Quick NPN BJT switch circuit – Bipolar Junction Transistor

Table of Contents

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.

2N3904 NPN BJT pin layout schematic symbol and current paths diagram by electronzap electronzapdotcom
2N3904 NPN BJT pin layout schematic symbol and current paths diagram by electronzap electronzapdotcom

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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 and direction of voltage and conventional current flow (positive to negative) is shown in the first diagram.

Diagram

NPN Bipolar Junction Transistor switch basic circuit schematic electronics diagram by electronzap electronzapdotcom
NPN Bipolar Junction Transistor switch basic circuit schematic electronics diagram by electronzap electronzapdotcom

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.

Video

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

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