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LED polarity

LEDs are one-way doors: long leg to plus, or nothing happens.

first shows up at Level 1 · Light

An LED (Light Emitting Diode) is a diode, which is a fancy word for a one-way door for electricity. Point it the right way and current flows and it glows. Point it the wrong way and nothing happens — no light, no damage at low voltages, just… nothing.

How to tell the two legs apart

  • The long leg is the anode (+). It goes toward the plus side.
  • The short leg is the cathode (−). It goes toward the minus side.
  • Look at the plastic body: one side has a tiny flat spot on the rim — that's the − side too. (Useful when the legs get bent or trimmed and you can't tell length anymore.)

Why one-way?

Inside the LED, two special materials meet at a junction. Electrons can only cross that junction in one direction — from − to + inside the part, which means current flows from + to − through it in the outside circuit. Flip it around and the junction blocks the flow.

If a project on this site isn't lighting up, checking the LED direction is almost always the first thing to try.

Projects that teach this

First Light Build a real flashlight circuit and learn why every circuit is a loop. L1 15 min Morse Code Messenger A push button, an LED, and a secret code — send real messages with light. L1 20 min Twin Reds Two red LEDs on one loop — same current, half the leftover push, and a lesson in how voltages stack when parts sit end to end. L1 15 min Tabletop Traffic Light Three LEDs on three parallel lanes, all lit by one switch — build a tabletop traffic light and discover parallel circuits. L1 25 min The Vault Two buttons, one loop — the LED only lights when BOTH are pressed at the same time. L1 15 min Do Not Disturb Sign A green "chill" LED plus a switched red "leave me alone" LED — two lanes on one battery, and a switch that only touches one of them. L1 20 min Handhold Chain Three switches in a line — the LED only lights when all three are closed at once. AND, one step past The Vault. L1 15 min Two-Door Signal Two slide switches in parallel, one shared LED — either switch alone lights the lamp. The OR gate, at Level 1. L1 15 min Signal Light A slide switch and a push button in parallel — the slide is the steady beacon, the button is the extra flash. Either lights the lamp. L1 15 min Escape Hatch The Vault with a big red emergency button wired around both locks — turn BOTH keys, or hit the button, and the light comes on. L1 20 min The Brightness Lab Run a real experiment: predict, measure with your eyes, and prove Ohm's law. L1 30 min Panic Doorbell Two buttons wired so either one fires the same buzzer and light — your first OR gate. L2 20 min Two-Tone Doorbell Two call buttons, one doorbell: LOUD rings the buzzer and lights the porch LED; QUIET starves the buzzer and shows why it can't just be dimmed. L2 25 min Arm & Ring A doorbell that ignores you unless a hidden slide switch is armed first — series-AND with sound, plus an 'armed' LED that shows the state. L2 20 min Two-Hand Bell Two push buttons in series feeding a buzzer and an LED — both hands must stay pressed for the alarm to sound. L2 20 min Silent Alarm Three panic buttons wired in parallel, all inside an armed lane — any one of them fires the buzzer, and a small LED tells you the alarm is watching. L2 25 min Quiz Buzzer Two teams, two big buttons: yours lights your LED and rings your buzzer, theirs does the same on their side. Two independent doorbells sharing one battery. L2 20 min Dusk Chorus The Curfew Beeper with a red LED wired in parallel with the buzzer — one dusk sensor drives a chime and a lamp at the same time. L3 30 min Warm Hand Chorus Fever Beeper with a red LED added in parallel with the buzzer — one warm-hand sensor drives a chime and a lamp at the same time. L3 30 min Party Spinner One button, three mouths: a spinning motor, a red LED, and a buzzer all fire together. One transistor drives them all. L4 25 min

Parts you'll meet along the way

L1 1 kΩ resistorL1 10 kΩ resistorL1 220 Ω resistorL1 3×AA battery pack (4.5 V)L1 Green LED (5 mm)L1 Jumper wiresL1 Push button (tactile)L1 Red LED (5 mm)L1 Slide switchL1 Solderless breadboardL1 Yellow LED (5 mm)L2 Active buzzer (3–5 V)L3 Light sensor (LDR)L3 NPN transistor (2N2222)L3 Temperature sensor (10 kΩ thermistor)L4 DC motor (3 V)L4 Diode (1N4001)