lasting long? Remember too that not all that current will be useful as the voltage sags linearly over the 33 seconds from 5V to 0V.
Go with batteries
Check here for some circuits for solar chargers:
http://www.discovercircuits.com/S/solar-cell.htm
Cheers,
FoxyRick.