Transistor
BC547
Small-signal NPN BJT, TO-92 — 45V, 100mA, general-purpose switching and amplification.
BC547 · onsemi
Overview
A general-purpose NPN bipolar transistor for small-signal amplification and low-current switching. Unlike a MOSFET it is current-controlled: the base needs a continuous current roughly equal to the collector current divided by the gain, so driving one from a GPIO always means a base resistor, and that resistor's value is a calculation rather than a default. In exchange it is cheap, forgiving of static, and saturates to a low collector-emitter voltage. The 100mA collector limit puts it in the same niche as the 2N7000 — an LED bank, a small relay coil, the input of another stage — and it is the wrong part for a motor. Suffix letters select the gain bin: BC547A is the low-gain group, BC547B the middle, BC547C the highest. Kits generally ship the B.
Wiring tip: base → GPIO through a resistor (size it for Ic/hFE), emitter → GND, collector → load return
Pins & wiring
BC547
TO-92
Physical pin order shown left to right.
All pins
Hover · click to selectSelected pin
COLLECTOR
- Direction
- Passive
Connects to the load's return side, not directly to a board pin.
Full pin table
| Pin | Name | Role | Direction | Note |
|---|---|---|---|---|
| 1 | Other | Passive | Connects to the load's return side, not directly to a board pin. | |
| 2 | GPIO | In | TO-92, flat side facing you with the leads pointing down: left-to-right is Collector, Base, Emitter. This is the opposite order to the 2N3904 (E-B-C) in the identical package — swapping the two without re-checking is a classic way to spend an evening debugging a circuit that is wired backwards. Always confirm against the datasheet's package drawing. | |
| 3 | Ground | Passive | — |
Wiring examples
Low-side switch for a small DC load
Any 5V hostGPIO drives the base through a resistor sized for saturation; the load sits between the positive rail and the collector; a flyback diode handles inductive loads.
| BC547 pin | Connects to | Note |
|---|---|---|
| BASE | host Digital out (any matching pin) | Through a base resistor — see the passive below. |
| EMITTER | host Ground (any matching pin) | — |
You also need
- Required1 kΩ resistor between gpio and base — Sets the base current. Worked for a 50mA load from a 5V GPIO: force at least Ic/10 = 5mA of base current to guarantee saturation regardless of gain bin, so R = (5V - 0.7V) / 5mA ≈ 860Ω, and 1kΩ is the nearest sensible value. Recompute this for YOUR load current — a resistor copied from another circuit is how a transistor ends up in its linear region, dissipating heat instead of switching.
- Required diode between load+ and collector — Clamps the inductive kick when a relay coil or motor switches off, which would otherwise exceed the 45V Vceo and punch through the transistor.
- The load's ground must be common with the MCU's ground.
- 100mA is the ceiling. Beyond that use a MOSFET — a BJT at its current limit runs hot and needs ever more base drive.
- Unlike a MOSFET the base draws current continuously while the load is on. Budget it: several of these switched at once is real load on a GPIO bank.
Specifications
Footprint
No independently verified physical footprint is on file for this part yet.
Alternatives
Upgrades
Strictly better on the headline spec, but expect to touch firmware or wiring to actually use the difference.
The load is well beyond 100mA — a motor, a heater, an LED strip.
Compare with BC547- Load current ceiling: 100 mA → 47 A — Different package (TO-220) and a different footprint entirely.
Similar parts
Does the same job with a different part. Expect wiring and library changes — a starting point, not a guarantee.
A MOSFET doing the same job — no continuous drive current and no gain calculation, at the cost of caring about gate threshold voltage.
Compare with BC547- Switching element: BJT (current-controlled) → MOSFET (voltage-controlled) — The base resistor calculation disappears; a gate pull-down appears in its place.
- Load current ceiling: 100 mA → 200 mA
Documents
Datasheets
Frequently asked questions
What voltage does the BC547 run at?
Banana Board does not have a verified operating voltage on file for the BC547 yet — check the datasheet in Documents before wiring it up.
How many pins does the BC547 have?
The BC547 has 3 pins, communicating over Digital. See the full pinout table in Pins & wiring for each pin's role.
Can I simulate the BC547 before ordering hardware?
Not yet — the BC547 does not have a live simulation model in Banana Board today, so validate the wiring here and test on real hardware.
Where is the BC547 datasheet?
Linked in the Documents section on this page, and cited on every spec that came from it.
Related references
Build it, do not just look it up
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