Components · #6 of 48

Diodes + LEDs

Current Limiting, Polarity, Brightness

Why it matters

LEDs are diodes that emit light. They have polarity (direction matters) and need current limiting (resistors) to prevent destruction.

The idea

What Is a Diode?

A diode is a one-way valve for electricity:

Forward bias: Current flows (low resistance)
Reverse bias: Current blocked (high resistance)
Diodes have a voltage drop (~0.7V for silicon, ~2V for LEDs)

LEDs (Light Emitting Diodes)

LEDs are diodes that emit light when current flows:

Anode (+): Longer leg, connects to positive
Cathode (-): Shorter leg, connects to ground
Voltage drop: ~2V (red) to ~3.5V (blue/white)
Current: Typically 20mA max (check datasheet)

Current Limiting

LEDs must have a current-limiting resistor:

Without a resistor, LED draws too much current → destroys itself
Formula: R = (V_supply - V_LED) / I_desired
Example: 3.3V supply, 2V LED, 15mA desired → R = (3.3 - 2) / 0.015 = 87Ω (use 100Ω)

Brightness Myths

Higher voltage ≠ brighter — current determines brightness
PWM dimming — change duty cycle, not voltage
Color affects voltage drop — blue/white need more voltage than red

Demo

LEDs are simple but critical. Review polarity and current limiting before connecting.

Key takeaways

Diodes are one-way valves — direction matters
LEDs have polarity: anode (+) to positive, cathode (-) to ground
Always use a current-limiting resistor with LEDs
Brightness is controlled by current (or PWM duty cycle), not voltage

Going deeper

LEDs have a forward voltage (V_f) that varies by color. Red: ~1.8V, Green: ~2.1V, Blue/White: ~3.0-3.5V. For PWM dimming, use frequencies >100Hz to avoid visible flicker. For battery-powered devices, use lower current (5-10mA) to save power.

Math details

Current-limiting resistor:
  R = (V_supply - V_LED) / I_desired

Example calculations:
  V_supply = 3.3V
  V_LED = 2V (red LED)
  I_desired = 15mA = 0.015A

  R = (3.3V - 2V) / 0.015A = 1.3V / 0.015A = 87Ω

  Use standard value: 100Ω (slightly safer, I = 13mA)

Power in resistor:
  P = I² × R = (0.013A)² × 100Ω = 0.017W (safe, <0.25W)

Power in LED:
  P = V_LED × I = 2V × 0.013A = 0.026W

Implementation

LLM Prompt: LED Resistor Calculator

Write a Rust function that calculates current-limiting resistor for LED.
Input: V_supply, V_LED, I_desired. Output: resistor value (round to nearest
standard value: 220Ω, 330Ω, 470Ω, 1kΩ, etc.). Include validation: warn if
calculated resistor < 100Ω (may be too low) or if I_desired > 20mA.

Lab Exercise

Identify LED polarity (longer leg = anode)
Calculate resistor: 3.3V supply, 2V LED, 15mA → R = 87Ω (use 100Ω)
Build circuit: 3.3V → resistor → LED anode → LED cathode → GND
Measure current with multimeter (should be ~13mA)
Try reversing LED — it won’t light (proves polarity matters!)

💡 Diodes + LEDs