Level 1 · Light · project 1 of 24 · 15 min · ●○○○○
First Light
Build a real flashlight circuit and learn why every circuit is a loop.
What you need
- 1× 3×AA battery pack (4.5 V)
- 1× Solderless breadboard
- 3× Jumper wires
- 1× Red LED (5 mm) long leg toward plus
- 1× 220 Ω resistor red-red-brown — see the math below
- 1× Slide switch
you'll learn: Circuits are loops · LED polarity · Ohm's law · Switches
Circuit map
The circuit you'll build, running on the Workbench right now.
simulating…
If it doesn't work
The boring ways this circuit actually breaks — check these first before you unwire everything.
- Nothing at all. The AA pack has its own on/off switch — check it. Then check that both wires (red +, black −) actually reach the breadboard rails.
- LED stays dark. Flip the LED. The long leg (anode) has to face the + side. LEDs are one-way doors — reversing them is harmless, so just try the other way.
- LED super bright, then dies with a pop. You skipped the resistor or grabbed the wrong one. 220 Ω is red-red-brown. A wire alone in that slot fries the LED in about a second.
- Slide switch does nothing. The slide switch has three legs. Use the middle leg and ONE outer leg — the other outer leg is off. Sliding then connects middle-to-outer.
- Works only sometimes. A leg is barely in the hole. Push each part all the way down — you should feel the clip grab. Loose legs make random circuits.
What you're building
A flashlight. Slide the switch, light comes on. Slide it back, light goes off. It sounds simple, but it teaches the idea every circuit needs: a loop.
The big idea: circuits are loops
Electricity only flows if it can travel in a complete circle: out of the battery's + side, through your parts, and back into the battery's − side. Break the circle anywhere and everything stops. That's all a switch is — a gap in the loop that you control.
Build it
- Push the slide switch into the breadboard so its three legs sit in three different rows.
- Push the LED in nearby. Look closely: one leg is longer. The long leg is the + side (its fancy name is the anode).
- Connect the battery pack's red (+) wire to the switch's middle leg.
- Wire one outer leg of the switch to the LED's long leg... but wait — put the 220 Ω resistor in between. (Why? Math below. Never skip it.)
- Connect the LED's short leg to the battery pack's black (−) wire.
- Slide the switch. Light!
If nothing happens: is the loop really complete? Is the LED in backwards? (Flipping it won't hurt it — it's a one-way door, not a fuse.)
Why the resistor? Do the math
An LED is greedy. Connected straight to the battery it gulps far too much current and burns out — sometimes instantly. The resistor is a speed bump that sets how much current flows, and Ohm's law tells us how big a bump we need:
current = voltage ÷ resistance
- Your battery pack pushes with 4.5 V.
- A red LED uses up about 2.0 V of that just being on (its forward voltage).
- That leaves 4.5 − 2.0 = 2.5 V pushing current through the resistor.
- Through 220 Ω: 2.5 V ÷ 220 Ω ≈ 0.011 A, which is 11 mA.
A 5 mm LED is happiest around 10 mA and must stay under about 20 mA. Our 11 mA sits right in the comfy zone. You built a flashlight and chose the resistor on purpose. That's engineering.
🧠 Your challenge
No single right answer. That's the point.
- Swap the 220 Ω resistor for the 1 kΩ one. Predict first: brighter or dimmer? Then check. Can you calculate the new current before you look? (Hint: 2.5 V ÷ 1000 Ω.)
- Can you move the switch to the other side of the LED — between the short leg and the battery? Does it still work? What does that tell you about loops?
- Design a flashlight that uses the push button instead. When would a button be better than a slide switch? When would it be worse?
For grown-ups: safety notes
- 4.5 V from AA batteries is safe to touch — this whole level is skin-safe.
- Never connect the battery's red wire directly to its black wire (a short circuit). The wires and batteries get hot fast.
- An LED without its resistor can pop. It won't hurt you, but it's a dead LED.
- Keep loose batteries and small parts away from little kids and pets.
Checked against
Keep building
You might also like
shares ideas with this one