PAGE 064 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
PNP Transistor as an LED Switch
PNP switches the positive side of the load.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
High-side switching
A PNP transistor switches the positive side. Its emitter goes to the supply and the LED with resistor connects between the collector and ground.
Operation
- The LED lights when the base is pulled lower than the emitter by about 0.7 V.
- Use a base resistor, again 1 to 4.7 kilohms.
- The control signal must rise close to the supply voltage to switch it fully off.
PNP types such as the 2N3906 or BC557 are useful when the load must have a permanent ground connection.
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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