Week 09: Input Devices

Assignment

Group Assignment

Probe the analog levels and digital signals of an input device.
Use lab test equipment to observe the signal.
As a minimum, demonstrate the use of a multimeter and an oscilloscope.
Document the group work on the group work page.
Reflect on what I learned from the group work on my individual page.

Individual Assignment

Measure something.
Add a sensor to a microcontroller board that I have designed.
Read the sensor data from the board.

Overview

This week, I explored creating a low-cost input sensor for my final project. I developed a binary contact switch using copper foil and a XIAO ESP32-C3, focusing on the basic logic of digital input before moving to complex pressure sensing.

Group Assignment Reflection

For the group assignment, we used lab test equipment to observe how input devices generate electrical signals. The goal was not only to check whether a sensor works, but also to understand what kind of signal the microcontroller is actually receiving.

We focused on the difference between analog levels and digital signals:

Analog signal: the voltage can change continuously within a range.
Digital signal: the voltage switches between two clear states, usually HIGH and LOW.

In this test, the copper foil contact sensor behaved as a digital input device. When the two copper surfaces were separated, the input pin stayed in one state. When the two copper surfaces touched each other, the circuit was closed and the signal changed clearly.

This helped me understand that an input device is not only about the sensor material itself. The circuit structure is also very important. A sensor needs a complete and stable electrical path so the microcontroller can read the state correctly.

Individual Assignment: Copper Foil Contact Switch

References & Resources

Resource	Link
Adrián Torres — FabriXiao	View Project
Adrián Torres — Step Response	View Project
XIAO ESP32-C3 Pin Multiplexing	Official Wiki

Materials & BOM

Item	Description
① XIAO ESP32-C3	Main controller for input processing.
② Copper Foil Tape	Used as conductive contact surfaces.
③ Spacer Material	Paper and tape to create an air gap.
④ Connection	Jumper wires and USB-C cable.
⑤ Software	Arduino IDE for coding and Serial monitoring.

Note: This version does NOT use LEDs, external resistors, Velostat, or conductive fabric.

Board Used for This Test

For this input test, I used my own designed microcontroller board as the main controller board. The copper foil contact sensor was connected to the board through the input pin and GND.

Signal pin → upper copper foil
GND → lower copper foil

The board reads the state of the copper foil sensor and sends the result to the Serial Monitor. This allowed me to test whether the sensor could be used as an input device for my final project.

Wiring & Logic

Wiring Diagram

D4 ➡ Upper copper foil (Signal)
GND ➡ Lower copper foil (Ground)
USB-C ➡ Computer (Power & Data)

Logic Explanation

I used the internal INPUT_PULLUP resistor, which simplifies the circuit by removing the need for external resistors.

Idle State: D4 is pulled HIGH by the internal resistor ➡ pressed:0.
Contact State: Two copper layers touch, connecting D4 directly to GND ➡ pressed:1.

Circuit Logic: Open Circuit and Closed Circuit

This sensor works by changing the circuit between an open state and a closed state.

Open Circuit

When the two copper foil layers are not touching, the circuit is open. The signal pin is not connected to GND. Because I used INPUT_PULLUP, the microcontroller keeps the input pin at HIGH.

Copper foil separated
→ circuit open
→ input pin pulled HIGH
→ pressed:0

Closed Circuit

When the two copper foil layers touch, the signal pin is directly connected to GND. The input becomes LOW. In the code, LOW is treated as the pressed state.

Copper foil touching
→ circuit closed
→ signal pin connected to GND
→ input reads LOW
→ pressed:1

This is why the sensor gives a clear digital result. It is not trying to measure a changing resistance. It only checks whether the electrical path is connected or disconnected.

Why I Used `INPUT_PULLUP`

In this test, I did not use an external resistor. Instead, I used the internal pull-up resistor inside the microcontroller by setting the pin mode to:

pinMode(PRESS_PIN, INPUT_PULLUP);

This is important because a digital input pin should not be left floating. If a pin is floating, it may randomly read HIGH or LOW because it is not connected to a stable voltage.

The internal pull-up resistor gives the input pin a default state:

Default state → HIGH
Touched state → LOW

This makes the circuit simple and stable for a contact switch.

This also means that the circuit still uses a resistor. It is just inside the microcontroller, not added externally on the board.

