3-Aspect Signal LED Wiring

LED signals are reliable, bright, and last virtually forever. Here's how to wire them correctly.

LED Basics for Signals

• **Forward Voltage:**
• Red: 2.2V
• Yellow: 2.2V
• Green: 2.2V (some: 3.0V)

• **Current:** 10-20mA typical, 5-10mA for scale appearance

Common Cathode Configuration

All LEDs share a negative (ground) connection:

`

+12V ──┬── 1K ── Red LED ────┐

├── 1K ── Yellow LED ──┤── GND (common)

└── 1K ── Green LED ───┘

`

Control: Apply +12V to light each LED

Common Anode Configuration

All LEDs share a positive connection:

`

+12V (common) ──┬── Red LED ── 1K ────┐

├── Yellow LED ── 1K ──┤── Control

└── Green LED ── 1K ───┘

`

Control: Connect to ground to light each LED

Resistor Selection

Using R = (Vsupply - Vled) / I:

• 12V supply, 10mA: (12 - 2.2) / 0.01 = 980Ω → use 1KΩ
• 5V supply, 10mA: (5 - 2.2) / 0.01 = 280Ω → use 330Ω

Multi-Head Signals

For signals with multiple heads:

• Use separate resistors per LED
• Consider LED driver ICs for many signals
• MAX7219 can drive 64 LEDs