gen-define-a-transistor-as-a-switch-and-give-one-short-worked-ex-ae07

Define a transistor as a switch and give one short worked example.

# Transistor as a Switch A **transistor** acts as an electrically controlled switch that can turn current flow ON or OFF between two terminals (collector and emitter for BJT, or drain and source for MOSFET) based on the signal applied to a third terminal (base or gate). ## Key Principle: - **Small control signal → Large current switched** - **ON state**: Control signal present → current flows (switch closed) - **OFF state**: No control signal → no current flows (switch open) --- ## Worked Example: LED Control with NPN Transistor **Goal**: Use a 5V microcontroller (max 20mA output) to control a high-power LED (100mA). **Circuit**: - NPN transistor (e.g., 2N2222) - LED + 330Ω resistor from +12V to collector - Emitter to ground - 10kΩ resistor from microcontroller pin to base **Operation**: - **Microcontroller outputs 5V** → Small base current (~0.5mA) flows → Transistor saturates → 100mA flows through LED → **LED ON** - **Microcontroller outputs 0V** → No base current → Transistor off → No collector current → **LED OFF** **Result**: The transistor switches 100mA (for the LED) while the microcontroller only provides 0.5mA—acting as a current-amplifying switch.