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NodeMCU NodeMCU is formed by an ESP12E

Si7021 Sensor  for measuring humidity and temperature The  Si7021  Sensor  integrates   humidity   and   temperature   sensor   elements,   an   analog-to-digital converter, signal processing, calibration data, and an I2C Interface. The Si7021 offers an accurate, low-power, factory-calibrated digital solution ideal for measuring humidity and temperature Features Precision Relative Humidity Sensor ± 3% RH (max), 0–80% RH High Accuracy Temperature Sensor ±0.4 °C (max), –10 to 85 °C 0 to 100% RH operating range Up to –40 to +125 °C operating range Wide operating voltage – (1.9 to 3.6 V) Low Power Consumption – 150 μA active current How to wire https://learn.sparkfun.com/tutorials/si7021-humidity-and-temperature-sensor-hookup-guide/all Library https://github.com/adafruit/Adafruit_Si7021/blob/master/examples/si7021/si7021.ino#L36

I built few thermometers powered by battery to check temperature's rooms in my house. For my tests I used : Webmos d1 mini 18650 battery  Sensor Si7021

Attiny : arduino's little brother Arduino is a very good card but if you need a very smaller card size or if you don't need alls pins and less memory, Attiny is a good alternative VCC =Voltage is 2.7-5.5 v Notice : Pins output high voltage is 4.3 volts with a Vcc of 5 volts. For 3.3 Vcc, pins o utput  voltage is 2.5 V  So be carefull, this is not 3.3 volts like Arduino How to program Attiny  To program attiny, just use Arduino IDE.  To do this, you will have to connevt Attiny to arduino.  Arduino will now be used as bridge Initial step Upload ArduinoISP sketch on your arduino : Select card Arduino Uno Load sketch Samples > 11.ArduinoISP  Tips : be sure to unplug all wires before to upload the sketch First step First time and only once, you have to write bootloader on Attiny : Choose 'Arduino as ISP' as the programmer prior to burning the Arduino bootloader onto the ATtiny Select card ATtinyXX Verify Board Properties...