part · Level 1 · Light · output
LED forward voltage explained
Red LED (5 mm) — A tiny light with a one-way door — current only flows long-leg-in.
What it is
An LED — light-emitting diode — is a tiny chip of semiconductor crystal that glows when current passes through it in the right direction. No filament, no heat worth mentioning, and it lasts for decades if you treat it right.
A diode is a one-way door for current. Your LED has two legs: the longer one is the anode (+) and goes toward the battery's red wire; the shorter one, next to a flat spot on the rim, is the cathode (−). Wire it backwards and it simply stays dark — no harm done.
The catch: an LED has almost no resistance of its own. Give it 4.5 V directly and it will take whatever current the battery can push, far past its limit, and die — sometimes with a tiny pop. So an LED always travels with a resistor.
How it works
Every diode needs a certain push before it opens up. For a red LED that push is its forward voltage, about 2.0 V. Below 2.0 V it barely conducts; once it is on, the voltage across it stays stuck near 2.0 V no matter how much current flows. That stubbornness is why the resistor gets to set the current.
Work it through with the 4.5 V pack and a 220 Ω resistor:
- Voltage left for the resistor: 4.5 V − 2.0 V = 2.5 V
- Ohm's law, current = voltage ÷ resistance: 2.5 V ÷ 220 Ω ≈ 0.011 A = 11 mA
A 5 mm LED is bright and happy at 10 mA and is rated for a maximum of 20 mA. The smallest resistor that stays legal is 2.5 V ÷ 0.020 A = 125 Ω; 220 Ω leaves comfortable margin.
Go the other way with 1 kΩ: 2.5 V ÷ 1000 Ω = 2.5 mA — still clearly lit, just softer. With 10 kΩ: 0.25 mA, a faint glow in a dark room. Brightness tracks current, not voltage, and the resistor is the dial.
The numbers
| Forward voltage | ≈ 2.0 V — at 10 mA; varies a little between LEDs |
|---|---|
| Maximum current | 20 mA — aim for 10–15 mA |
| Current with 220 Ω at 4.5 V | ≈ 11 mA — (4.5 − 2.0) ÷ 220 |
| Smallest safe resistor at 4.5 V | 125 Ω — 2.5 V ÷ 20 mA |
| Long leg | anode (+) — short leg / flat rim edge = cathode (−) |
See it working
The First Light circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- No resistor. The LED runs at whatever the battery can push, overheats and dies. Fix: 220 Ω in series with every LED, on its own lane.
- Backwards. Nothing lights. Fix: flip it so the long leg faces +. It is not damaged.
- Two LEDs sharing one resistor in parallel. One hogs the current and the other is dim. Fix: one resistor per LED.
- Legs trimmed equal so you cannot tell which is which. Fix: find the flat edge on the plastic rim — that side is −.
- Resistor on the wrong side? There is no wrong side. Before or after the LED, a series resistor limits the same current.
Questions people ask
- Which leg of an LED is positive?
- The longer leg is the anode (+). The shorter leg, on the side with the flat edge on the rim, is the cathode (−) and goes toward the battery black wire.
- Why does my LED burn out?
- It was connected without a series resistor, or with one that was too small. An LED cannot limit its own current; above about 20 mA it overheats. (4.5 − 2.0) ÷ 220 Ω ≈ 11 mA is the safe target.
- What resistor do I need for a red LED on 4.5 volts?
- 220 Ω is the standard choice: it gives about 11 mA. Anything from 150 Ω to 1 kΩ works — smaller is brighter, and 125 Ω is the absolute minimum.
- Does an LED work in both directions?
- No. It is a diode, so current flows only from anode to cathode. Reversed, it stays dark but is not harmed at battery voltages.
Projects that use this part
- 🔦 First Light — Build a real flashlight circuit and learn why every circuit is a loop. L1 · 15 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
- 🎛️ Dial-a-Brightness — Twist a knob and watch the LED fade up and down — your first "how much" input. L2 · 20 min
- 🔧 Continuity Tester — Your first real tool — two probe wires that beep and light up whenever the tips touch anything that conducts. L2 · 20 min
- 🚨 Panic Doorbell — Two buttons wired so either one fires the same buzzer and light — your first OR gate. L2 · 20 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
- 🌙 Automatic Night Light — A light that switches itself on when the room gets dark — no hands, no code. L3 · 35 min
- 🌅 Sunrise Alarm — The night light — flipped. Swap two parts and the LED wakes up with the sun instead. L3 · 25 min
- 🌡️ Warm Hand Alarm — Cup your hand over the sensor and the LED lights up — a thermometer that turns into a switch. L3 · 30 min
- 📖 Reading Light — The Automatic Night Light with a knob on top — turn it to dial in how dark the room has to get before the lamp fires. Your first tunable threshold. L3 · 30 min
- 🏝️ Vacation Nightlight — The Automatic Night Light with a manual override — flip the slide switch off and the lamp stays dark even in a pitch-black room. L3 · 25 min
- 📚 Study Lamp — The Automatic Night Light with a force-on override — flip the slide switch and the lamp stays lit even in daylight, ignoring the sensor. L3 · 25 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
verified against: SparkFun — Light-Emitting Diodes (LEDs) · Adafruit — All About LEDs · SparkFun — Voltage, Current, Resistance, and Ohm's Law