Project 4 — The Light That Fades by Itself

Text written by Claude Opus 4.8 (claude-opus-4-8).

Project 4 — The Light That Fades by Itself

The one idea: you can store a little electricity in a “bucket” and let it trickle back out, so the light fades instead of snapping off.

This is the first project worth moving onto the breadboard — the wiring has a branch in it, and the breadboard makes that branch tidy.

What you need

• 3×AA battery holder (4.5 V)
• A largish electrolytic capacitor: 470 µF to 1000 µF
• One LED
• One resistor, about 1 kΩ
• Breadboard + jumper wires (or alligator clips if you prefer to stay visible)

Capacitor polarity matters. An electrolytic capacitor has a − stripe down one side and a shorter − leg. The longer leg is +. Plug it in the right way and don’t exceed its voltage rating (anything ≥10 V is fine here).

Build it

The capacitor sits across the battery to charge; the LED + resistor sit across the capacitor to drain it:

                +-----[ + capacitor − ]-----+
                |                            |
  [Battery +]---+                            +---[Battery -]
                |                            |
                +---[resistor]---[LED]-------+

1. With the battery connected, the capacitor charges (near-instantly) and the LED lights.
2. Now disconnect the battery (pull both battery clips).
3. The LED keeps glowing — and slowly fades to dark. The capacitor is powering it on its own.

Let the child charge and disconnect it over and over. Then change one thing at a time and let them predict:

• Bigger capacitor (1000 µF instead of 470 µF) → fades slower.
• Bigger resistor → fades slower too.

What to say to the child

“This little part is a bucket for electricity. When the battery’s connected, the bucket fills right up. Now watch — I’ll take the battery away completely… and the light’s still on! The bucket is slowly pouring its electricity into the light. See it fading? When the bucket’s empty, the light goes dark. What do you think a bigger bucket would do?”

The slow fade is the wow moment — it’s the first time electricity does something over time instead of instantly.

For you — the physics

This is where your background pays off, because the fade is a clean exponential you can actually measure with a phone stopwatch.

Discharging through resistance $R$, the capacitor voltage decays as

With $R \approx 1\ \text{k}\Omega$ and $C = 1000\ \mu\text{F}$, $\tau = 1$ second — but the LED stops emitting once $V$ drops below its forward voltage (~1.8 V), so the visible fade is a fraction of $\tau$ and feels quick; bump $R$ to 10 kΩ and $C$ to 1000 µF for a long, lazy fade ($\tau = 10$ s). Letting the child vary $R$ and $C$ and watch the fade time track $RC$ is a genuine first encounter with a time constant.

The stored energy is

which at 4.5 V and 1000 µF is about 10 mJ — tiny, which is exactly why it’s safe to play with. (“The bucket only holds a thimbleful, so it can’t hurt you.”)

Note the asymmetry: charging here is near-instant because the charge path has almost no resistance, while the discharge is slowed deliberately by the resistor. If you want a slow fill too, put a resistor in the charge path and the LED will fade in as well as out.

Try next

• Fade-in and fade-out: add a charge-path resistor and use a switch (Project 2!) to connect/disconnect — the LED swells up and dies away like a breath.
• Touch-discharge: charge it, disconnect, then let the child bridge the capacitor’s legs with the LED by hand — they’re “pouring the bucket out” on demand.

→ Next: Project 5 — Steady-hand buzz-wire game, the capstone that puts the loop, the switch, and the buzzer together into a toy.