9. Input devices¶

Image source from fabacademy mattermost page

Group Assignment¶

probe an input device’s analog levels and digital signals

Link to group assignment is here

💡 Note: Some Key takeaways from the Group Assignment

📊 The logic analyzer shows the signals as lines or patterns on a screen so you can see what’s happening .
🐞 Logic analyzer helps to debug communication protocols.
🕵️‍♂️ It's like a detective for electronics, helping to solve problems and make sure everything is working.

Individual Assignment¶

Here is my schedule for the week09

Measure something: add a sensor to a microcontroller board that you have designed and read it

This week I tired different input devices like ultrasonic,potentiometer and capacitive sensor. Before jumping into the input devices, it is must to know about the digital and analog pin. For this input week, I am going to use the board that I designed in week06 and fabricated in week08.

Feature	Analog Signal	Digital Signal
Nature	Continuous	Discrete
Values	Infinite possible values	Finite possible values (0 and 1)
Representation	Sine waves, smooth curves	Square waves, binary code
Noise Immunity	Susceptible to noise	Resistant to noise
Transmission	Degrades over long distances	Can be transmitted over long distances without degradation
Complexity	Simpler to generate	Requires complex processing
Examples	Audio signals, temperature sensors	USB, Ethernet, digital sensors

Understanding XIOA RP2040 Analog and digital pins¶

Analog Pins¶

XIOA RP2040 has four analog pins.These pins are connected to the RP2040’s 12-bit ADC (Analog-to-Digital Converter).

Pin Label	Function	ADC Channel	Notes
A0	Analog Input	ADC0	Can read voltages (0–3.3V).
A1	Analog Input	ADC1	Can read voltages (0–3.3V).
A2	Analog Input	ADC2	Can read voltages (0–3.3V).
A3	Analog Input	ADC3	Can read voltages (0–3.3V).

Note:
The ADC has a 12-bit resolution, meaning it can represent analog voltages as digital values between **0** and **4095**

Warning
The input voltage range is **0V to 3.3V**. Exceeding this range can damage the board.

Digital Pins¶

The Xiao RP2040 has 11 digital pins that can be used for GPIO (General Purpose Input/Output), PWM (Pulse Width Modulation), and communication protocols like I2C, SPI, and UART.

Pin Label	Function	Notes
D0	Digital I/O	Can be used for GPIO, PWM, or UART TX.
D1	Digital I/O	Can be used for GPIO, PWM, or UART RX.
D2	Digital I/O	Can be used for GPIO or PWM.
D3	Digital I/O	Can be used for GPIO or PWM.
D4	Digital I/O	Can be used for GPIO or PWM.
D5	Digital I/O	Can be used for GPIO or PWM.
D6	Digital I/O	Can be used for GPIO or PWM.
D7	Digital I/O	Can be used for GPIO or PWM.
D8	Digital I/O	Can be used for GPIO or PWM.
D9	Digital I/O	Can be used for GPIO or PWM.
D10	Digital I/O	Can be used for GPIO or PWM.

Note:
All digital pins support GPIO and can be configured as input or output.

Note:
Most digital pins support PWM (except D0 and D1, which are primarily used for UART).

Special Functions:

D0 (TX) and D1 (RX): Used for UART communication.
D2 (SDA) and D3 (SCL): Used for I2C communication.
D9 (MISO), D7 (CS), D8 (SCK), D10 (MOSI): Used for SPI communication.

I refer this pinout

Potentiometer¶

A potentiometer is a type of variable resistor commonly used in electronics to control electrical signals. It allows you to manually adjust the resistance,brightness and position of the motor. I used potentiometer as the input to control the brightness of the led.

I refer the link to learn about the potentiometer

Connecting a Potentiometer to Guard X

Potentiometer has three pins:

Connect one end of the potentiometer to 3.3v .

Connect the other end to GND .

Connect the middle pin signal pin (wiper) to an analog input pin (e.g., A0).

I made the connection as shown below.

