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.. _ar_transistor:

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.

|img_NPN&PNP|

* **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: 

.. list-table::
    :widths: 20 20 20
    :header-rows: 1

    *   - Name	
        - ITEMS IN THIS KIT
        - LINK
    *   - Newton Lab Kit	
        - 450+
        - |link_newton_lab_kit|

You can also buy them separately from the links below.


.. list-table::
    :widths: 5 20 5 20
    :header-rows: 1

    *   - SN
        - COMPONENT	
        - QUANTITY
        - LINK

    *   - 1
        - :ref:`cpn_pico_2`
        - 1
        - |link_pico2_buy|
    *   - 2
        - Micro USB Cable
        - 1
        - 
    *   - 3
        - :ref:`cpn_breadboard`
        - 1
        - |link_breadboard_buy|
    *   - 4
        - :ref:`cpn_wire`
        - Several
        - |link_wires_buy|
    *   - 5
        - :ref:`cpn_resistor`
        - 3(220Ω, 1KΩ, 10KΩ)
        - |link_resistor_buy|
    *   - 6
        - :ref:`cpn_led`
        - 1
        - |link_led_buy|
    *   - 7
        - :ref:`cpn_button`
        - 1
        - |link_button_buy|
    *   - 8
        - :ref:`cpn_transistor`
        - 1(S8050/S8550)
        - |link_transistor_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".

.. code-block:: arduino

    // 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.

   .. code-block:: arduino

        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.

   .. code-block:: arduino

        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.