Output Devices

← See Week 8: Input Devices (Initial Design)

Project Overview

This week focused on creating a complete temperature monitoring system that combines an input sensor (NTC thermistor) with an output display (OLED screen). The system uses a Seeed XIAO RP2040 microcontroller to read temperature from a thermistor via analog input and display the readings on a 0.98-inch OLED screen via I2C communication. The entire system is integrated on a custom-designed and milled PCB.

System Architecture:

• Microcontroller: Seeed XIAO RP2040
• Input Sensor: 10K NTC thermistor with 10K series resistor (voltage divider circuit)
• Output Display: 0.98" OLED (128x64 pixels, I2C interface, SSD1306 driver chip)

Expected Functionality: The thermistor measures ambient temperature, the microcontroller converts the resistance reading to temperature using the Beta parameter equation, and displays the result in both Celsius and Fahrenheit on the OLED screen, updating every second.

PCB Design & Fabrication

Design software used: Fusion

Materials:

• Xiao RP2040
• NTC 10k 3750k Thermistor
• 10kΩ Resistor
• 0.1 µF Ceramic Capacitor
• 0.98" OLED
• Two 4.99kΩ pull up resistors

Files:
PCB Traces.png
PCB Edges.png

Front of PCB

Back of PCB

Bridge between GND and 5V

Improved PCB Design - fixed width of GND traces and NTC connections

Improved Schematic

Hardware Configuration

OLED Display Specifications

Thermistor Circuit

3.3V --- [10K Resistor] --- A0 (ADC) --- [25K Thermistor] --- GND

OLED Display Troubleshooting

Initial Testing: I2C Communication

The first test confirmed the display controller was responding to I2C commands:

#include <Wire.h>

void setup() {
  Serial.begin(115200);
  delay(3000);
  Wire.begin();
  
  Serial.println("Scanning I2C bus...");
  for(byte addr = 1; addr < 127; addr++) {
    Wire.beginTransmission(addr);
    byte error = Wire.endTransmission();
    
    if (error == 0) {
      Serial.print("Device found at 0x");
      if (addr < 16) Serial.print("0");
      Serial.println(addr, HEX);
    }
  }
}

void loop() {}

Serial Monitor Output: Device found at 0x3C

This confirmed:

• ✓ I2C wiring is correct (SDA and SCL properly connected)
• ✓ Pull-up resistors are functioning
• ✓ Display controller chip is powered and responding
• ✓ No address conflicts on the I2C bus

Display Initialization: Success Without Output

Test code that initialized successfully but produced no visual output:

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
#define OLED_RESET -1

Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);

void setup() {
  Serial.begin(115200);
  delay(2000);
  
  Wire.begin();
  
  if(!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println("Display failed!");
    while(1);
  }
  
  Serial.println("SUCCESS at 0x3C!");
  Serial.println("Screen should be WHITE");
  
  display.fillScreen(WHITE);
  display.display();
}

void loop() {}

Serial Monitor showed:

SUCCESS at 0x3C!
Screen should be WHITE

Display showed: Nothing. Completely dark.

Systematic Debugging: Display Tests

To isolate the problem, a series of systematic tests were performed:

Test 1: Alternative I2C Address

Hypothesis: Display might be at 0x3D instead of 0x3C

Action: Tested initialization at both 0x3C and 0x3D

Result: Only responds at 0x3C, still no output ✗

Test 2: Display Resolution Verification

Hypothesis: 0.98" displays might be 128x32 instead of 128x64

Action: Tried both Adafruit_SSD1306(128, 64, ...) and Adafruit_SSD1306(128, 32, ...)

Result: Both initialize successfully, neither shows output ✗

Test 3: Initialization Mode Testing

Hypothesis: Display needs external charge pump mode

// Tried both initialization modes:
display.begin(SSD1306_SWITCHCAPVCC, 0x3C);  // Internal charge pump
display.begin(SSD1306_EXTERNALVCC, 0x3C);   // External power supply

Result: Both modes initialize successfully, no visual output ✗

Test 5: Maximum Contrast & Force Display On

Hypothesis: Display contrast too low or display sleep mode

// Force maximum contrast
display.ssd1306_command(0x81);  // Contrast control
display.ssd1306_command(0xFF);  // Maximum (255)

// Force display ON
display.ssd1306_command(0xAF);  // Display ON command

// Force all pixels ON (test pattern)
display.ssd1306_command(0xA5);  // Entire display ON

Result: Commands execute without error, no visible change ✗

Test 6: Alternative Driver Chips

Hypothesis: Display might not be SSD1306, could be SH1106 or similar variant

Installed U8g2 library and tested multiple driver options:

Conclusion: All drivers initialize successfully but produce no visual output

Test 7: Second Display Unit

Root Cause Analysis: Display Issues

Thermistor Troubleshooting

After establishing that the display controller was communicating (even though not displaying), attention turned to testing the thermistor input circuit to verify the complete system functionality.

