Jan 302011

Transistors are very handy little components that have allowed many advancements in electronics. In the circuit below we use a capacitor to “tell” the transistor to switch on an LED (as opposed to using the capacitor to power the LED directly).

To build the circuit you’ll need:

  • 470 Ohm resistor
  • 20k ohm resistor (we used two 10K in series)
  • red LED
  • NPN transistor (we used 2n3904)
  • push-button switch
  • jumper wires
  • 100 uF capacitor
  • 9v battery or power source


  1. Notice that the capacitor can dump its charge through the resistors to the base of the transistor. Explain why this would allow the LED to stay on longer than if the LED drew current from the capacitor directly. (think: the transistor base only needs a tiny current!)
  2. What happens if we switch the 100 uF capactor with a 10 uF or 1000 uF?

  2 Responses to “Using a capacitor with a transistor: Lighting an LED longer”

  1. Can you, please, explain what is happening here?


    • The capacitor is like a storage tank for electrons. When you press the button it quickly charges or “fills” the capacitor. When you release the button the electrons will “empty out”, but must pass through the base pin of the transistor (which uses only a small amount of current). The transistor is really just a switch that “stays on” as long as the capacitor is feeding it, and lights the LED.

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