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Parts:

Arduino Nano or Arduino Pro Mini
Nokia 5110 84x48 LCD
DHT11 temperature/humidity sensor
DS1307 or DS3231 RTC module with built-in AT24C32 EEPROM
Cheap encoder with 3 debouncing capacitors
Features:

GUI based on Nokia LCD and encoder
temperature, humidity, date and time can be stored every 1 to 120 minutes
each record is compressed to 39 bitfield only so 32kbit flash (4KB) can fit 819 records
optional AT24C256 chip can store even 6553 records
deep sleep used to save battery, ATMEGA is woken up by interrupts mainly
DHT11 is powered only during measurements
powered by single 18650 or other lithium cell
a few display “faces”
6 fonts
battery level meter
data review and graphs
min/max with date/time
all recorded data dump via serial port in CSV format
LCD back light
own fast and low resource N5110 library used
own low level DHT11 data reading
own DS1307, DS3231 and AT24C32 I2C EEPROM handling code
the code uses almost all 32KB Arduino flash
all internal registers can be stored in external EEPROM or DS1307 internal RAM
Data compression

Following values are recorded:

time (hr,min)
date (d,m,y)
temperature
humidity
Above data is compressed to 39-bit bitfield:

hr 0..23 5b
min 0..59 6b
d 1..31 5b
m 1..12 4b
y 2018..2021 2b
temp -40.0..64.0 = 1024values = 10b
hum 0..100 7b
total 39 bits
bits 76543210
byte0 hhhhhmmm
byte1 mmmddddd
byte2 mmmmyytt
byte3 tttttttt
byte4 hhhhhhh0
Only 5 bytes are used for 1 record”

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