Fabrication Process

Sandwich Structure

I cut two pieces of copper foil and used paper/tape to create a simple layered structure:

Top Layer: Copper foil connected to D4.
Middle Layer: Paper spacer with an air gap.
Bottom Layer: Copper foil connected to GND.

Testing the Contact

Initially, I tried using a sponge to press the layers, but it failed to trigger. I realized that since the sponge is non-conductive, it only works if the force is strong enough to make the two copper surfaces physically touch.

Signal Observation

The copper foil sensor showed a very clear signal change when the two conductive surfaces touched.

In the idle state, the two copper foil layers were separated. The input pin was pulled HIGH by the internal pull-up resistor, so the Serial Monitor showed:

pressed:0, value:0, holdTime:0

When the two copper foil layers touched, the signal pin was connected to GND. The input changed to LOW, and the code interpreted this as a pressed state:

pressed:1, value:100, holdTime:[ms]

The result was stable for detecting contact. This means the copper foil structure works well as a simple digital switch.

However, this test does not measure pressure intensity. It only detects whether the circuit is open or closed. Therefore, it can clearly answer:

Is there contact? → Yes / No

But it cannot answer:

How much pressure is applied? → Not yet

Code Implementation

// Ambient Nodes V1
// Copper foil contact switch with debounce and hold time
// Board: XIAO ESP32-C3

const int PRESS_PIN = D4;

bool stablePressed = false;
bool lastReading = false;

unsigned long lastDebounceTime = 0;
unsigned long pressStartTime = 0;

const unsigned long debounceDelay = 30; // 30ms debounce

void setup() {
  Serial.begin(115200);
  delay(1000);

  pinMode(PRESS_PIN, INPUT_PULLUP); // Use internal pull-up
}

void loop() {
  // Read the raw state (LOW means pressed due to PULLUP)
  bool readingPressed = digitalRead(PRESS_PIN) == LOW;

  // Debounce logic
  if (readingPressed != lastReading) {
    lastDebounceTime = millis();
    lastReading = readingPressed;
  }

  if ((millis() - lastDebounceTime) > debounceDelay) {
    if (readingPressed != stablePressed) {
      stablePressed = readingPressed;

      if (stablePressed) {
        pressStartTime = millis(); // Record start time
      }
    }
  }

  // Output values
  int value = stablePressed ? 100 : 0;
  unsigned long holdTime = stablePressed ? millis() - pressStartTime : 0;

  Serial.print("pressed:");
  Serial.print(stablePressed ? 1 : 0);
  Serial.print(", value:");
  Serial.print(value);
  Serial.print(", holdTime:");
  Serial.println(holdTime);

  delay(50);
}

Code Explanation

The code reads the copper foil contact sensor as a digital input.

Pin Definition

const int PRESS_PIN = D4;

This line defines which pin is connected to the copper foil sensor. In this test, the signal layer of the copper foil is connected to D4.

State Variables

bool stablePressed = false;
bool lastReading = false;

These variables store the sensor state. lastReading stores the raw reading from the previous loop. stablePressed stores the confirmed state after debounce.

Timing Variables

unsigned long lastDebounceTime = 0;
unsigned long pressStartTime = 0;
const unsigned long debounceDelay = 30;

These variables are used for timing. lastDebounceTime records when the sensor reading last changed. pressStartTime records when the press started. debounceDelay is set to 30 ms to avoid false triggering caused by unstable contact.

Setup

Serial.begin(115200);
pinMode(PRESS_PIN, INPUT_PULLUP);

Serial.begin(115200) starts serial communication so I can see the sensor data in the Serial Monitor.

INPUT_PULLUP turns on the internal pull-up resistor. This means the pin stays HIGH when the copper layers are not touching. When the copper layers touch, the pin connects to GND and reads LOW.

Reading the Sensor

bool readingPressed = digitalRead(PRESS_PIN) == LOW;

Because I used INPUT_PULLUP, the logic is inverted. LOW means the sensor is pressed, and HIGH means the sensor is not pressed.

Debounce Logic

if (readingPressed != lastReading) {
  lastDebounceTime = millis();
  lastReading = readingPressed;
}

When the reading changes, the code records the time of the change. This helps filter out unstable contact noise.

if ((millis() - lastDebounceTime) > debounceDelay) {
  if (readingPressed != stablePressed) {
    stablePressed = readingPressed;

    if (stablePressed) {
      pressStartTime = millis();
    }
  }
}

Only when the reading stays stable for more than 30 ms does the code accept it as the real sensor state. If the sensor becomes pressed, the code records the start time.

