The term **LED** stands for **Light Emitting Diode**. It is a semiconductor light source that emits light when current flows through it. Below is a detailed breakdown of its electronic properties, construction, and common types. --- ## 1. Physical Structure and Pinout A standard through-hole LED has two main terminals. Because an LED is a diode, it is **polarized**, meaning it only works when connected in the correct direction. | Part | Name | Description | | :--- | :--- | :--- | | **Long Leg** | **Anode (+)** | The positive terminal. | | **Short Leg** | **Cathode (-)** | The negative terminal (often marked by a flat side on the plastic casing). | | **Semiconductor Die** | **PN Junction** | The "heart" of the LED where light is generated. | | **Epoxy Lens** | **Casing** | Protects the die and focuses the light beam. | --- ## 2. Key Electrical Characteristics To use an LED correctly in a circuit, you must understand these three fundamental parameters: ### A. Forward Voltage ($V_f$) This is the amount of voltage required to "turn on" the LED. It varies significantly by the color of the LED: * **Red/Orange:** 1.8V – 2.2V * **Yellow:** 2.1V – 2.4V * **Green:** 2.0V – 3.5V * **Blue/White:** 3.0V – 3.6V ### B. Forward Current ($I_f$) Most standard 5mm LEDs operate safely at **20mA (0.02 Amps)**. Exceeding this limit will cause the LED to overheat and burn out. ### C. Current Limiting Resistor Because LEDs have very low internal resistance once they reach their forward voltage, you **must** use a resistor in series to prevent them from drawing too much current. **The Formula (Ohm's Law):** ```text R = (Source Voltage - Forward Voltage) / Desired Current ``` *Example: For a 5V supply, a Red LED (2V), and 20mA current:* `R = (5V - 2V) / 0.02A = 150 Ohms` --- ## 3. Types of LEDs LEDs come in various packages depending on the application: | Type | Full Name | Common Use | | :--- | :--- | :--- | | **Through-Hole** | Dual In-line Package (DIP) | Breadboarding, indicator lights, toys. | | **SMD** | Surface Mount Device | Smartphones, PCB backlighting, LED strips. | | **RGB** | Red-Green-Blue LED | Multi-color displays and mood lighting. | | **COB** | Chip on Board | High-power floodlights and street lamps. | --- ## 4. How It Works (The Physics) When power is applied, electrons cross the **P-N junction** from the negative side to the positive side. As electrons fall into "holes" (lower energy levels), they release energy in the form of **photons** (light). This process is called **Electroluminescence**.