Is the 2N3904 the same pinout as the BC547?

No — and this is the classic trap. Held with the flat printed face toward you and the pins pointing down, read left to right: the 2N3904 (like the 2N2222) is E-B-C (emitter, base, collector), while the BC547 is C-B-E (collector, base, emitter). The collector and emitter are swapped end-for-end. Drop a BC547 into breadboard holes wired for a 2N3904 and the transistor is reversed — always check the datasheet for your exact part.

How do I tell which pin is the base?

Use a multimeter in diode mode. The base sits between the two PN junctions, so it is the one pin that reads a forward drop of about 0.6–0.7V to both of the other two pins. For an NPN part, keep the red (+) probe on the base and touch each other pin in turn — both should read ~0.6–0.7V. For a PNP, the black (−) probe goes on the base instead. The pin that is common to both forward-biased junctions is the base; of the remaining two, the datasheet tells you which is emitter and which is collector.

Does the package change the pinout?

Yes. This guide covers the TO-92 plastic package only. The same die in a metal-can TO-18 package uses a completely different pin layout (often with a locating tab), so the E-B-C / C-B-E order here does not apply. Pin assignments vary by both part number and package — never assume; read the pinout drawing in the datasheet for your specific part and package.

Pinout Reference · Quick Reference · 6 min

Transistor Pinout (E/B/C): which pin is the emitter, base and collector — and how not to plug it in backwards

Circflow Editorial · Published 2026-06-02

A TO-92 transistor has three legs — emitter (E), base (B) and collector (C) — and getting them in the wrong order is one of the most common beginner mistakes. The trap that catches everyone: the 2N2222 and 2N3904 are E-B-C, but the BC547 is C-B-E — reversed. Swap one for the other in the same breadboard holes and the collector and emitter are backwards. This guide shows how to orient the package, gives the pin table for the common parts, and explains how to verify the pins with a multimeter before you commit. When you wire that transistor to switch an LED, size the parts with the LED Resistor Calculator and Ohm's Law Calculator.

How to orient a TO-92 package

A pinout is only meaningful once you fix the viewing orientation. For the TO-92 plastic package the convention is simple: hold the transistor with the flat (printed) face toward you and the pins pointing down, then read the three pins left → right. That left-to-right reading is what every "Pin 1 / Pin 2 / Pin 3" row below refers to.

Figure 1: TO-92 seen from the flat printed face with pins pointing down. Pins 1-2-3 are read left → right; pin 2 (the middle pin) is the base on all the parts in this guide.

The pin table (2N2222 / 2N3904 / BC547)

With the package oriented as above, the common NPN small-signal transistors map as follows. Pin 2 (the middle pin) is the base in every case; only the outer two pins move:

Part	Pin 1	Pin 2	Pin 3	Order
2N2222 (NPN)	E — emitter	B — base	C — collector	Standard E-B-C
2N3904 (NPN)	E — emitter	B — base	C — collector	Same as 2N2222
BC547 (NPN)	C — collector	B — base	E — emitter	REVERSED (C-B-E)

⚠️ The middle pin is always the base here, but that is not a universal rule across all transistors — some parts put the base on an end pin. Treat the table as the reference for these specific TO-92 parts only, and always confirm against the datasheet for your exact part number.

The 2N vs BC gotcha

The 2N2222 and 2N3904 share the standard E-B-C order, so they drop into the same footprint interchangeably. The BC547 looks identical — same TO-92 body, same three legs — but its order is C-B-E, the exact reverse. The base stays in the middle, but the collector and emitter are swapped end-for-end.

🛑 A 2N3904 and a BC547 sitting in the same breadboard holes are wired backwards relative to each other. Swapping one brand of "NPN transistor" for another without checking the pinout will quietly reverse collector and emitter — the circuit may barely work, behave oddly, or not work at all. Always check the datasheet for your part and package before wiring.

