How do I choose between NPN and PNP transistors?

Short Answer: Use NPN for switching the low side (between load and ground). Use PNP for switching the high side (between power and load).

Detailed: The choice depends on where you place the load relative to the transistor.

NPN (Low-side switch)

text

    VCC ────[LOAD]──── Collector
                        │
                       NPN
                        │
                    Emitter ──── GND
                    Base ─────── Control signal (2–5V)

• Advantage: Easy to drive (control signal referenced to ground)

• Use when: Load connects to VCC, transistor connects load to ground

PNP (High-side switch)

text

    VCC ──── Emitter
              │
             PNP
              │
    Collector ────[LOAD]──── GND
    Base ─────────────────── Control signal (needs pull-up)

• Advantage: Load is grounded on one side (safer for some applications)

• Disadvantage: Control signal must be near VCC to turn OFF (0V to turn ON)

Quick decision chart:

If you want to switch...	Use	Control signal	Example
Load to ground	NPN	HIGH (2–5V) turns ON	LED, relay, motor
Load to VCC (grounded load)	PNP	LOW (0V) turns ON	Common-anode LED, some sensors
Both sides floating	Relay or optocoupler	Isolated	AC loads, different voltage domains

Pro tip: For microcontrollers (3.3V or 5V), NPN is easier because a GPIO HIGH directly turns it on. PNP requires the GPIO to go LOW to turn on — fine, but you need a pull-up resistor to keep it OFF by default.