9 - Input Devices

This week we will learn the importance of the input signals received by devices, focusing mainly on sensors and how they interact with the environment to provide information (in our case) to the microcontroller.

INTRODUCTION

Sensors are devices that detect and respond to physical or chemical stimuli from the environment. These stimuli can include light, temperature, motion, pressure, humidity, sound, among others. Sensors convert these stimuli into electrical signals that can be processed and analyzed.
The most common types of sensors are shown in the following links:

Temperature Sensors : Measure the temperature of the environment.
Motion Sensors : Detect movement and the presence of objects or people.
Light Sensors : Measure light intensity.
Pressure Sensors : Detect changes in pressure.
Humidity Sensors : Measure humidity in the air or materials.

It is important to note that there are two important concepts when making a measurement: "Sensors" and "Transducers." Below is a comparison table to help you understand their similarities and differences.

Feature	Sensor	Transducer
Definition	A device that detects and measures physical properties from the environment	A device that converts one form of energy into another
Function	Measures physical quantities and converts them into readable signals	Converts physical quantities into electrical signals or vice versa
Output	Typically provides a readable signal (e.g., voltage, current)	Provides an electrical signal corresponding to the physical quantity
Power Source	May or may not require an external power source	Often requires an external power source
Examples	Temperature sensor (measures temperature) Light sensor (measures light intensity) Pressure sensor (measures pressure)	Thermocouple (converts temperature difference into voltage) Piezoelectric transducer (converts mechanical stress into electrical signal) Microphone (converts sound waves into electrical signals)

Transducers can be classified as passive and active, below is a table showing their operation.

Feature	Active Transducers	Passive Transducers
How they work	Convert physical energy into electrical energy by generating their own voltage	Convert physical energy into electrical energy by modifying an external voltage
Power source	Do not require an external power source; generate their own electrical output	Require an external power source to operatey
Examples	Thermocouples (generate voltage from temperature difference) Piezoelectric sensors (generate voltage from mechanical stress) Photovoltaic cells (generate voltage from light)	Resistive sensors (change resistance with physical changes) Capacitive sensors (change capacitance with physical changes) Inductive sensors (change inductance with physical changes)

A comparison of active and passive transducers is shown below.

Feature	Active Transducers	Passive Transducers
Applications	Medical imaging (ultrasound) Pressure sensors Vibration sensors	Displacement measurement (LVDT) Light sensors (LDR) Temperature measurement (thermistor)
Pros	Do not require external power source High sensitivity and accuracy Can generate their own electrical output	Simple design Low cost Reliable and durable
Cons	May have complex design Can be expensive Limited to specific applications	Require external power source May have lower sensitivity Limited to specific applications

Ultrasonic Sensor

When we ask to copilot "What is an ultrasonic sensor?", it gives us the following information.

An ultrasonic sensor is a device that measures distance by using ultrasonic sound waves. It works by emitting high-frequency sound waves (typically above 20 kHz) and then measuring the time it takes for the echo to return after bouncing off an object. This time interval is used to calculate the distance to the object.
How Ultrasonic Sensors Work

Transmission: The sensor emits ultrasonic sound waves using a transmitter.
Reception: The sound waves travel through the air, hit an object, and reflect back to the sensor.
Calculation: The sensor measures the time taken for the echo to return and calculates the distance using the formula:
\( D=\frac{1}{2}TC \)
where \(T\) is the time measured in seconds and \(C\) is the speed of sound (approximately \(343\) meters per second).

The "Ultrasonic Sensor HC-SR04" will be used in this project. The sensor and its pinout are shown in the following figure.

As you can see in the image, the sensor needs a 5V power supply, so an external source will be necessary to connect it to the "XIAO RP2040", the problem is that the signal coming from the "Echo" can damage the controller, so it is necessary to place a resistor and protect the device, so through a voltage divider, we have to make the resistor drop a voltage of \(1.8v\). According to the data sheets, the maximum current supported by the input pins of the "XIAO RP2040" is \(4mA\). Based on that we can make the following calculations:

\(R=\frac{V}{I}=\frac{1.8}{4m A}=450\Omega\) Which would be the minimum resistance we should put, however we do not want to work with the largest amount of current that the pins allow, so working with half of the maximum current is a good idea. \(R=\frac{V}{I}=\frac{1.8}{2m A}=900\Omega\) Which we can approximate to a \(1KΩ\) resistor, since it is popular and easy to obtain.

In the case of "Trig," it is connected to a controller output pin, which is used to trigger ultrasonic pulses. Setting this pin to HIGH for \(10 µs\) triggers the sensor to initiate an ultrasonic burst. An animation ( MCI educación ) of the sensor's operation is shown below.

The following links provide data sheets and other important information about the sensor.

NEW TEST BOARD

The idea is that depending on the distance between an object and the sensor, several LEDs will light up as the thermometer form. To do this, it's necessary to design a new test board, which will be connected to our main board made during the "Electronics Production" week.
The test board will consist of 5 LEDs of different colors and 1kΩ resistors that will be connected to each LED respectively.

The following image shows the LED and master boards together.

Code

The goal is to be able to visualize a strip of LEDs as an object approaches or moves away, so the first thing to do is declare all the variables, which include the output LEDs and the "Trigger" and "Echo".

The next step is to define the pins as inputs and outputs using the "pinMode" instruction within the "void setup()".

In the "void loop()" we start by turning the trigger on and off with a pulse width of \(10 μs\). Then, using the formulas mentioned in the data sheets, we calculate the duration (time in which the sound returns) and the distance.

The last step is to set the conditions for turning the LEDs on and off depending on the distance.

The complete code is shown below, and the file is at the bottom of the page.

RESULT

The following images show the "Ultrasonic Sensor HC-SR04" with the LED board.

Video of how it works when moving an object closer or further away from the sensor

Learning outcome

This week we learned about the importance of input elements such as sensors. It's important to understand how a sensor works before using it, since "we can't program something we don't understand."

Group Task

Group Task