Bipolar Junction Transistors (BJTs) use a weak signal (low current and/or voltage) to control a larger voltage and/or current. The switch circuit is the simplest BJT circuit.

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The 2N3904 is very popular NPN Bipolar Junction Transistor (BJT). Therefore, I use them in many of my videos.

The pin layout for the 2N3904 and the direction of voltage and conventional current flow (positive to negative) is shown in the first diagram.

Diagram

The NPN BJT collector must be able to handle the amount of current needed by the load. This maximum collector current (Ic) varies widely between different transistors. Always check the datasheet for the particular part number you are using to learn all it’s limitations.

The 2N3904 is rated at a maximum of 200mA collector current. Should keep it under half that current and it’s maximum wattage rating of 625mW for the vast majority of the time.

Video

Quick NPN BJT switch circuit build using 2N3904 Bipolar Junction Transistor schematic to breadboard



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