part · Level 1 · Light · passive
What a 220 ohm resistor is for
220 Ω resistor — A speed bump for electricity. This one protects LEDs.
What it is
A resistor is a part that resists the flow of current — a deliberate bottleneck. Its size is measured in ohms (Ω): more ohms, less current for the same voltage. Inside the little beige cylinder is a film of carbon or metal trimmed to exactly the right amount of resistance.
Resistors have no polarity. Either way round is fine, and they do not care whether they sit before or after the part they protect, as long as they are in the same loop.
220 Ω is the most-used value in beginner electronics for one reason: it is the right size to keep a 5 mm LED under its 20 mA limit on a 3–5 V supply. Yours is marked with three colour bands — red, red, brown — plus a gold tolerance band.
How it works
Reading the bands. Each colour is a digit: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. The first two bands are digits, the third is how many zeros. Red-red-brown = 2, 2, one zero = 220 Ω. Gold means ±5 %, so the real value is somewhere between 209 and 231 Ω.
Ohm's law. Current = voltage ÷ resistance. With a red LED (2.0 V forward voltage) on the 4.5 V pack, the resistor sees 4.5 − 2.0 = 2.5 V, so 2.5 ÷ 220 ≈ 11 mA. With a green LED at 2.1 V: 2.4 ÷ 220 ≈ 10.9 mA. Both well under 20 mA and well above the few mA where an LED looks dull.
Power. Resistors turn the energy they block into heat. Power = current × voltage across them = 0.011 A × 2.5 V ≈ 0.028 W. Your resistor is rated for 0.25 W, so it runs nine times below its limit and stays cool. Even the worst case — the full 4.5 V straight across it — is 4.5 ÷ 220 = 20 mA and 4.5 × 0.020 = 0.09 W. Still fine.
The numbers
| Resistance | 220 Ω — bands: red, red, brown (+ gold ±5 %) |
|---|---|
| Current with a red LED at 4.5 V | ≈ 11 mA |
| Current if 4.5 V is straight across it | ≈ 20 mA — still safe for the resistor |
| Power in normal use | ≈ 0.03 W — rated 0.25 W |
| Polarity | none |
See it working
The First Light circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Reading the bands from the wrong end. Brown-red-red is 1.2 kΩ, not 220 Ω. Fix: the gold band goes on the right; read away from it.
- One 220 Ω "shared" by two parallel LEDs. Each LED gets an unequal share. Fix: one resistor per LED lane.
- Both legs in the same breadboard strip. The resistor is bypassed and the LED sees the full 4.5 V. Fix: legs in different rows.
- Thinking a bigger resistor is "safer" for everything. 10 kΩ on an LED gives 0.25 mA — practically off. Fix: 220 Ω for LEDs, bigger values for sensing.
Questions people ask
- What are the colour bands for a 220 ohm resistor?
- Red, red, brown, then gold: 2, 2, one zero = 220 Ω at ±5 %.
- Why do LEDs use a 220 ohm resistor?
- Because on a 3–5 V supply it lands the LED current in the safe 10–15 mA range. At 4.5 V with a 2.0 V LED: (4.5 − 2.0) ÷ 220 ≈ 11 mA, under the 20 mA maximum.
- Does it matter which side of the LED the resistor goes on?
- No. In a series loop the same current flows through every part, so the resistor limits it from either side.
Projects that use this part
- 🔦 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
- 🔔 Door Buzzer — Your first sound circuit — a real doorbell for your bedroom door. L2 · 20 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
- 🔈 Whisper Alarm — A doorbell with a volume knob — the pot lives in the buzzer lane, so twisting it changes the beep without touching the light. L2 · 25 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
- 🌙 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 — Resistors · SparkFun — Voltage, Current, Resistance, and Ohm's Law · Kuphaldt — Lessons in Electric Circuits, Ohm's Law