part · Level 1 · Light · output
Why green LEDs need 2.1 volts
Green LED (5 mm) — Like the red LED, in go-light green.
What it is
A green LED is a light-emitting diode whose crystal is tuned to give off light at about 565 nanometres. It works exactly like the red and yellow ones — current in the long leg (anode, +), out the short leg (cathode, −), resistor always in series — with one small difference in the numbers.
Colour and voltage are linked in an LED. Shorter-wavelength light carries more energy per photon, so the crystal needs a bigger push to make it. Red needs about 2.0 V; this green needs about 2.1 V; blue and white LEDs need around 3 V.
Practically that means a green LED can share resistor values with red and yellow, but not a resistor.
How it works
Redo the Ohm's-law sum with the green forward voltage of 2.1 V:
- Voltage across the resistor: 4.5 V − 2.1 V = 2.4 V
- Current with 220 Ω: 2.4 ÷ 220 ≈ 10.9 mA — call it 11 mA, essentially the same as red.
That tenth of a volt barely matters at 220 Ω. It matters a lot when LEDs are wired in parallel on one resistor: current takes the easiest path, and the lower-voltage red LED opens up first and steals the current while the green one stays dim. Give each LED its own 220 Ω and each gets its own 11 mA.
Two LEDs in series (nose to tail on one resistor) add their voltages instead: 2.0 + 2.1 = 4.1 V, leaving only 0.4 V for the resistor — 0.4 ÷ 220 ≈ 1.8 mA, dim. That is why a 4.5 V pack drives LEDs in parallel lanes, never stacked.
The numbers
| Forward voltage | ≈ 2.1 V — slightly higher than red or yellow |
|---|---|
| Maximum current | 20 mA |
| Current with 220 Ω at 4.5 V | ≈ 10.9 mA — (4.5 − 2.1) ÷ 220 |
| Two in series at 4.5 V | ≈ 1.8 mA — too dim — use parallel lanes |
| Wavelength | ≈ 565 nm |
See it working
The Tabletop Traffic Light circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Green and red in parallel on one resistor. Red hogs it, green sulks. Fix: one resistor per LED, always.
- Expecting green to be as bright as red on 220 Ω. It gets nearly the same current; any difference is efficiency. Fix: try 150 Ω (≈ 16 mA) if you want more.
- Reading a data sheet for a "true green" or blue LED and using 2.1 V. Some greens are 3 V parts. Fix: check the packet; if it is a 3 V LED, the resistor maths changes to (4.5 − 3.0) ÷ 220 ≈ 6.8 mA.
Questions people ask
- What resistor for a green LED on 4.5 V?
- 220 Ω. The forward voltage is about 2.1 V, so (4.5 − 2.1) ÷ 220 ≈ 11 mA — safely under 20 mA and nicely bright.
- Why do different colour LEDs have different forward voltages?
- Light colour is set by the energy of each photon, and the crystal has to be given at least that energy per electron. Higher-energy colours (green, blue, white) need a higher voltage than red.
- Can I put a red and a green LED in series?
- On 4.5 V it is too weak: 2.0 + 2.1 = 4.1 V leaves only 0.4 V for the resistor, about 1.8 mA. Wire them as two parallel lanes with a 220 Ω each instead.
Projects that use this part
- 🚦 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
- 🚪 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
- 🔔 Door Buzzer — Your first sound circuit — a real doorbell for your bedroom door. 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
- 🎮 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
verified against: Adafruit — All About LEDs · SparkFun — Light-Emitting Diodes (LEDs) · SparkFun — Series and Parallel Circuits