Material Master

 Material Measurement Device

Materials

• Adafruit Trinket M0 - $9
  
• .9" I2C OLED Display - $3
• Hall effect Sensor - $2
• Magnets!
• Perfboard
• Repurposed Lithium Battery - $0
• Charge/Discharge IC $2
• USB breakout board

Display will need 2(3)/5 of the pinouts. "SDA on pad 0 and SCL on pad 1." Since we are going for low power, I'll need a transistor to control the power. This drawing is wrong: 

    "Unique pad capabilities Digital #0 / A2  - this is connected to PA08 on the ATSAMD21. This pin can be used as a digital I/O with selectable pullup or pulldown, analog input (use 'A2'),  PWM output, and is also used for I2C data (SDA) Digital #1 / A0  - this is connected to PA02 on the ATSAMD21. This pin can be used as a digital I/O with selectable pullup or pulldown, capacitive touch, analog input (use 'A0'),  and true analog (10-bit DAC) output. It cannot be used as PWM output. Digital #2 / A1  - this is connected to PA09 on the ATSAMD21. This pin can be used as a digital I/O with selectable pullup or pulldown, analog input (use 'A1'),  PWM output, and is also used for I2C clock (SCL), and hardware SPI MISO Digital #3 / A3  - this is connected to PA07 on the ATSAMD21. This pin can be used as a digital I/O with selectable pullup or pulldown, analog input (use 'A3'),  capacitive touch, PWM output, and is also used for UART RX (Serial1 in Arduino), and hardware SPI SCK Digital #4 / A4  - this is connected to PA06 on the ATSAMD21. This pin can be used as a digital I/O with selectable pullup or pulldown, analog input (use 'A4'),  capacitive touch, PWM output, and is also used for UART TX (Serial1 in Arduino), and hardware SPI MOSI"

Battery display:

I'll need to create a voltage divider that scales down the battery's max 4.2V:

Battery + (4.2V max) ---- R1 (100k) ----+---- R2 (330k) ---- GND

                                        |

                                        +----> Trinket analog input pin (e.g., A1(Pad 2)

This will result in a max battery reading of 3.22V. It will use minimal current:

I=430k4.2​≈9.8μA

"All of the I/O pins can be used for 12-bit analog input" means I'll get values from 0 to 4095. The voltage divider reduced the reading to 76.7% of the actual voltage:
Vout​=Vbat​×R1+R2R2​=Vbat​×100k+330k330k​=Vbat​×0.767

Pseudocode, but insert real values of R1 & R2 from DMM:

const int analogPin = A1;  // ADC pin

const float R1 = 100000.0; // 100k nominal 

const float R2 = 330000.0; // 330k nominal

const float ADC_MAX = 4095.0;

const float V_REF = 3.3;

void setup() {

  Serial.begin(115200);

  analogReadResolution(12);  // 12-bit resolution for Trinket M0

}

void loop() {

  int adcValue = analogRead(analogPin);

  

  // Convert ADC to voltage at divider output

  float V_out = (adcValue / ADC_MAX) * V_REF;

  // Calculate battery voltage

  float V_bat = V_out * (R1 + R2) / R2;

  Serial.print("ADC: ");

  Serial.print(adcValue);

  Serial.print("  V_out: ");

  Serial.print(V_out, 3);

  Serial.print(" V  Battery Voltage: ");

  Serial.print(V_bat, 3);

  Serial.println(" V");

  delay(1000);

}

Hall Effect Sensor:

Testing with 5V, I got a sense distance of around 2.5mm. This might have to be embedded in the wall of a main body with magnets very near in the outer ring. Although it looks like I was testing it incorrectly. From the datasheet: "The UA package is south pole active: Applying a south magnetic pole greater than BOP facing the branded side of the package switches the output low."