Output Values

int value = stablePressed ? 100 : 0;
unsigned long holdTime = stablePressed ? millis() - pressStartTime : 0;

The code converts the digital state into a simple value:

Not pressed → value:0
Pressed → value:100

It also calculates holdTime, which shows how long the sensor has been pressed.

Serial Monitor Output

Serial.print("pressed:");
Serial.print(stablePressed ? 1 : 0);
Serial.print(", value:");
Serial.print(value);
Serial.print(", holdTime:");
Serial.println(holdTime);

The Serial Monitor prints three values:

pressed → whether the copper foil is touching
value → simple 0 / 100 output
holdTime → how long the contact has lasted

This output is useful for my final project because sitting is not only a pressure event. The duration of sitting can also become part of the interaction logic.

Problem & Solution

Problem: Non-conductive Trigger Failure

Pressing the surface with non-conductive materials, like a sponge, did not change the Serial Monitor values.

Solution: Physical Contact Verification

The current version is a digital contact switch, not an analog pressure sensor. It requires the two copper surfaces to physically touch to close the circuit.

✅ Working: Copper surface touches copper surface ➡ pressed:1.
❌ Not working: Sponge presses the surface but copper layers remain separated ➡ pressed:0.

Results & Limitations

Test Results

The test was successful. The Serial Monitor correctly identified the binary states:

Idle: pressed:0, value:0, holdTime:0
Pressed: pressed:1, value:100, holdTime:[ms]

Limitations

✅ Binary only (0 or 100).
❌ No pressure intensity measurement.
⚠️ Sensitivity depends entirely on the mechanical contact and spacer thickness.

Why This Test Does Not Give Pressure Values

The current copper foil sensor is a digital contact switch. It only has two states:

0 → not touching
1 → touching

There is no gradual resistance change in this version, so it cannot produce a continuous pressure value.

This is different from a pressure sensor such as an FSR. An FSR changes resistance when pressure changes. To read that kind of changing resistance, I need to use a voltage divider circuit.

A voltage divider usually includes:

FSR + fixed resistor

The microcontroller then reads the changing voltage from the middle point of the circuit.

Pressure changes
→ FSR resistance changes
→ voltage changes
→ analog value changes

This is why my next commercial pressure sensor tests need an external resistor or a resistor module. Without the voltage divider, the microcontroller cannot reliably convert the sensor’s changing resistance into a useful analog voltage.

Difference Between Contact Switch and Pressure Sensor

Through this test, I understood the difference between a contact switch and a pressure sensor.

Type	How it works	Output	Useful for
Copper foil contact switch	Detects whether two conductive surfaces touch	Digital: 0 / 1	Sitting / not sitting detection
FSR pressure sensor	Changes resistance under pressure	Analog value	Pressure level detection
Step response sensor	Measures charge / discharge timing	Time-based value	Capacitive or material sensing

The copper foil sensor is simple, cheap, and sensitive. It is useful for detecting whether contact happens. However, it is not enough for measuring pressure intensity.

For my final project, this test is still useful because the cushion first needs to know whether someone is sitting. Later, I can combine this contact logic with commercial pressure sensors to read more detailed pressure changes.

What I Learned

This test helped me understand the basic relationship between material contact, circuit state, and microcontroller reading.

The copper foil itself does not automatically become a pressure sensor. In this version, it works because two conductive surfaces physically touch and close the circuit. The microcontroller then reads the change as a digital signal.

I also learned that a stable input needs a defined default state. INPUT_PULLUP gives the input pin a stable HIGH state when the sensor is not touched. When the copper foil touches GND, the signal changes clearly to LOW.

This is a useful foundation for my next pressure sensor tests. For commercial pressure sensors such as FSRs, I will need to use a resistor or voltage divider circuit to convert resistance change into voltage change. This will allow the microcontroller to read more detailed analog values instead of only 0 and 1.

Commercial FSR Pressure Sensor Tests

After finishing the copper foil contact switch test, I continued testing commercial FSR pressure sensors for my final project.

The copper foil test helped me understand digital contact input. In this part, I focused on analog pressure input, because the final cushion needs to detect not only whether contact happens, but also pressure trends under a soft cushion structure.

