RC Circuit LED Persistence

See how capacitors store energy by building a circuit where an LED stays lit after power is disconnected—demonstrating the RC time constant in action.

Overview

When you disconnect power from a circuit, things usually turn off instantly. But add a capacitor, and you can store enough energy to keep an LED glowing for a noticeable amount of time! In this project, you'll compare different capacitor values and see firsthand how larger capacitors store more energy and keep the LED lit longer. This is the RC time constant in action—a fundamental concept used in timing circuits, filters, and power supplies.

Components Needed

  • severalx Wires
  • 1x 33 ohm resistor
  • 1x LED
  • 1x 1000µF electrolytic capacitor
  • 1x 220µF electrolytic capacitor
  • 1x 47µF electrolytic capacitor

Instructions

  1. Gather Your Components

    Collect all the materials for this project: wires, a 33 ohm resistor, an LED, and three different electrolytic capacitors (1000µF, 220µF, and 47µF). You'll also need a breadboard and a 5V power source. Having multiple capacitor values lets you compare how capacitance affects energy storage.

  2. Identify the Resistor

    The 33 ohm resistor limits current through the LED and sets the discharge rate for the RC circuit. Reading the color bands: Orange (3), Orange (3), Black (×1) = 33 ohms. This resistor protects the LED from excessive current while allowing enough brightness to see clearly.

  3. Understand Your Capacitors

    Electrolytic capacitors are polarized—they have a positive and negative terminal. The negative side is marked with a stripe and shorter lead. The capacitance value is printed on the body. Larger capacitance (measured in microfarads, µF) means more energy storage.

  4. Test Without a Capacitor (Baseline)

    First, build the circuit without any capacitor: connect the power source through the resistor to the LED, then back to ground. When you connect power, the LED turns on. When you disconnect power, the LED turns off instantly. This is your baseline for comparison.

  5. Add the 47µF Capacitor

    Now add the 47µF capacitor in parallel with the LED (positive to positive, negative to negative). Connect power to charge the capacitor, then disconnect. The LED stays lit briefly before fading out—the capacitor is releasing its stored energy!

  6. Try the 220µF Capacitor

    Replace the 47µF capacitor with the 220µF capacitor. Repeat the test—connect power, then disconnect. Notice how the LED stays lit noticeably longer! The larger capacitor stores more charge and releases it over a longer time period.

  7. Test the 1000µF Capacitor

    Finally, try the 1000µF capacitor—the largest value. The difference is dramatic! The LED stays lit much longer after power is disconnected, clearly demonstrating that larger capacitance equals more stored energy and a longer discharge time.

Challenges

  • Calculate the theoretical time constant for each capacitor and compare to what you observe
  • Try different resistor values—how does resistance affect the discharge time?
  • Use a multimeter to measure the capacitor voltage as it discharges
  • Build a circuit where the LED fades on slowly when power is connected (hint: put the capacitor in a different position)
  • Combine multiple capacitors in parallel—does it equal the sum of their capacitances?