Initial Thermistor Test: Incorrect Readings

The thermistor was reading 94K ohms instead of the expected ~10K ohms at room temperature. This indicated either:

• Wrong thermistor specification in code
• Inverted voltage divider calculation
• Incorrect beta coefficient

Specification Mismatch Discovery

Using a multimeter to directly measure the thermistor's resistance revealed the actual specification:

• Measured resistance: ~25,000 ohms at room temperature (20-22°C)
• Code assumed: 10,000 ohms at 25°C
• This 2.5× difference caused massive temperature calculation errors

After updating the code to THERMISTOR_NOMINAL = 25000, initial readings showed proper room temperature (~20°C).

Intermittent Connection Problems

20.1°C → 96.9K ohms → -4.0°C
-2.1°C → 87.9K ohms
-2.1°C → 87.7K ohms  
-1.9°C → 86.8K ohms

Diagnosis: Cold Solder Joints

The erratic readings were classic symptoms of intermittent electrical connections:

• Readings would jump when board was moved or touched
• Connection would make/break randomly
• Visual inspection showed dull, cracked solder joints

Attempted Fix: Reflow Solder Joints

Reheated the thermistor solder joints with fresh solder. Initially this worked:

Raw ADC: 3040 | Voltage: 2.45V | Resistance: 28815Ω | Temp: 21.7°C  
Raw ADC: 3116 | Voltage: 2.51V | Resistance: 31828Ω | Temp: 19.4°C  
Raw ADC: 3177 | Voltage: 2.56V | Resistance: 34608Ω | Temp: 17.5°C  

The readings were now stable and showed a consistent cooling curve (thermistor was cooling down after being heated by the soldering iron).

Continuous Decrease: Cooling Behavior

After the solder reflow, temperature readings showed a steady decrease:

21.7°C → 19.4°C → 17.5°C → 15.7°C → 14.3°C → 12.9°C → 11.7°C → 10.7°C → 9.7°C

Analysis: This is actually correct behavior! The thermistor had been heated during soldering and was naturally cooling back to ambient temperature. The smooth, consistent cooling curve proved:

• ✓ Thermistor was working properly
• ✓ ADC reading was accurate
• ✓ Mathematical conversion was correct
• ✓ The component was responding to temperature changes

Thermistor Complete Failure

Likely causes:

• Thermistor damaged by heat: NTC thermistors are sensitive components. Repeated soldering with excessive heat can permanently damage them:
  • Recommended soldering: 300-320°C for <3 seconds per joint
  • Multiple reflow attempts without cooling may have exceeded thermal limits
  • Internal element can short circuit when overheated
• Solder bridge: Accidental connection between thermistor pads creating short
• PCB trace damage: Hidden short circuit in PCB between power rails

Working Test Code (When Functional)

Complete Temperature Display System

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

// Display settings
#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
#define OLED_RESET -1
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);

// Thermistor settings
#define THERMISTOR_PIN A0
#define SERIES_RESISTOR 10000       // 10K ohm resistor in voltage divider
#define THERMISTOR_NOMINAL 25000    // 25K at 25°C (NOT 10K!)
#define TEMPERATURE_NOMINAL 25
#define BETA_COEFFICIENT 3750

void setup() {
  Serial.begin(115200);
  delay(2000);
  
  // Initialize I2C and display
  Wire.begin();
  
  if(!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println("Display initialization failed!");
    while(1);
  }
  
  // Configure ADC
  analogReadResolution(12);  // 12-bit ADC (0-4095)
  
  display.clearDisplay();
  display.setTextSize(1);
  display.setTextColor(WHITE);
  display.setCursor(0, 0);
  display.println("Temperature Monitor");
  display.println("Ready!");
  display.display();
  delay(2000);
}

void loop() {
  // Read thermistor
  int rawValue = analogRead(THERMISTOR_PIN);
  
  // Calculate voltage and resistance
  float voltage = rawValue * (3.3 / 4095.0);
  
  // Calculate resistance (inverted voltage divider: thermistor on bottom)
  float resistance = SERIES_RESISTOR * voltage / (3.3 - voltage);
  
  // Convert to temperature using Beta equation
  float steinhart;
  steinhart = resistance / THERMISTOR_NOMINAL;
  steinhart = log(steinhart);
  steinhart /= BETA_COEFFICIENT;
  steinhart += 1.0 / (TEMPERATURE_NOMINAL + 273.15);
  steinhart = 1.0 / steinhart;
  steinhart -= 273.15;  // Celsius
  
  float fahrenheit = steinhart * 9.0 / 5.0 + 32.0;
  
  // Display on OLED (if working)
  display.clearDisplay();
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println("Temperature:");
  
  display.setTextSize(3);
  display.setCursor(0, 20);
  display.print(steinhart, 1);
  display.println("C");
  
  display.setTextSize(2);
  display.setCursor(0, 45);
  display.print(fahrenheit, 1);
  display.println("F");
  display.display();
  
  // Serial output for debugging
  Serial.print("ADC: ");
  Serial.print(rawValue);
  Serial.print(" | V: ");
  Serial.print(voltage, 2);
  Serial.print("V | R: ");
  Serial.print(resistance, 0);
  Serial.print("Ω | Temp: ");
  Serial.print(steinhart, 1);
  Serial.println("°C");
  
  delay(1000);
}