The Fading Light
Press a button, let go, and watch an LED fade slowly as a capacitor empties.
What you’ll make
Press a button and an LED lights. Let go, and instead of snapping off, it slowly fades away. A capacitor is storing energy while you hold the button, then feeding it to the LED after you let go.
What you need
- A battery holder for 3 AA batteries, with a switch, and 3 AA batteries (4.5 V)
- A breadboard and jumper wires
- 1 pushbutton (momentary, normally open) that fits a breadboard
- 1 electrolytic capacitor, 1,000 µF, rated 10 V or higher (16 V or 25 V is fine)
- 1 resistor, 100 Ω (bands: brown, black, brown; five-band: brown, black, black, black, brown)
- 1 resistor, 330 Ω (bands: orange, orange, brown; five-band: orange, orange, black, black, brown)
- 1 red LED, 5 mm
Cost: low. All common starter-kit parts.
Safety numbers for this build
| Supply | 4.5 V (3 AA batteries) |
|---|---|
| Capacitor | 1,000 µF, rated at least 10 V, installed the right way round |
| LED current | about 6 mA at most, fading to zero |
| Charging current | at most 45 mA for an instant, limited by the 100 Ω resistor |
| Stored energy | about 0.01 joules: harmless |
Build it, step by step
- Rails: battery pack off. Red lead to the red (+) rail, black lead to the blue (−) rail.
- Button: place the pushbutton across the centre gap. Connect one of its legs to the red (+) rail with a jumper wire.
- 100 Ω resistor: from the button leg diagonally opposite to an empty row. (On most small 4-leg buttons, diagonal legs only connect when the button is pressed.) We’ll call this the capacitor row.
- Capacitor: long leg (+) into the capacitor row, striped leg (−) into the blue (−) rail.
- 330 Ω resistor: from the capacitor row to another empty row.
- LED: long leg into the 330 Ω resistor’s row, short leg into the blue (−) rail.
- Switch the battery pack on. Nothing happens yet.
Try it
- Press and hold the button for two or three seconds. The LED lights.
- Let go and watch: the LED fades out over a second or two.
Not working? If the LED is always on, or never comes on, you’re probably using two button legs that are always connected (or never). Use diagonally opposite legs. Check the LED and capacitor polarity.
Why it works
BeginnerThe capacitor takes over
While you hold the button, the battery charges the capacitor and lights the LED. When you let go, the battery is disconnected, but the capacitor is full of charge. It pushes that charge out through the 330 Ω resistor and the LED. As it empties, its voltage falls, the current falls, and the LED dims. Once the voltage drops below the LED’s forward voltage (about 2 V), the light goes out.
IntermediateThe numbers
While the button is held, the capacitor charges to about 3.9 V (a little voltage is lost across the 100 Ω resistor). The time constant is τ = R × C = 330 × 0.001 = 0.33 seconds. The LED current starts at about (3.9 − 2.0) ÷ 330 ≈ 6 mA, and the part above the LED’s forward voltage shrinks by about 63% every 0.33 seconds, so it’s down to a faint glow in roughly a second and a half. Double the resistor or the capacitor, and the fade takes about twice as long (though with a bigger resistor the LED starts dimmer).
The 100 Ω resistor limits the charging current when you first press the button: at most 4.5 ÷ 100 = 45 mA for an instant, which protects the button and batteries.
For teachers and parents
Open the teacher guide
Time: 30 to 45 minutes.
Learners will: see a capacitor store and release energy; relate fade time to resistance and capacitance; install a polarized part correctly.
Risk and supervision: low. Check every electrolytic capacitor’s orientation before students switch on, and use parts rated 10 V or higher.
Discussion questions: Where did the LED’s energy come from after the button was released? Why does the light fade instead of stopping all at once? Where else might a device need a short burst of stored energy?
Common misconceptions: “A capacitor is a tiny battery” (it stores charge directly, not chemically, and empties much faster). “The capacitor makes electricity” (it only stores what the battery gave it).
Worksheet with answers:
- Calculate the time constant with 330 Ω and 1,000 µF. (0.33 seconds.)
- Predict the time constant with 1 kΩ. (1 second; the fade lasts about three times longer, but starts dimmer.)
- Why must the capacitor go in the right way round? (Electrolytic capacitors are polarized; reversed, they can bulge, leak or fail.)
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.