Project 1 — Make the Light Come On

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

Project 1 — Make the Light Come On

The one idea: electricity flows only when the loop is unbroken.

What you need

3×AA battery holder (4.5 V)
One LED (a red one is easiest — it lights at the lowest voltage)
One resistor, about 330 Ω (anything from 220 Ω to 1 kΩ works)
Three alligator-clip leads

Build it

Make one ring, in this order:

  [Battery +] --clip--> [resistor] --clip--> [LED long leg]
                                                  |
                                              [LED short leg]
                                                  |
  [Battery -] <--------------- clip --------------+

Clip the battery’s + wire to one end of the resistor.
Clip the resistor’s other end to the LED’s long leg (the long leg is +).
Leave the last connection — from the LED’s short leg back to the battery’s − — for the child to make.

Hand the child that last clip. When they connect it, the loop closes and the LED lights. Let them pull it off (light dies) and put it back (light returns) a few times. That open-and-close is the whole lesson.

What to say to the child

“Electricity runs around and around in a ring, like a little train on a track. The light only glows when the train can get all the way around. If there’s a gap anywhere — anywhere — the train is stuck and the light stays off. Watch: you close the gap… and YOU just turned on the light.”

Then let them be the one who opens and closes the loop. Six-year-olds love being the cause.

If it doesn’t light

LED in backwards? The long leg must point toward +. Backwards is harmless — it just won’t light. Great first debugging lesson: “let’s flip it around and try.”
A clip not gripping metal? Alligator clips sometimes bite onto plastic insulation instead of bare wire. Wiggle them onto the shiny metal.
Battery dead or backwards in the holder? Check the little + / − marks.

For you — the physics

The resistor is there for current limiting, and this is the one piece of “why” worth holding in your head:

An LED is a diode. Past its forward voltage (≈1.8–2.2 V for red) its current rises almost vertically with voltage — its dynamic resistance is tiny. Connect it straight across 4.5 V and the current spikes into thermal runaway and the LED dies (often with a satisfying-but-expensive pop). The series resistor sets the current to something safe:

I \;=\; \frac{V_\text{batt} - V_\text{LED}}{R} \;\approx\; \frac{4.5 - 2.0}{330} \;\approx\; 7.5\ \text{mA}

— bright enough to see, gentle on the LED. This is the moment to mention, if the child asks, that the resistor is “a narrow part of the track that keeps the train from going too fast and burning out the light.”

Try next

Swap the 330 Ω for a much bigger resistor (say 4.7 kΩ) → dimmer. The child can see “more squeeze on the track = less light.”
Add a second LED in the loop, one after the other (series). Both light, both a bit dimmer — the loop is shared.

→ Next: Project 2 — Press-to-buzz doorbell, where the gap becomes a button you control on purpose.