The goal of this test was not to measure exact body weight. The goal was to compare:

sensor shape
sensing area
analog reading range
signal direction
sensitivity
saturation behavior
performance under foam layers

Material Context: Foam Layer Test

The final cushion will use a soft layered structure, not a hard surface. The pressure sensors will be embedded inside the cushion, so the foam structure strongly affects the readings.

My planned cushion structure is:

Main support layer: thicker 60D high-resilience foam
Upper soft layer: around 2–3mm thin foam or another soft material
Sensor layer: placed between foam / support layers
Bottom structure: later combined with LED groove and electronics packaging

Before testing with the final 60D foam, I used two pieces of 2mm EPE foam as a temporary test layer. I placed the pressure sensor between the EPE foam layers and pressed or sat on the structure.

This test is important because foam spreads pressure. A small round sensor may miss pressure or saturate quickly depending on where the body touches it. A longer strip sensor covers a wider area, so it is more suitable for the cushion’s left, right, and hip sensing zones.

Pressure Sensor Materials Prepared

These parts were prepared for testing and comparison. They are not all final BOM items.

Part	Model / Spec	Unit Price	Qty	Role
Strip FSR	110mm strip pressure sensor	¥18.5 / pc	2	Main sitting pressure test
Strip FSR	SF15-130, 130mm	¥19.0 / pc	2	Wider area pressure comparison
Strip FSR	SF15-54, 54mm	¥13.8 / pc	2	Smaller zone comparison
Round FSR	FSR402 / RFP602	¥10.8 / pc	1	Backup / local trigger
Pressure Sensor Adapter Module	Module A, one potentiometer, VCC / GND / DO / AO	¥4.2 / pc	1+	Preferred module for strip FSR
Linear Voltage Conversion Module	Module B, two adjustment points, VCC / GND / DO / AO	¥11.88 / pc	1	Comparison / backup module
XH2.54 Cable	XH2.54 / 3P / 30cm, 5 pcs per set	¥1.98 / set	2 sets	More stable connection
EPE Foam	1m × 0.5m × 2mm, white EPE foam	¥14.83 / sheet	1	Temporary pressure diffusion test layer

I prepared multiple sensor types because the final cushion may need different sensing zones. The strip FSRs are the main candidates for wider area sensing. The FSR402 round sensor and shorter strip sensor are kept for comparison or local trigger testing.

Sensor Interface Module Comparison

I tested two different sensor interface modules. This became important because the same type of FSR sensor can behave differently depending on the module.

Module	Name	Unit Price	Tested With	Signal Behavior	Current Judgment
Module A	Pressure Sensor Adapter Module	¥4.2 / pc	110mm strip FSR	No-pressure value was high; pressure made raw decrease	More suitable for strip FSR and cushion zone sensing
Module B	Linear Voltage Conversion Module	¥11.88 / pc	FSR402 round sensor	No-pressure value was low; pressure made raw increase	Works, but better for comparison / local trigger

Module A: Pressure Sensor Adapter Module

This module has one blue potentiometer and provides:

VCC
GND
DO
AO

I mainly used this module with the 110mm strip FSR. In my test, this module gave clearer analog readings through XIAO A0, so it is currently the preferred module for the main cushion sensing route.

Module B: Linear Voltage Conversion Module

This module has two adjustment points and also provides:

VCC
GND
DO
AO

I tested this module with the FSR402 round sensor. It also worked, but the signal direction was different. Without pressure, the raw value was low. When pressure was applied, the raw value increased and could reach saturation quickly.

The sensor and module must be documented together. Different modules can change the signal direction, reading range, and sensitivity.

Wiring

For both modules, I used the analog output pin for the test.

Sensor → sensor input side of the module

Module VCC → XIAO power pin
Module GND → XIAO GND
Module AO  → XIAO A0

Module DO → not used in this analog test

I used AO instead of DO because AO provides continuous analog readings. DO only gives a digital threshold output, which is less useful for pressure trend testing.

For modules that require 5V power, the AO voltage needs to be checked before connecting to XIAO A0. The XIAO ESP32-C3 is a 3.3V logic board, so the analog input should not receive 5V directly.

Reading Logic

In the Serial Monitor, I used several values to understand the sensor behavior.

