2N3904 NPN Transistor

Description

DESCRIPTION The 2N3904 are a good general purpose NPN transistor for low power amplification and switching. PACKAGE INCLUDES: Qty 5 ? 2N3904 NPN General Purpose Transistors KEY FEATURES OF 2N3904 NPN GENERAL PURPOSE TRANSISTOR: NPN Bipolar Silicon construction Standard EBC pin-out in TO-92 package Up to 40V collector-emitter and 60V collector-base Up to 200mA current 100MHz switching speed The 2N3904 are a good inexpensive general purpose NPN transistor for low power amplification and switching and should be part of every parts bin. These can be very handy when you need to boost the output of a uC pin to drive something that it can not drive directly.? NPN transistors are generally used in low-side switching applications where they are connected between the load and ground. Here is a simple example circuit.? We have an LED indicator that we want to drive.? The indicator consists of 3 LEDs which each draw 20mA of current. A uC pin can easily drive an LED at 20mA, but this circuit will require 60mA which is outside the range of the typical uC drive capability.? You could use 3 pins on the uC to drive the 3 LEDs, but that is a waste of pins unless you have pins to spare.? This simple circuit allows you to drive all three LEDs off the same uC pin. The 4.7K resistor on the base is there to limit the current out of the uC pin to safe levels and the value is not overly critical.? A 1K or 10K will also work. The 50 ohm resistor sets the overall current for the 3 LEDs.? This value may