I used the IDE example code

📌

  /*
  Analog input, analog output, serial output

  Reads an analog input pin, maps the result to a range from 0 to 255 and uses
 the result to set the pulse width modulation (PWM) of an output pin.
 Also prints the results to the Serial Monitor.

  The circuit:
  - potentiometer connected to analog pin 0.
  Center pin of the potentiometer goes to the analog pin.
  side pins of the potentiometer go to +5V and ground
  - LED connected from digital pin 9 to ground through 220 ohm resistor

   created 29 Dec. 2008
  modified 9 Apr 2012
 by Tom Igoe

 This example code is in the public domain.

 https://www.arduino.cc/en/Tutorial/BuiltInExamples/AnalogInOutSerial
 */

 // These constants won't change. They're used to give names to the pins used:
 const int analogInPin = A0;  // Analog input pin that the potentiometer is attached to
 const int analogOutPin = 9;  // Analog output pin that the LED is attached to

 int sensorValue = A0;  // value read from the pot
 int outputValue = 0;  // value output to the PWM (analog out)

 void setup() {
 // initialize serial communications at 9600 bps:
 Serial.begin(9600);
 }

 void loop() {
 // read the analog in value:
 sensorValue = analogRead(analogInPin);
 // map it to the range of the analog out:
 outputValue = map(sensorValue, 0, 1023, 0, 255);
  // change the analog out value:
 analogWrite(analogOutPin, outputValue);

 // print the results to the Serial Monitor:
 Serial.print("sensor = ");
 Serial.print(sensorValue);
 Serial.print("\t output = ");
 Serial.println(outputValue);

 // wait 2 milliseconds before the next loop for the analog-to-digital
 // converter to settle after the last reading:
  delay(2);
 }

I opened up the serial Monitor at 115200 baud rate and I was able to observe the output on the serial monitor. The change in potentiometer in pin number A0 from 0-1024 analog signal was mapped to the output signal of 0-255 digital sign.

HC-SR04 Ultrasonic Distance Sensor¶

An ultrasonic sensor is a sensor that uses ultrasonic waves to detect an object, measure the distance and determine if it is in motion.

Here’s how it works:

The sensor sends out a sound wave.

The sound wave bounces off an object.

The sensor listens for the sound wave to come back.

The ultrasonic sensor has a transmitter and a receiver.

I refer this link to learn about the HC-SR04 Ultrasonic Distance Sensor

I made the connection as shown below:

VCC - 5V
GND - GND
Trig pin - D4
Echo pin - D5

I upload the example code.

📌

   /*
   * Created by ArduinoGetStarted, https://arduinogetstarted.com
   *
   * Arduino - Ultrasonic Sensor HC-SR04
   *
   * Wiring: Ultrasonic Sensor -> Arduino:
   * - VCC  -> 5VDC
   * - TRIG -> Pin 9
   * - ECHO -> Pin 8
   * - GND  -> GND
   *
   * Tutorial is available here: https://arduinogetstarted.com/tutorials/arduino-ultrasonic-sensor
   */

  int trigPin = A0;    // TRIG pin
 int echoPin = A1;    // ECHO pin

 float duration_us, distance_cm;

 void setup() {
  // begin serial port
 Serial.begin (9600);

 // configure the trigger pin to output mode
 pinMode(trigPin, OUTPUT);
 // configure the echo pin to input mode
 pinMode(echoPin, INPUT);
 }

 void loop() {
 // generate 10-microsecond pulse to TRIG pin
 digitalWrite(trigPin, HIGH);
 delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  // measure duration of pulse from ECHO pin
  duration_us = pulseIn(echoPin, HIGH);

 // calculate the distance
 distance_cm = 0.017 * duration_us;

 // print the value to Serial Monitor
 Serial.print("distance: ");
 Serial.print(distance_cm);
 Serial.println(" cm");

 delay(500);
 }

How the code works?