Value	Meaning
`raw`	Original ADC value read from XIAO A0
`filtered`	Smoothed version of the raw value
`signedDiff`	`filtered - baseline`, used to see whether the signal goes up or down
`pressureAbs`	Absolute difference from baseline
`pressureValue`	Normalized 0–100 pressure trend value
`state`	Text label such as no pressure, light pressure, or strong pressure

The raw value is not a body weight measurement. It is only an ADC reading. The value depends on the sensor, module, foam thickness, contact area, sensor position, and module sensitivity.

My body weight is around 58kg, but the Serial Monitor value does not mean 58kg. It only shows the pressure response under this specific sensor and foam setup.

Test 02: 110mm Strip FSR + Pressure Sensor Adapter Module

In this test, I connected the 110mm strip FSR to the pressure sensor adapter module. The module AO pin was connected to XIAO A0.

I placed the strip sensor between two 2mm EPE foam layers and used a notebook / flat board to apply more even pressure. I also tested sitting on the foam structure directly.

This setup was closer to the final cushion structure than direct finger pressing.

Observation

The 110mm strip FSR worked clearly through the foam structure.

In the no-pressure state, the raw value was close to the baseline. When I sat on the foam structure, the raw value changed clearly, and the calculated pressure value increased strongly.

During a relatively still sitting position, the reading stayed in a small fluctuation range. It was also sensitive enough to detect small body movements while I was talking or shifting slightly.

Example Reading

Condition	Raw / Filtered Behavior	Pressure Trend	State	Meaning
No pressure	Baseline / no-load value	Close to 0	No pressure	Empty cushion
Notebook / flat pressure	Clear change from baseline	Strong increase	Strong / saturated	Pressure detected
Sitting on EPE foam	Stable high value	Strong sitting pressure	Sitting detected	Useful for sitting detection
Sitting still	Small fluctuation	Stable range	Stable sitting	Good for detecting stillness
Talking / slight body movement	Visible fluctuation	Sensitive movement response	Movement tendency	Useful later for motion logic

Interpretation

The 110mm strip FSR is suitable for the final cushion because it covers a wider area than a round sensor. It is better for left / right / hip pressure zones and sitting-state detection.

However, the final system will need smoothing, threshold tuning, and calibration so the light does not react too much to small movement noise.

Test 03: FSR402 Round Sensor + Linear Voltage Conversion Module

In this test, I connected the FSR402 round thin-film pressure sensor to the linear voltage conversion module.

This setup also worked, but its signal behavior was different from the strip FSR test.

Without pressure, the raw value stayed around a low value, around 100–150. When pressure was applied, the raw value increased and could reach 4095.

Observation

Condition	Reading Behavior	Interpretation
No pressure	Raw around 100–150	Stable no-pressure baseline
Pressed through foam	Raw increased quickly	Pressure detected
Strong local pressure	Raw could reach 4095	Sensor / module saturated
Old mapping code	`pressureChange` stayed at 0	Code direction did not match this module

Interpretation

The FSR402 round sensor is useful for local pressure detection. However, because it is a small single-point sensor, the pressure can be too concentrated and the signal can saturate quickly.

For my final cushion, this sensor is less suitable as the main pressure sensing method. It can be kept as a backup or local trigger sensor.

Test 04: 130mm and 54mm Strip FSR Preparation

I also prepared 130mm and 54mm strip pressure sensors.

The 130mm strip sensor may be useful for wider pressure zones because it covers a larger area than the 110mm sensor. It can be tested for the left / right / hip zones if I need more sensing coverage.

The 54mm strip sensor is smaller and may be useful for local zone testing or backup sensing, but it is less suitable for large-area sitting pressure detection.

The final choice between 110mm and 130mm will depend on the cushion size, sensor placement, and how much area I want each sensor zone to cover.

FSR Test Code

This code was used to compare different FSR sensors and interface modules. It reads the analog value from A0, calibrates a baseline, smooths the raw value, and maps the pressure change into a normalized value.

