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2.8 Press Gently

In this lesson, we’ll learn how to use a micro switch (also known as a limit switch) with the Raspberry Pi Pico 2 to detect when it’s pressed or released. Micro switches are commonly used in devices like microwave oven doors, printer covers, or as end stops in 3D printers because they are reliable and can handle frequent activation.

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	1(10KΩ)	BUY
6	Capacitor	1(104)	BUY
7	Micro Switch	1

Understanding the Micro Switch

A micro switch typically has three pins:

Common (C): The middle pin.
Normally Open (NO): Connected to the common pin when the switch is pressed.
Normally Closed (NC): Connected to the common pin when the switch is not pressed.

By connecting the switch appropriately, we can detect when it’s pressed by reading the voltage level on a GPIO pin.

Circuit Diagram

sch_limit_sw

By default, GP14 is low and when pressed, GP14 is high.

The purpose of the 10K resistor is to keep the GP14 low during pressing.

When you press a mechanical switch, the contacts may bounce, causing multiple rapid transitions between open and closed states. The capacitor connected between GP14 and GND helps filter out this noise.

Switch Not Pressed:
- The Common (C) pin is connected to the NC pin, which is connected to GND.
- GP14 reads LOW (0V).
Switch Pressed:
- The Common (C) pin is connected to the NO pin, which is connected to 3.3V.
- GP14 reads HIGH (3.3V).

Wiring Diagram

wiring_limit_sw

Writing the Code

We’ll write a MicroPython program that detects when the micro switch is pressed and prints a message accordingly.

Note

Open the 2.8_micro_switch.py from newton-lab-kit/micropython or copy the code into Thonny, then click “Run” or press F5.
Ensure the correct interpreter is selected: MicroPython (Raspberry Pi Pico).COMxx.

import machine
import utime

# Initialize GP14 as an input pin
switch = machine.Pin(14, machine.Pin.IN)

while True:
    if switch.value() == 1:
        print("The switch is pressed!")
        utime.sleep(0.5)  # Debounce delay

When the code is running, you will observe the following phenomenon:

Not Pressed: No message should appear.
Pressed: “The switch is pressed!” should appear in the console each time you press the switch.

Understanding the Code

Import Modules:
- import machine: Access to hardware functions.
- import utime: Time-related functions.
Initialize the Switch Pin:
- switch = machine.Pin(14, machine.Pin.IN): Sets up GP14 as an input pin.
Main Loop:
- while True: Starts an infinite loop.
- if switch.value() == 1: Checks if the switch is pressed (GP14 reads HIGH).
- print("The switch is pressed!"): Outputs a message to the console.
- utime.sleep(0.5): Adds a delay to debounce the switch and prevent multiple detections from a single press.

Alternative Wiring: Using Internal Pull-Down Resistor

If you prefer to simplify the wiring even further, you can rely solely on the internal pull-down resistor:

Modify the Circuit:
- Remove the external 10 kΩ resistor and 0.1 µF capacitor.
- Micro Switch Connections:
  - Common (C) Terminal: Connect to GP14 on the Pico.
  - Normally Open (NO) Terminal: Connect to 3.3V on the Pico.
  - Normally Closed (NC) Terminal: Leave unconnected.

Modified Code:

import machine
import utime

# Initialize GP14 as an input pin with an internal pull-down resistor
switch = machine.Pin(14, machine.Pin.IN, machine.Pin.PULL_DOWN)

while True:
    if switch.value() == 1:
        print("The switch is pressed!")
        utime.sleep(0.5)  # Debounce delay

Practical Applications

Limit Detection: Use the micro switch as an end stop in CNC machines or 3D printers to detect the limit of movement.
Safety Interlocks: Ensure a device operates only when certain conditions are met (e.g., a door is closed).
User Input: Incorporate into projects where a robust and reliable button is needed.

Experimenting Further

Control an LED:

Connect an LED to another GPIO pin (e.g., GP15) with a suitable resistor. Modify the code to turn the LED on when the switch is pressed.

import machine
import utime

switch = machine.Pin(14, machine.Pin.IN, machine.Pin.PULL_DOWN)
led = machine.Pin(15, machine.Pin.OUT)

while True:
    if switch.value() == 1:
        led.value(1)  # Turn on the LED
        print("The switch is pressed!")
        utime.sleep(0.5)
    else:
        led.value(0)  # Turn off the LED

Counting Presses:

Modify the code to count how many times the switch has been pressed.

Control an LED:

import machine
import utime

switch = machine.Pin(14, machine.Pin.IN, machine.Pin.PULL_DOWN)
count = 0

while True:
    if switch.value() == 1:
        count += 1
        print("Switch pressed {} times".format(count))
        utime.sleep(0.5)

Conclusion

Using a micro switch with the Raspberry Pi Pico 2 allows you to detect physical interactions reliably. Understanding how to wire the switch and read its state in your code is essential for creating responsive and interactive projects.