Week09 | Input Devices – Signal Probing

Group members

Week 09 – Input Devices

Group Assignment

Probe an input device’s analog levels and digital signals.
Use both a multimeter and an oscilloscope to measure and analyze the signals.

Reference material:
https://gitlab.fabcloud.org/academany/fabacademy/2026/labs/oulu/site/-/wikis/Classes/Input%20devices

1. Introduction

The objective of this assignment was to analyze and understand the electrical behavior of input devices at the signal level.

We investigated:

  • Analog sensors (light sensor, potentiometer, NTC thermistor)
  • Digital communication (I²C protocol)
  • Signal levels using:
  • Digital multimeter
  • Oscilloscope (Keysight DSOX1204A)

The goal was to observe real voltage behavior rather than relying only on software output.

2. Experimental Setup

We used:

  • XIAO RP2040 breakout board
  • Light sensor (phototransistor)
  • Potentiometer
  • NTC thermistor
  • Oscilloscope
  • Multimeter

The breakout board provides:

  • Analog inputs (A0–A3)
  • I²C interface (SDA, SCL)
  • 3.3V logic level
  • SPI header

Sensors were connected either:

  • Directly to ADC pins (analog measurement)
  • Through I²C communication (digital measurement)

3. Analog Signal Investigation

3.1 Light Sensor (Phototransistor)

Principle

The phototransistor was connected in a voltage divider configuration.
Its resistance changes depending on light intensity.

  • More light → Higher conduction
  • Output voltage shifts accordingly

Observation

The oscilloscope showed:

  • Continuous voltage variation
  • Small amplitude noise
  • Rapid transitions when light intensity changed

This demonstrates:

  • Analog signals are continuous
  • They are sensitive to environmental noise
  • They do not have sharp threshold transitions

3.2 Potentiometer

Theory

The potentiometer acts as a voltage divider:

$$ V_{out} = V_{cc} \cdot \frac{R_{variable}}{R_{total}} $$

By rotating the knob, the resistance ratio changes.

Measurement

Observed voltage range:

  • Minimum ≈ 0 V
  • Maximum ≈ 3.3 V

Oscilloscope showed:

  • Smooth ramp behavior
  • Stable waveform
  • No oscillatory behavior

This confirms correct divider operation.

3.3 NTC Thermistor

Principle

An NTC (Negative Temperature Coefficient) thermistor decreases resistance as temperature increases.

Its resistance follows approximately:

$$ R(T) = R_0 \, \exp!\left[ B \left( \frac{1}{T} - \frac{1}{T_0} \right) \right] $$

Where:

  • $R_0$ = resistance at reference temperature $T_0$
  • $B$ = thermistor material constant
  • $T$ = absolute temperature (Kelvin)

As temperature increases:

  • Resistance decreases
  • Divider voltage shifts accordingly

Observation

Oscilloscope showed:

  • Slow voltage drift
  • Stable analog response
  • No abrupt transitions

This reflects thermal inertia and continuous analog behavior.

4. ADC Conversion (RP2040)

The RP2040 microcontroller uses a 12-bit ADC.

Resolution:

$$ \text{Resolution} = \frac{V_{\text{ref}}}{2^{12}} = \frac{3.3\,\text{V}}{4096} \approx 0.8\,\text{mV} $$

Voltage conversion formula:

$$ V_{\text{in}} = \frac{\text{ADC}{\text{value}}}{2^{12} - 1} \times V $$}

Comparison between:

  • Multimeter measurement
  • Oscilloscope waveform
  • ADC value from serial monitor

confirmed correct signal-to-digital conversion.

5. Digital Signal Investigation – I²C Communication

5.1 I²C Overview

I²C (Inter-Integrated Circuit) uses two lines:

  • SDA (Serial Data)
  • SCL (Serial Clock)

Characteristics:

  • Open-drain configuration
  • Pull-up resistors required
  • 3.3V logic levels

Logic thresholds:

$$ V_{\text{HIGH}} > 0.7 V_{cc} \quad , \quad V_{\text{LOW}} < 0.3 V_{cc} $$

5.2 I²C Frame Structure

Each I²C transmission includes:

  1. Start condition
  2. 7-bit address + R/W bit
  3. ACK bit
  4. 8-bit data
  5. ACK bit
  6. Stop condition

Rules:

