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2.15 Two Types of Transistors: NPN and PNP

In this lesson, we’ll explore two types of transistors: the S8050 (NPN) and the S8550 (PNP). Transistors are commonly used as electronic switches, and we’ll see how both types can be used to control an LED with a button.

NPN (S8050): This type of transistor allows current to flow from the collector to the emitter when a high signal is applied to the base.
PNP (S8550): For PNP transistors, current flows from the emitter to the collector when a low signal is applied to the base.

While both transistors serve similar purposes, they behave oppositely when it comes to signal control. Let’s use these transistors to control an LED based on button input.

What You’ll Need

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Newton Lab Kit	450+	Newton Lab Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Raspberry Pi Pico 2	1	BUY
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	Resistor	3(220Ω, 1KΩ, 10KΩ)	BUY
6	LED	1	BUY
7	Button	1	BUY
8	Transistor	1(S8050/S8550)	BUY

Wiring the NPN (S8050) Transistor

sch_s8050

In this circuit, pressing the button sends a high signal to the GP14 pin. When GP15 outputs a high signal, the NPN transistor conducts, allowing current to flow through the LED, lighting it up.

wiring_s8050

Wiring the PNP (S8550) Transistor

sch_s8550

For the PNP transistor circuit, the button starts with a low signal on GP14 and changes to high when pressed. When GP15 outputs a low signal, the PNP transistor conducts, allowing current to flow and lighting up the LED.

wiring_s8550

Writing the Code

Note

You can open the file 2.15_transistor.ino from newton-lab-kit/arduino/2.15_transistor.
Or copy this code into Arduino IDE.
Select the Raspberry Pi Pico 2 board and the correct port, then click “Upload”.

// Define the pins
const int buttonPin = 14;  // Button connected to GP14
const int transistorPin = 15;  // Transistor base connected to GP15

int buttonState = 0;  // Variable to hold the button state

void setup() {
  pinMode(buttonPin, INPUT);
  pinMode(transistorPin, OUTPUT);
}

void loop() {
  // Read the state of the button
  buttonState = digitalRead(buttonPin);

  // control the transistor
  digitalWrite(transistorPin, buttonState);

  delay(10);  // Small delay for debouncing
}

Results

For NPN Transistor (S8050):

When you press the button, the LED should turn on. When you release the button, the LED should turn off.
For PNP Transistor (S8550):

When you press the button, the LED should turn off. When you release the button, the LED should turn on.

Understanding the Code

Reading the Button State:

Reads the current state of the button.
```
buttonState = digitalRead(buttonPin);
```
Controlling the Transistor:
- For NPN Transistor: When the button is pressed (buttonState is HIGH), the transistor is turned on, allowing current to flow and lighting up the LED.
- For PNP Transistor: When the button is pressed (buttonState is HIGH), the transistor is turned off (LOW), and when the button is not pressed, the transistor is turned on.
```
digitalWrite(transistorPin, buttonState);
```

Further Exploration

Control Larger Loads:

Use transistors to control devices that require more current than the Pico can provide directly, such as motors or relays.
Transistor as an Amplifier:

Explore how transistors can be used to amplify signals.
Experiment with Darlington Pair:

Use two transistors to create a Darlington pair for higher current gain.

Conclusion

In this lesson, you’ve learned how to use both NPN and PNP transistors to control an LED using a Raspberry Pi Pico and a button. Understanding the differences between NPN and PNP transistors is crucial for designing circuits that require switching or amplification.