An **LED (Light Emitting Diode)** is a semiconductor light source that emits light when current flows through it. It is one of the most fundamental components in modern electronics. --- ## 1. Physical Structure and Polarity Unlike resistors, LEDs are **polarized**, meaning they only work when connected in one specific direction. | Part | Description | Identification | | :--- | :--- | :--- | | **Anode (+)** | The positive terminal. | Longer leg / Connected to the smaller internal metal part. | | **Cathode (-)** | The negative terminal. | Shorter leg / Flat side on the plastic casing / Larger internal metal part. | | **Semiconductor Die** | The heart of the LED. | A tiny chip that converts electricity into light. | | **Epoxy Lens** | The plastic body. | Protects the die and focuses the light beam. | --- ## 2. Technical Specifications To use an LED safely, you must understand its electrical limits: ### A. Forward Voltage ($V_f$) This is the amount of voltage required to "turn on" the LED. If the voltage is too low, it won't light up. If it is too high without a resistor, the LED will burn out. * **Red/Yellow:** ~1.8V to 2.2V * **Blue/White/Green:** ~3.0V to 3.4V ### 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 fail. --- ## 3. The Current Limiting Resistor Because LEDs have very low internal resistance once they start conducting, you must always use a resistor in series to limit the current. **The Formula (Ohm's Law):** $$R = \frac{V_{source} - V_{forward}}{I_{forward}}$$ **Example calculation:** If you have a 9V battery and a Red LED (2V, 20mA): 1. $(9V - 2V) = 7V$ 2. $7V / 0.02A = 350 \Omega$ *Result: Use a **350 Ohm** resistor (or the closest standard value like 330Ω).* --- ## 4. Types of LEDs LEDs come in various packages depending on the application: * **Through-Hole:** The classic 3mm or 5mm bulbs with long legs used in breadboards. * **SMD (Surface Mount Device):** Tiny, flat LEDs used on circuit boards (e.g., in smartphones). * **RGB LED:** Contains Red, Green, and Blue chips in one package to create any color. * **7-Segment Display:** An arrangement of 8 LEDs used to display numbers (0-9). ---