A worked example: say you build a transistor low-side switch that turns an LED on and off. The base goes through a resistor sized with the Ohm's Law Calculator, the LED gets its own current-limiting resistor from the LED Resistor Calculator, the collector drives the load and the emitter goes to ground. If you prototyped it with a 2N3904 (E-B-C) and later substitute a BC547 (C-B-E) into the same holes, the collector and emitter are now reversed — the LED no longer switches the way you expect. For the full pin drawings of each part see the 2N2222 and BC547 pinout pages.

PNP complements

Each NPN part has a PNP complement with matching characteristics. The complement keeps the same package and the same physical pin order — only the transistor type (and so the load/supply polarity) flips:

NPN part	PNP complement	Pin order
2N2222	PN2907 / 2N2907	E-B-C
2N3904	2N3906	E-B-C
BC547	BC557	C-B-E (same as BC547)

Note that the BC557 keeps the BC547's C-B-E order — the complement does not "fix" the reversal, it inherits it. So the same 2N-vs-BC trap carries over to the PNP side.

How to verify with a multimeter

Never trust a pinout you are unsure of — confirm it in 30 seconds with a multimeter in diode mode. A bipolar transistor is effectively two back-to-back PN junctions that meet at the base, so the base is the one pin that forward-biases against both of the others:

Set the meter to diode mode (the symbol that looks like a diode / arrow).
For an NPN part, put the red (+) probe on the suspected base and touch each other pin in turn — both junctions should read about 0.6–0.7V forward.
The pin that gives ~0.6–0.7V to both others is the base. If you get an open (OL) reading both ways, that pin is not the base — try another.
Reverse the probes: from base to either outer pin in the blocking direction you should read OL (open). That confirms it is an NPN (red probe on base forward-conducts).
For a PNP part the same test works with the black (−) probe on the base instead.

⚠️ This identifies the base reliably and confirms NPN vs PNP. Telling the emitter from the collector by meter alone is harder (the B-E junction often shows a slightly higher forward drop than B-C, but the difference is small) — so use the diode test to find the base, then read the datasheet pin drawing to assign emitter and collector.

This guide is for the TO-92 plastic package. Metal-can TO-18 parts use a different pin layout (often keyed by a small tab next to the emitter), so the E-B-C / C-B-E orders above do not carry over — check that package's own datasheet drawing.

FAQ

Is the 2N3904 the same pinout as the BC547?: No — and this is the classic trap. Held with the flat printed face toward you and the pins pointing down, read left to right: the 2N3904 (like the 2N2222) is E-B-C (emitter, base, collector), while the BC547 is C-B-E (collector, base, emitter). The collector and emitter are swapped end-for-end. Drop a BC547 into breadboard holes wired for a 2N3904 and the transistor is reversed — always check the datasheet for your exact part.
How do I tell which pin is the base?: Use a multimeter in diode mode. The base sits between the two PN junctions, so it is the one pin that reads a forward drop of about 0.6–0.7V to both of the other two pins. For an NPN part, keep the red (+) probe on the base and touch each other pin in turn — both should read ~0.6–0.7V. For a PNP, the black (−) probe goes on the base instead. The pin that is common to both forward-biased junctions is the base; of the remaining two, the datasheet tells you which is emitter and which is collector.
What's the PNP version of these transistors?: Each NPN part has a PNP complement: the 2N2222 pairs with the PN2907 / 2N2907, the 2N3904 pairs with the 2N3906, and the BC547 pairs with the BC557. The complement keeps the same package and the same physical pin order — for example the BC557 is C-B-E just like the BC547 — but the transistor type (and so the supply/load polarity and the multimeter base probe) is reversed.
Does the package change the pinout?: Yes. This guide covers the TO-92 plastic package only. The same die in a metal-can TO-18 package uses a completely different pin layout (often with a locating tab), so the E-B-C / C-B-E order here does not apply. Pin assignments vary by both part number and package — never assume; read the pinout drawing in the datasheet for your specific part and package.