Input Devices

Input Devices

This week focuses on understanding and working with input devices. The objective is to measure signals from sensors and integrate them with a microcontroller board.

Group Assignment

This week, we focused on testing an analog and a digital input using two biomedical devices. For the analog input, we used the Sparkfun AD8232 Heart Rate Monitor, which required wiring to an oscilloscope to measure the heart signal. The sensor was placed on a person’s body following the Einthoven triangle for heart rate monitoring, and we observed the signal on the oscilloscope, which appeared as a noisy version of the expected pattern.

For the digital input, we used the MAX30102 Blood Oxygen Sensor, which communicates via I2C protocol. We connected the sensor to a microcontroller and uploaded a code to read heart rate and oxygen saturation (SPO2) values. The data was then sent to the serial monitor for analysis, where we observed the clock and data signals on an oscilloscope. The week’s activity helped us explore the wiring, signal reading, and data interpretation for both types of inputs in biomedical applications.

If you want to learn more about the group assignment, visit the following link: Group Assignment - Fab Academy 2025

Individual Assignment

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

Understanding Input Devices

Input devices are electronic components that capture data from the environment and convert it into electrical signals that can be processed by a microcontroller. These devices include sensors such as temperature sensors, light sensors, and motion sensors.

Choosing a Sensor

Before selecting the sensor for my project, I decided to compare some of the available sensors in our laboratory. During my research, I found two potential options: the KY-013 and the LM35. To better understand their operation, I reviewed their datasheets and highlighted key characteristics that differentiate them.

Sensor Comparison: KY-013 vs LM35

1. Sensor Overview

The KY-013 is a low-cost analog temperature sensor that provides a voltage output corresponding to the temperature. It operates over a wider temperature range compared to the LM35 but with lower accuracy.

The LM35 is a precision analog temperature sensor that provides an output voltage proportional to the centigrade temperature. It is known for its high accuracy and is commonly used for temperature measurements in biomedical and environmental systems.

2. Features Comparison

Feature KY-013 LM35
Output Type Analog Voltage Analog Voltage
Voltage Range 0-5V 0-5V
Accuracy ±2°C (typical) ±0.5°C
Operating Temperature -40°C to 125°C -55°C to 150°C
Power Supply 5V 5V
Output Voltage at 0°C Approx. 1.5V 0.5V
Applications Basic temperature measurements in low-cost projects Precision temperature measurement in various applications (environmental monitoring, biomedical, etc.)

3. Why I Chose the LM35

After evaluating both sensors, I decided to use the LM35 for my project because it offers much higher precision, which is crucial when monitoring human body temperature. While the KY-013 typically has an accuracy of ±2°C, the LM35 provides a much more precise accuracy of ±0.5°C. This level of precision is essential for healthcare applications or wearable devices where even small temperature variations can be significant.

The LM35's output is directly proportional to the temperature in Celsius, making it easier to convert the analog output into meaningful temperature data. Since the LM35 is more reliable and accurate, it ensures that the readings are consistent, making it ideal for tasks such as health monitoring systems that require precise body temperature measurements.

Using My Custom PCB from Week 8

For this project, I used the custom PCB that I designed and fabricated during Week 8 of the Fab Academy, as part of the Electronics Production assignment. You can find more details about its development here.

  • Microcontroller: Xiao, which operates at 3.3V and includes its own voltage regulator.
  • Sensor Connection: The LM35 temperature sensor is connected to an analog pin for data acquisition.
  • Power Distribution: The board requires both 5V and 3.3V to function correctly.

I connected the components in the following way:

Custom PCB

Power Supply Considerations

Currently, I still need to add a 5V voltage regulator to properly power the board from a battery. However, for now, I am using a DC power supply set to 5V to provide power to this part of the circuit.

Since the Xiao microcontroller has its own 3.3V regulator, I am powering it directly from my computer via USB, while the rest of the circuit operates at 5V. Once I integrate the voltage regulator, I will be able to power the entire system from a standalone battery.

Custom PCB Custom PCB

Connecting the Sensor to the Microcontroller

I connected the LM35 temperature sensor to my custom-designed microcontroller board. The connections were as follows:

  • VCC (5V) to the LM35 VCC pin.
  • GND to the LM35 GND pin.
  • Analog Output to an ADC pin on the microcontroller.

Reading Sensor Data

What is Arduino?

Arduino Logo

Arduino is an open-source electronics platform that allows users to build interactive projects by using microcontrollers. It provides a simple and flexible environment for programming and controlling hardware components. The official Arduino IDE can be downloaded from here.

To read the temperature data, I wrote a simple Arduino program that captures the analog voltage from the LM35 and converts it into a temperature reading. This program reads the sensor data, converts it to Celsius, and prints the values to the Serial Monitor every second. Additionally, it checks whether the temperature is within the normal range for fingers (28-34°C).

Why No Additional Libraries Are Needed?

The LM35 sensor provides a direct analog voltage output that can be read using the built-in analogRead() function in Arduino. Unlike digital sensors that require communication protocols such as I2C or SPI, the LM35 simply outputs a voltage proportional to the temperature, making additional libraries unnecessary.

LM35 Sensor Formula

The LM35 temperature sensor provides an output of 500 mV at 0°C, with a 10 mV per degree Celsius increase. To convert the sensor’s analog output to a temperature reading in Celsius, the following formula is used:

Temperature (°C) = (Voltage (V) - 0.5) × 100

Where:

  • Voltage is the output from the LM35 sensor, typically ranging from 0.5V at 0°C to 3.0V at 250°C.
  • 0.5V is the baseline output of the LM35 at 0°C.
  • 100 is the scaling factor, as the sensor increases 10 mV per °C.

    • See the Pen Untitled by Paula Rivero (@Paula-Rivero) on CodePen.

    Results and Observations

    • In the video, we can observe how the temperature of the fingers is being measured, ranging from 24°C to 30°C. Normally, the finger temperature is between 28°C and 34°C, depending on environmental factors.
    • The LM35 sensor is highly sensitive to external noise, which can affect the readings.
    • The readings were stable, but probably adding a capacitor across the power lines , will helped reduce noise.

    Learning Outcome

    • Understand how to accurately measure body temperature using the LM35 sensor, with particular focus on finger temperature ranging from 24°C to 30°C.
    • Learn how environmental factors can affect the temperature readings of the sensor and the typical finger temperature range of 28°C to 34°C.
    • Gain experience comparing sensor readings with standard instruments (e.g., thermometers) to validate sensor performance.
    • Understand the limitations of the LM35 sensor, particularly its sensitivity to external noise and how to mitigate this issue for reliable measurements.