NPN transistor · TO-92
2N3904 Pinout
Flat face toward you, legs pointing down, left to right: 1 emitter, 2 base, 3 collector (E-B-C).
A general-purpose NPN up to 200 mA, popular in US kits and Arduino starter sets.
- Pin 1: Emitter (E)Goes to 0 V (ground) when the transistor is used as a switch.
- Pin 2: Base (B)The control pin. A small current into it, always through a resistor, switches the transistor on.
- Pin 3: Collector (C)Goes to the load (LED, relay, buzzer). Current flows in here and out of the emitter.
Check your part before wiring. Read the maker and the full part number printed on it, open that maker's datasheet, or test the legs with a multimeter's diode setting.
PNG 1440 × 1280 pixels, SVG to any size. Both keep the wirecora.com mark along the bottom.
Pinout table
Which leg of the 2N3904 is which?
| Pin | Name | What it does |
|---|---|---|
| 1 | Emitter E | Goes to 0 V (ground) when the transistor is used as a switch. |
| 2 | Base B | The control pin. A small current into it, always through a resistor, switches the transistor on. |
| 3 | Collector C | Goes to the load (LED, relay, buzzer). Current flows in here and out of the emitter. |
Key ratings
How much can the 2N3904 take?
From the onsemi datasheet: 2N3903, 2N3904 General Purpose Transistors, NPN Silicon (2N3903/D, Rev. 9). Stay well inside these limits: they are the most the part takes, not what it should run at.
Open the onsemi datasheet (PDF)Pin order by maker
Is every 2N3904 wired E-B-C?
The onsemi datasheet numbers the legs 1 emitter, 2 base, 3 collector: E-B-C with the flat face toward you, the same order as its PNP partner, the 2N3906.
That is the opposite of the BC547 (C-B-E), the part it most often replaces. The surface-mount MMBT3904 in SOT-23 has a different layout altogether.
A first circuit
Sound a 5 V buzzer from an Arduino pin
- An active 5 V buzzer (the kind that beeps on its own) from +5 V to the collector, its plus leg to +5 V.
- Emitter to 0 V.
- A 2.2 kΩ resistor from the Arduino pin to the base: about 2 mA, more than enough for a buzzer of around 30 mA.
- Pin high: beep. Pin low: quiet.
The pin never carries the buzzer's current; it only drives the base, so it stays well inside its own limit.
2N3904 questions
What is the 2N3904 pinout?
With the flat face toward you and the legs down, from left to right: emitter, base, collector (E-B-C). Pins 1, 2 and 3 in the onsemi datasheet.
How much current can a 2N3904 handle?
200 mA continuous at most and up to 40 V, with 625 mW of heat in free air. Comfortable for LEDs, buzzers and small relays.
What is the PNP version of the 2N3904?
The 2N3906. It has the same E-B-C leg order, so a pair of them can sit side by side the same way round.
Can I swap a 2N3904 and a BC547?
For most small switching circuits, yes, but the legs are mirrored: the 2N3904 is E-B-C and the BC547 is C-B-E. Turn the part round, or the emitter and collector swap.
Can I use this pinout picture?
Yes. Download it as PNG or SVG and use it in notes, lessons or posts. The wirecora.com mark along the bottom stays in the picture.
Pin order and ratings from the onsemi datasheet named above. Last reviewed .