Lecture

ESP32 ADC

04 Mar 2022 • Richard Kuo

Introduction to ADC functions, and Joystick, Thermistor, Photoresistor, Gas sensors (MQ-2, MQ-3, MQ-135), eNose.

ADC (Analog Digital Converter)

Vin is input voltage for ADC to sample

Vref is reference voltage for ADC to compare with Vin

3-bit ADC example:

12-bit ADC : output is a 12-bit binary code, so its value = 0 ~ 4095

ADC architecture

Direct-Conversion ADC

Successive-Approximation ADC

Sigma-Delta ADC

ESP32 ADC

ESP32 ADC documentation

Analog Input Tutorial

Vout = Vin * R2 / (R1+R2)

With a potentiometer

With a photoresistor

NodeMCU-32S pinout

Sketch> ESP32> ESP32_ADC

Connect NodeMCU-32S with a NTC-Thermistor and a 10K ohm to GND
Download ESP32_ADC.ino to ~/Documents/Arduino/examples/
Verify, then Tools>Serial Monitor>

Sketch> ESP32 > ESP32_ADC_Joystick

ESP32_ADC_Joystick.ino

Examples> 03.Analog>

AnalogInOutSerial - Read an analog input pin, map the result, and then use that data to dim or brighten an LED.
AnalogWriteMega - Fade 12 LEDs on and off, one by one, using an Arduino Mega board.
Calibration - Define a maximum and minimum for expected analog sensor values.
Fading - Use an analog output (PWM pin) to fade an LED.
Smoothing - Smooth multiple readings of an analog input.

Examples> 03.Analog> AnalogInput

Connect NodeMCU-32S with a NTC-Thermistor and a 10K ohm pulldown.
AnalogInput.ino read sensor value as delay to turn LED on & off
To show sensorValue
- setup() to add `Serial.begin(9600);
- loop() to add Serial.println(sensorValue);
Verify, then Tools>Serial Monitor>

Examples> 03.Analog> AnalogInOutSerial

Connect NodeMCU-32S with a NTC Thermistor and a 10K ohm pulldown.
Modify AnalogInOutSerial.ino
- analogOutPin = 2
- replace analogWrite to ledcWrite (for ESP32 PWM)

const byte ledPin = 2;  // for ESP32 built-in LED

// setting PWM properties
const int freq = 5000;
const int ledChannel = 0;
const int resolution = 10; //Resolution 8, 10, 12, 15

ledcSetup(ledChannel, freq, resolution); // configure LED PWM functionalitites
ledcAttachPin(analogOutPin, ledChannel); // attach the channel to the GPIO2 to be controlled

ledcWrite(ledChannel, 102); //PWM Value varries from 0 to 1023 

Compile
Verify, then Tools>Serial Monitor>

NTC Thermistor

Negative Temperature Coefficient = Rising Temperature will decrease Resistance NTC thermistors can also be characterised with the B (or β) parameter equation,

where the temperatures and the B parameter are in kelvins, and R0 is the resistance at temperature T0 (25 °C = 298.15 K).
Solving for R yields

取log

log(R) = log(R0) + B * (1/T – 1/T0)

1/T = 1/T0 + (log(R) – log(R0)) / B

T = 1 / (1/T0 + (log(R) – log(R0)) /B)

[Homework] Thermistor.ino

write above formula in C++
create Thermistor.ino with the following codes

Read ADC (get ADCvalue)

int16_t ADCvalue;

void setup(){
  Serial.begin(9600);
}

void loop() {
  ADCvalue = analogRead(A0);
  Thermistor(ADCvalue);
}

Read Thermistor (use ADCvalue to calculate R & T)

void Thermistor(int16_t ADCvalue)
{
  double T, Temp;
  double T0 = 301.15;  // 273.15 + 28 (room temperature) 室溫換成絕對溫度
  double lnR;
  int16_t R;          // Thermistor resistence 
  int16_t R0 = 8805;  // calibrated by reading R at room temperature (=28 degree Celsius )
  int16_t B  = 3950;
  int16_t Pullup = 9930; // 10K ohm
	
  // R / (Pullup + R)   = adc / 4096
  R = (Pullup * ADCvalue) / (4096 - ADCvalue);
	
  // B = (log(R) - log(R0)) / (1/T - 1/T0) 
  T = 1 / (1/T0 + (log(R)-log(R0)) / B );
  Temp = T - 273.15;	
		
  Serial.println(Temp);
}

[Homework] Thermistor_accuracy.ino

Create Thermistor_accuracy.ino
Verify on NodeMCU-32S and, compare it to Thermistor.ino output

From Sketch/ESP32_ADC.ino

V = ReadVoltage(A0); //accuracy improved

double ReadVoltage(byte pin){
  double reading = analogRead(pin); // Reference voltage is 3v3 so maximum reading is 3v3 = 4095 in range 0 to 4095
  if(reading < 1 || reading > 4095) return 0;
  // return -0.000000000009824 * pow(reading,3) + 0.000000016557283 * pow(reading,2) + 0.000854596860691 * reading + 0.065440348345433;
  return -0.000000000000016 * pow(reading,4) + 0.000000000118171 * pow(reading,3)- 0.000000301211691 * pow(reading,2)+ 0.001109019271794 * reading + 0.034143524634089;
} // Added an improved polynomial, use either, comment out as required

Then,

V / 5 = R / (10K + R)

V = 5 * R / (10000+R)

V * (10000 + R) = 5 * R

10000 * V = 5 * R - V * R

10000 * V = (5 - V) * R

R = 10000 * V / (5-V)

V = ReadVoltage(A0); // ADC accuracy improved for ESP32
R = 9990 * V / (5 - V); // assuming 9990 is the measured resistance of 10K resistor by a multi-meter.
T = 1 / (1/T0 + (log(R)-log(R0)) / B ); // R0=8805 measured in room tempature at 28 celsius degree.
Temp = T - 273.15;

Serial.println(Temp);

Photoresistor

LDR arduino library

Photocell (Light Dependent Resistor) Library for Arduino

Sketchbook> GL5516>

Download files from github.com/rkuo2000/arduino/examples/GL5516
- GL5516.ino
- LightDependentResistor.cpp
- LightDependentResistor.h

Verify with NodeMCU-32S, and open Serial Monitor on ArduinoIDE

MQ Gas sensors

MQ2 300~10000ppm 煙幕感應器 (液化氧、丁烷、丙烷、甲烷、酒精、氫氣)

接線方式：VCC:接電源正極（5V）、GND:接電源負極、DO:TTL開關信號輸出、AO:模擬信號輸出
注意：感測器通電後，需要預熱20秒左右，測量的數據才穩定，感測器發熱屬於正常現象，因為內部有電熱絲，如果燙手就不正常了

MQ2 datasheet

MQ3 酒精蒸氣,乙醇,氣體檢測模組

MQ3 datasheet

The typical sensitivity characteristics of the MQ-3 for several gases. in their: Temp: 20°C、 Humidity: 65%、 O2 concentration 21% RL=200kΩ
- Ro: sensor resistance at 0.4mg/L of Alcohol in the clean air.
- Rs:sensor resistance at various concentrations of gases.