skill
LED polarity
LEDs are one-way doors: long leg to plus, or nothing happens.
first shows up at Level 1 · Light
See it working
simulating…
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. Morse Code Messenger A push button, an LED, and a secret code — send real messages with light. 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. Two Greens Twin Reds with green LEDs — bigger voltage bites, less leftover for the resistor, and a great lesson in how far a 4.5 V battery can actually push. Night Pin One 10 kΩ, one red LED, one slide — the simplest dimmer. A single big resistor runs the LED at 0.27 mA: invisible in daylight, perfect for a dark room. Tabletop Traffic Light Three LEDs on three parallel lanes, all lit by one switch — build a tabletop traffic light and discover parallel circuits. Bike Torch One yellow LED, two switches: a slide for a battery-friendly low beam, a push button for a bright flash, both lanes feeding the same lamp in parallel. Signal Pair Two push buttons, two LEDs, one battery — no sharing. Each button lights its own lamp with its own current, independent of the other. Dim Candle A 1 kΩ and a 10 kΩ in series throttle a red LED to a candle glow. Flip the bypass slide — the 10 kΩ gets shorted and the LED pops to a clear, steady glow. Three Sisters Three red LEDs side by side from one battery — each with its own 220 Ω — proving parallel lanes do their own Ohm's law and the battery adds the bill. One for All Three LEDs on ONE shared 220 Ω — the shortcut that fails. Red and yellow dim-split the current; green sits dark. Lesson: each LED needs its own resistor. Step Up Three 1 kΩ lanes in parallel into one red LED, each with its own slide switch. One slide = candle, two = glow, three = full lamp. The lanes stack. Longer Loop Three 1 kΩ speed bumps stacked end-to-end on one lane feed a single red LED. Resistors in series ADD up — the LED is throttled to a near-dark 0.87 mA. Bright and Dim Two red LEDs on one battery — one through 220 Ω (bright), one through 1 kΩ (candle). Same LEDs, two currents. The resistor sets each lane's brightness. Rainbow Loop Red, yellow and green LEDs on three parallel 220 Ω lanes under one slide. One flip lights all three, each lane running its own Ohm's law. Reserve Beacon A 10 kΩ in series with the LED keeps a dim always-on pilot glow. Flip the bypass slide — it shorts the 10 kΩ, and the pilot pops to full First-Light brightness. Short Cut First Light with a slide switch wired straight across the LED. Flip the slide and the LED goes dark even though the loop stays complete. The Vault Two buttons, one loop — the LED only lights when BOTH are pressed at the same time. 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. Handhold Chain Three switches in a line — the LED only lights when all three are closed at once. AND, one step past The Vault. Two-Door Signal Two slide switches in parallel, one shared LED — either switch alone lights the lamp. The OR gate, at Level 1. 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. 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. The Brightness Lab Run a real experiment: predict, measure with your eyes, and prove Ohm's law. Panic Doorbell Two buttons wired so either one fires the same buzzer and light — your first OR gate. 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. 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. Two-Hand Bell Two push buttons in series feeding a buzzer and an LED — both hands must stay pressed for the alarm to sound. 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. 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. Vault Chime Three slide switches in a line, a buzzer and a red LED hanging off the end — the chime only sings and glows when every single switch is closed. Hidden Mute A doorbell with a secret slide switch tucked inside just the buzzer lane: flip it and the ring button still lights the LED, but the sound stays off. Power Pilot A green pilot LED that glows the whole time the battery is plugged in, next to a red alarm LED and buzzer that only fire when the button is pressed. Shared Knob One pot sits above the merge of two parallel LED lanes (red + yellow). Twist it and BOTH LEDs fade together — the knob throttles the shared rail. 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. 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. Dawn Chorus Cookie Jar Alarm with a red LED added in parallel with the buzzer — one light sensor drives a chime and a lamp the moment the sun hits. Trip-Wire Chorus Trip Wire Alarm with a red LED added in parallel with the buzzer — one thin wire holds the whole chorus silent; snap it and both mouths fire. Night Doorbell A doorbell that only works at night. The ring button fires buzzer and LED together — but only once the LDR sees darkness. In daylight the button does nothing. Alley Lamp Automatic Night Light with a push-button override. Dark room OR button press fires the lamp — a wired-OR built straight onto the transistor's base. Party Spinner One button, three mouths: a spinning motor, a red LED, and a buzzer all fire together. One transistor drives them all.
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 10 kΩ potentiometerL2 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)