part · Level 1 · Light · output
Yellow LED resistor calculation
Yellow LED (5 mm) — Like the red LED, in caution-light yellow.
What it is
A yellow LED is built the same way as a red one — a chip of semiconductor that glows when current flows through it one way — but the crystal recipe is tuned so the light comes out at a shorter wavelength, around 590 nanometres, which your eye reads as yellow-amber.
Like every LED it has a long leg (anode, +) and a short leg beside a flat rim edge (cathode, −), and it needs a resistor in series or it will burn out.
Yellow is a great signalling colour: it stands out against most rooms and reads clearly across a distance, which is why the Morse Code Messenger uses it.
How it works
The maths is identical to the red LED because the forward voltage — the push needed before the LED conducts — is about 2.0 V for both:
- Voltage across the resistor: 4.5 V − 2.0 V = 2.5 V
- Current with 220 Ω: 2.5 ÷ 220 ≈ 11 mA — right in the 10–15 mA sweet spot, under the 20 mA maximum.
If yellow looks a little dimmer than red at the same 11 mA, that is your eyes and the LED's efficiency, not the circuit. Want more? Drop to 150 Ω: 2.5 ÷ 150 ≈ 17 mA, still legal. Do not go below 125 Ω (that is 20 mA).
Flashing it costs nothing extra: a push button in series simply breaks and remakes the loop. When the button is open the current is exactly 0 mA, so a Morse key on an LED can run for weeks on one pack.
Running yellow from another battery? The LED resistor calculator gives the exact resistor for any pack.
The numbers
| Forward voltage | ≈ 2.0 V — same as red |
|---|---|
| Maximum current | 20 mA |
| Current with 220 Ω at 4.5 V | ≈ 11 mA |
| Current with 150 Ω at 4.5 V | ≈ 17 mA — brighter, still safe |
| Wavelength | ≈ 590 nm — amber-yellow |
See it working
The Morse Code Messenger circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Mistaking it for a "bright" LED and skipping the resistor. Same 20 mA limit, same death. Fix: 220 Ω in series.
- Yellow next to red on one resistor. Their forward voltages are so close that this almost works — then one hogs the current. Fix: separate resistor per LED.
- Backwards. Dark, unharmed. Fix: long leg toward +.
Questions people ask
- What is the forward voltage of a yellow LED?
- About 2.0 V, the same as red. With a 4.5 V pack that leaves 2.5 V for the resistor, so 220 Ω gives (4.5 − 2.0) ÷ 220 ≈ 11 mA.
- Why does my yellow LED look dimmer than the red one?
- Yellow LEDs are usually a little less efficient, and eyes see colours differently. The current is the same. Use 150 Ω for ~17 mA if you want it brighter.
Projects that use this part
- 📡 Morse Code Messenger — A push button, an LED, and a secret code — send real messages with light. L1 · 20 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
- 🚲 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. L1 · 15 min
- 🚦 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. L1 · 15 min
- 🥀 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. L1 · 15 min
- 🌈 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. L1 · 15 min
- 🎚️ 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. L2 · 15 min
- 🌅 Sun Beacon — Sunrise Fan with a second lane — a yellow LED lights and a motor spins together the moment daylight hits the sensor. One brain, two mouths. L4 · 25 min
- 🌙 The Night Gate — A night light switched by a MOSFET. The gate takes no current, so the moment it turns on is plain voltage-divider math you can work out exactly. L5 · 30 min
- 🏃 The Four-Step Chaser — A 555 ticks five times a second and a CD4017 counter walks the light from LED to LED, round and round. Hold a button and the chase freezes. L6 · 60 min
- 💡 The Two-Switch Hall Light — A light with a switch at each end of the hall: flip either and the light changes. Four NAND gates in one CD4011 make an XOR. L6 · 60 min
verified against: SparkFun — Light-Emitting Diodes (LEDs) · Adafruit — All About LEDs