This Arduino code uses an HC-SR04 ultrasonic sensor, with D4 as the TRIG pin to send a 10µs pulse and D5 as the ECHO pin to measure the echo return time ( duration_us). The distance (duration_cm) is calculated by multiplying duration_us by 0.017 (accounting for the speed of sound and round-trip travel time) and printed to the Serial Monitor every 500ms. The sensor detects objects 2cm-400cm away, though accuracy depends on surface material and environmental factors like temperature and humidity.

Here a video of how it worked

RFID-RC522¶

For my final project, I am using RFID scanner for authentication. I have designed a board using the XIAO RP2040 to connect the RFID scanner and integrate it with the locking system.

Radio-Frequency Identification (.RFID) uses radio waves to read information from a tag on an object. The tag can be scanned from a few feet away, and you don’t need to point it directly at the reader, which makes it better than barcode.

What RFID reader?

A .RFID reader is a device used to gather information from an RFID tag, which is used to track individual objects. Radio waves are used to transfer data from the tag to a reader.

RC522 RF ID Reader Specifications¶

**Operating Frequency:** - 13.56 MHz (HF - High Frequency). **Communication Interface:** - SPI (Serial Peripheral Interface). - I2C (Inter-Integrated Circuit) - UART (Universal Asynchronous) - Receiver-Transmitter **Operating Voltage:** - 2.5V to 3.3V (for the RC522 chip). - Module typically operates at 3.3V or 5V (with onboard voltage regulator). **Read Range:** - Up to 5 cm (depending on the antenna design and tag type).

RC522 Pin Connections to XIAO RP2040¶

RFID-RC522 Module

Refer these two link and this to learn about the RFID RC-522.

How It Works:

The RC522 module emits a radio signal via its antenna.

When an RFID tag comes near, it gets powered by the electromagnetic field and sends back its UID (Unique Identifier).

The RC522 reads the UID and any stored data (if supported by the tag).

The data is sent to a microcontroller (Arduino, ESP, etc.) for processing.

I refer the week 08 for the connection where I have designed a board to be used for the rfid scanner. Here is how I connected the XIOA RP2040 with RFID. Then I tested the guard x with rfid.

RC522 Pin	XIAO RP2040 Pin	Description
SDA (SS/CS)	D7 (or any GPIO)	Chip Select (Slave Select)
SCK	D8	Serial Clock
MOSI	D10	Master Out Slave In
MISO	D9	Master In Slave Out
IRQ	Not connected	Interrupt (optional)
GND	GND	Ground
RST	D1 (or any GPIO)	Reset (optional)
3.3V	3.3V	Power (3.3V)

Add the rfid library for this program, Use the URL below

https://github.com/miguelbalboa/rfid.git

The process for adding the additional board is mentioned in week08. I followed the same step to add the boards.

I used the examples code. Examples > MFRC522 > Dumbinfo.

📌

  ## code edited from the deepseek

  #include <SPI.h>
  #include <MFRC522.h>

   #define SS_PIN D2  // Chip Select pin
   #define RST_PIN D1 // Reset pin

    MFRC522 mfrc522(SS_PIN, RST_PIN); // Create MFRC522 instance

    void setup() {
    Serial.begin(9600); // Initialize serial communication
     SPI.begin();        // Initialize SPI bus
     mfrc522.PCD_Init(); // Initialize MFRC522
     Serial.println("RFID Scanner Ready!");
    }

    void loop() {
    // Look for new cards
      if (mfrc522.PICC_IsNewCardPresent() && mfrc522.PICC_ReadCardSerial()) {
     Serial.println("Card detected!");

     // Print UID of the card
     Serial.print("UID: ");
     for (byte i = 0; i < mfrc522.uid.size; i++) {
     Serial.print(mfrc522.uid.uidByte[i] < 0x10 ? " 0" : " ");
      Serial.print(mfrc522.uid.uidByte[i], HEX);
     }
     Serial.println();

    mfrc522.PICC_HaltA(); // Halt PICC
     }
     }

How the code works?

Open serial monitor with baud rate settings of 9600 and move card near to the card Reader module. Observe serial monitor. It will show UID for that card.

Files¶

Code files for the week can be downloaded from here