PAGE 063 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
NPN Transistor as an LED Switch
A small base current allows a bigger collector current.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
How it works
An NPN transistor such as the 2N3904 or BC547 acts as an electronic switch. A small current into the base allows a larger current to flow from collector to emitter.
Wiring
- Emitter to ground.
- LED and its resistor between supply and the collector.
- Base through a resistor, such as 1 to 4.7 kilohms, to the control signal.
Limits
These small transistors handle roughly 100 to 200 mA. The transistor turns on when the base is about 0.7 V above the emitter.
REMEMBER
A transistor lets a tiny signal control a much larger LED current.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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