capacitorled
With a circuit of a charged capacitor, a resistor, and an LED, I can create a lighten up LED that gradually dims until the capacitor run out of charge.
With a combination of a battery, capacitors, resistors, and LED, is it possible to create the opposite effect, that is an LED that gradually lighten up? If not, what kind of components that I might need to produce this effect?
Short answer: Current stops when the capacitor gets charged up to the battery voltage.
When current flows through the circuit, the bulb lights up. In this case you can consider the bulb as a 'current detector'. Current in this case is flow of charge, and the charge carriers are electrons. They are pushed around because the battery pushes them with an electric field.
Now look at the capacitor symbol. It indicates that there is a gap between the plates. Electrons normally don't jump this gap. But you know there is current because the bulb initially turns on. So what is happening? Electrons are effectively being accumulated at one of the plates of the capacitor, while the opposite plate has electrons being removed, "charging it". But they can't accumulate indefinitely, because the more they accumulate, the more the repel each other. It basically takes energy/work to do this, and you can consider that the capacitor is storing this energy, and can effectively be released later on. As the capacitor gets charged, the voltage accross it augments, until the battery cannot push more electrons. At this point the capacitor voltage has equalized the battery voltage. No more electrons flow, the bulb finally turns off.
To release the energy stored in the capacitor, remove the battery from the circuit and connect the wires together. You should see the same effect (bulb turning on, then dimming until off), because the battery no longer keeps those electrons pushed and they return to neutrality via the bulb.
As cowbydan stated, placing the capacitor in series to the LED will block DC currents. To achieve the desired effect of a glowing LED after removing the batteries you need a circuit similar to this :
simulate this circuit – Schematic created using CircuitLab
The capacitor needs to be in parallel to the LED and the polarity of the capacitor needs to match the polarity of the connected battery. The resistor R1 is not mandatory in your particular application but it reduces the current which the LED consumes. A larger resistor value will increase the time the LED glows after you remove the battery but will also decrease its brightness. Just test it with some different values. Choosing a LED with a lower voltage rating would require a resistor in series to it to reduce the voltage.
Best Answer
The other component you need is a transistor. Try something like this, and it should take several seconds to turn on.
simulate this circuit – Schematic created using CircuitLab