// Universal FSR module test for XIAO ESP32-C3
// Board: My designed XIAO ESP32-C3 board
// Module AO  -> XIAO A0
// Module GND -> XIAO GND
// Module DO is not used.
//
// Serial Monitor: 115200 baud
// Send "r" to recalibrate baseline when there is NO pressure.

const int SENSOR_PIN = A0;

int baseline = 0;
float filtered = 0;

int maxChange = 3800;
int noiseLimit = 10;

int lightLimit = 8;
int mediumLimit = 30;
int strongLimit = 70;

void calibrateBaseline() {
  Serial.println();
  Serial.println("=== Calibrating baseline ===");
  Serial.println("Remove all pressure from the sensor.");

  delay(1000);

  long sum = 0;
  for (int i = 0; i < 120; i++) {
    sum += analogRead(SENSOR_PIN);
    delay(10);
  }

  baseline = sum / 120;
  filtered = baseline;

  Serial.print("baseline = ");
  Serial.println(baseline);
  Serial.println("raw,filtered,signedDiff,pressureAbs,pressureValue,direction,state");
}

void setup() {
  Serial.begin(115200);
  delay(1200);

  analogReadResolution(12);
  calibrateBaseline();
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read();
    if (c == 'r' || c == 'R') {
      calibrateBaseline();
    }
  }

  int raw = analogRead(SENSOR_PIN);

  filtered = filtered * 0.82 + raw * 0.18;

  int signedDiff = (int)filtered - baseline;
  int pressureAbs = abs(signedDiff);

  if (pressureAbs < noiseLimit) {
    pressureAbs = 0;
  }

  pressureAbs = constrain(pressureAbs, 0, maxChange);

  int pressureValue = map(pressureAbs, 0, maxChange, 0, 100);
  pressureValue = constrain(pressureValue, 0, 100);

  String direction;
  if (pressureAbs == 0) {
    direction = "none";
  } else if (signedDiff > 0) {
    direction = "positive";
  } else {
    direction = "reverse";
  }

  String state;
  if (pressureValue < lightLimit) {
    state = "no pressure";
  } else if (pressureValue < mediumLimit) {
    state = "light pressure";
  } else if (pressureValue < strongLimit) {
    state = "medium pressure";
  } else {
    state = "strong pressure";
  }

  Serial.print(raw);
  Serial.print(",");
  Serial.print((int)filtered);
  Serial.print(",");
  Serial.print(signedDiff);
  Serial.print(",");
  Serial.print(pressureAbs);
  Serial.print(",");
  Serial.print(pressureValue);
  Serial.print(",");
  Serial.print(direction);
  Serial.print(",");
  Serial.println(state);

  delay(100);
}

FSR Code Explanation

Baseline Calibration

The code first reads the sensor many times when there is no pressure and calculates a baseline.

baseline = sum / 120;

This is necessary because different sensors and modules start from different no-pressure values.

Software Smoothing

filtered = filtered * 0.82 + raw * 0.18;

This reduces small jumps in the raw analog reading.

Signal Direction

int signedDiff = (int)filtered - baseline;

This value shows whether the raw value goes up or down under pressure.

Unified Pressure Value

int pressureAbs = abs(signedDiff);
int pressureValue = map(pressureAbs, 0, maxChange, 0, 100);

Because different modules have different signal directions, I used the absolute difference from the baseline. This makes the final pressure value easier to compare.

Sensor and Module Comparison

Test	Sensor	Module	Setup	Result	Meaning for Final
Test 01	Copper foil contact sensor	Direct contact circuit	Conductive layers touched under pressure	Sensitive digital contact detection	Low-cost backup / reference
Test 02	110mm strip FSR	Pressure Sensor Adapter Module	Sensor placed between 2mm EPE foam layers	Clear sitting pressure response	Best current direction for main input
Test 03	FSR402 round FSR	Linear Voltage Conversion Module	Round sensor tested under foam layer	Very sensitive, but easy to saturate	Useful as local trigger / backup
Test 04	130mm strip FSR	To be compared with adapter module	Longer strip for wider sensing area	Works as larger sensing option	Candidate for left / right / hip zone
Test 05	54mm strip FSR	To be compared	Shorter strip for smaller area	Backup comparison	Useful for small local zone

Problems and Findings

1. Foam Changes the Reading

The pressure sensor does not behave the same when it is directly pressed by hand and when it is placed inside foam. Foam spreads the pressure and changes the contact area.

2. Single-Point Sensors Are Too Local

The FSR402 round sensor is sensitive, but its sensing area is small. In a cushion, body pressure is distributed over a larger area. A single-point sensor can miss pressure or saturate quickly depending on placement.

3. Long Strip Sensors Are More Suitable for the Cushion

The 110mm and 130mm strip FSR sensors cover a wider area. They are more suitable for detecting pressure trends across left, right, and hip zones.

