# Measure temperature with ESP32 , a Thermistor and MicroPython
# -------------------------------------------------------------------
# The used Thermistor is a NTC (Negative Temperature Coefficient).
# In a NTC Thermistor, resistance decreases as the temperature rises.
# There is a non-linear relationship between resistance and temperature.
#
# The Display ssd1306 is connected with the serial bus I2C .
# I2C stands for Inter-Integrated Circuit.
#
# During the temperature measurement with a NTC a led is switched on.
# -------------------------------------------------------------------

# libraries
from machine import ADC, Pin, I2C
from time import sleep
import ssd1306
from math import log

# a warm welcome :)
print("Hello, ESP32!")

# ESP32 Pin assignment for the LED.
# the Pin is defined as output to switch the LED on.
# the LED is protected with a 1 kOhm resistor 
led = Pin(15, Pin.OUT)

# ESP32 Pin assignment and size definitions for OLED display (ssd1306)
# I2C : serial bus with SCL (serial clock) and SDA (serial data) 
i2c        = I2C(0, scl=Pin(22), sda=Pin(21))
oled_width = 128
oled_height = 64
oled = ssd1306.SSD1306_I2C(oled_width, oled_height, i2c)

# ESP32 Pin assignment for the Thermistor, using the integrated 
# ADC (Analog Digital Converter);
# the signal is not attenuated (ATTN_0DB)
ntc_pin = ADC(Pin(35))
ntc_pin.atten(ADC.ATTN_0DB)

# Beta Coefficient of the Thermistor
beta = 3950.0   

# write a first text on the OLED display
oled.text('Hello, David!', 10, 10)      
oled.show()
sleep(3)

# measure the temperature every second in an endless loop 
while True:
   # begin new measurement : switch led on 
   led.on()
  
   # delete old content and initialize OLED display 
   oled.init_display()
   
   # read thermistor and calculate temperature in Celsius
   # resolution is 12 bit (4095)
   # calculate Kelvin to Celsius : 273.15
   ntc_data = ntc_pin.read()
   print('ntc_data = ' + str(ntc_data))
   temp = 1.0 / (log(1.0 / (4095.0 / ntc_data - 1)) / beta + 1.0 / 298.15) - 273.15;
   print('temp     = ' + str(temp))
   print('---------------------------')

   # display the measured temperature in Celsius in Console and OLED display
   
   oled.text(str(temp), 30, 30)  
   oled.show()

   # end of measurement : switch led off
   led.off()
   # wait 1 second before new measurement
   sleep(1)