  • Data is valid when SCL is HIGH
  • SDA changes only when SCL is LOW
  • ACK bit is pulled LOW by the slave

5.3 Oscilloscope Observation of I²C

When probing SDA and SCL:

Observed:

  • Square wave clock on SCL
  • Data transitions on SDA
  • Clear ACK pulse
  • Stable HIGH ≈ 3.3 V
  • Clean LOW ≈ 0 V

Clock frequency estimated as:

$$ f = \frac{1}{T_{clock}} $$

The oscilloscope confirmed:

  • Proper synchronization between SDA and SCL
  • Correct ACK behavior
  • Stable digital thresholds
  • Clean signal edges without ringing

6. Analog vs Digital Signal Comparison

Analog Signals Digital Signals
Continuous voltage Discrete 0/1 states
Noise visible Threshold-based logic
Requires ADC Direct logic interpretation
Smooth waveform Square waveform
Sensitive to interference More robust

7. Signal Integrity and Noise

From oscilloscope measurements:

  • Analog lines showed minor ripple
  • Rapid light changes caused transient spikes
  • Digital lines showed sharp transitions
  • No significant ringing observed

Possible noise sources:

  • Environmental interference
  • Probe grounding quality
  • ADC sampling effects

Understanding signal integrity is essential before debugging at the software level.

Input Devices – Group Assignment

Probing Analog and Digital Signals

This assignment investigates both analog input signals and digital I2C communication using oscilloscope measurements.

1. System Overview

Complete measurement setup including XIAO RP2040 board, sensors, and oscilloscope probes.

System Overview

2. Board Design and Connections

2.1 I2C Schematic

SDA and SCL lines use 10kΩ pull-up resistors connected to 3.3V.

I2C Schematic

2.2 Custom Board with XIAO RP2040

Board powered via USB-C. Sensors connected through headers.

Board Closeup

Board Vertical View

3. Analog Signal Observation

Analog signals were measured from: - Light sensor
- Potentiometer
- NTC thermistor

Signals were connected to oscilloscope channel and observed as voltage variations.

3.1 Light Sensor

Voltage increases as light intensity increases.

Light Sensor Analog Signal

3.2 Potentiometer

Manual rotation causes smooth voltage rise and fall.

Potentiometer Analog Signal

3.3 NTC Thermistor

NTC resistance decreases as temperature increases.

Thermistor equation:

$$ R(T) = R_0 e^{B\left(\frac{1}{T} - \frac{1}{T_0}\right)} $$

Observed behavior: - Slow voltage drift
- Stable analog output

NTC Analog Signal

4. Serial Data Output

ADC values printed through USB serial communication.

Example output: - Light: xxx
- Potentiometer: xxx
- NTC: xxx

Serial Monitor Output

5. I2C Communication Analysis

Digital communication between XIAO RP2040 and VL53L1X distance sensor was captured.

Measured: - SDA line
- SCL line
- ACK bit
- Start and Stop conditions

5.1 I2C Clock and Data Signals

SDA (yellow) and SCL (green) signals.

I2C Signal 1

5.2 8-bit Transfer and ACK Bit

After 8 data bits, receiver pulls SDA low for ACK.

I2C ACK Bit

5.3 Frequency Measurement

Measured I2C clock frequency ≈ 6 kHz
Peak-to-peak voltage ≈ 3.8 V

I2C Frequency

6. VL53L1X Time-of-Flight Sensor

Distance sensor connected via I2C interface.

Pins used: - SDA
- SCL
- 3.3V
- GND

VL53L1X Closeup

Conclusion

This experiment demonstrated:

  • Measurement of analog voltage levels from multiple sensors
  • ADC monitoring through serial interface
  • Detailed analysis of I2C digital communication
  • Observation of 8-bit transmission and ACK handshake
  • Verification of clock frequency and signal integrity

Both analog and digital input signals were successfully probed and analyzed using an oscilloscope.

8. Reflection

This assignment demonstrated the importance of analyzing signals at the electrical level.

Key learnings:

  • Analog signals are continuous and noise-sensitive.
  • Digital signals rely on timing and voltage thresholds.
  • Oscilloscope provides deeper insight than serial monitor output.
  • I²C communication can be verified by observing ACK timing and clock synchronization.
  • Hardware-level verification strengthens debugging skills.

This exercise strengthened understanding of both sensor physics and communication protocols.