4. Signal Direction Depends on the Module

Different modules produced different signal directions. One setup could output a high value without pressure and decrease under pressure, while another setup could output a low value without pressure and increase under pressure.

Because of this, the code should calibrate a baseline first and then calculate the change from that baseline, instead of assuming one fixed direction.

5. The Sensors Are Sensitive

The sensors were sensitive enough to detect pressure through foam layers. They could also detect small body movements while sitting. This is useful, but the final system will need smoothing, thresholds, and calibration so the light does not react too much to small noise.

Final Input Decision

After testing different pressure sensors and modules, I confirmed that all tested sensors can detect pressure changes.

For the final cushion, I will prioritize strip FSR sensors, especially the 110mm or 130mm type, for the main sensing zones. They are more suitable for area-based pressure detection because they cover more surface area.

The FSR402 round sensor is sensitive and useful, but it saturates easily under concentrated pressure. It is better as a local trigger or backup sensor.

The copper foil contact sensor will remain as a low-cost backup or reference contact detector.

The final input direction is:

Main sensing: 110mm / 130mm strip FSR sensors
Sensor zones: left side, right side, hip / center area
Backup sensor: FSR402 round sensor or copper foil contact sensor
Reading method: analog input through XIAO A0
Physical structure: sensor layer tested first with 2mm EPE foam; final structure will use 60D high-resilience foam and a thin upper layer
Future improvement: smoothing, threshold tuning, calibration, and web-based sensitivity adjustment

Updated Input Reflection

The copper foil test helped me understand digital contact input. It is simple and useful for detecting whether contact happens, but it cannot measure pressure intensity.

The commercial FSR tests helped me understand analog pressure input. FSR readings are more useful for my final cushion because they can show pressure trends, but they are also more sensitive to sensor shape, module type, foam structure, and calibration.

This means the final input system should not react directly to every raw value. It needs baseline calibration, smoothing, threshold tuning, and later web-based adjustment.

Next Steps

Test the selected strip FSR under the final 60D high-resilience foam and thin upper foam layer.
Decide the final position of the left, right, and hip sensing zones.
Replace crocodile clips with more stable connectors or soldered wires.
Connect the pressure input logic to the LED output system.
Later connect the pressure input values to the web interface for sensitivity and threshold adjustment.

Week 09: Input Devices

Assignment

Group Assignment

Individual Assignment

Overview

Group Assignment Reflection

Individual Assignment: Copper Foil Contact Switch

References & Resources

Materials & BOM

Board Used for This Test

Wiring & Logic

Wiring Diagram

Logic Explanation

Circuit Logic: Open Circuit and Closed Circuit

Open Circuit

Closed Circuit

Why I Used INPUT_PULLUP

Fabrication Process

Sandwich Structure

Testing the Contact

Signal Observation

Code Implementation

Code Explanation

Pin Definition

State Variables

Timing Variables

Setup

Reading the Sensor

Debounce Logic

Output Values

Serial Monitor Output

Problem & Solution

Problem: Non-conductive Trigger Failure

Solution: Physical Contact Verification

Results & Limitations

Test Results

Limitations

Why This Test Does Not Give Pressure Values

Difference Between Contact Switch and Pressure Sensor

What I Learned

Commercial FSR Pressure Sensor Tests

Material Context: Foam Layer Test

Pressure Sensor Materials Prepared

Sensor Interface Module Comparison

Module A: Pressure Sensor Adapter Module

Module B: Linear Voltage Conversion Module

Wiring

Reading Logic

Test 02: 110mm Strip FSR + Pressure Sensor Adapter Module

Observation

Example Reading

Interpretation

Test 03: FSR402 Round Sensor + Linear Voltage Conversion Module

Observation

Interpretation

Test 04: 130mm and 54mm Strip FSR Preparation

FSR Test Code

FSR Code Explanation

Baseline Calibration

Software Smoothing

Signal Direction

Unified Pressure Value

Sensor and Module Comparison

Problems and Findings

1. Foam Changes the Reading

2. Single-Point Sensors Are Too Local

3. Long Strip Sensors Are More Suitable for the Cushion

4. Signal Direction Depends on the Module

5. The Sensors Are Sensitive

Final Input Decision

Updated Input Reflection

Next Steps

Why I Used `INPUT_PULLUP`