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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...

How to wire You need : ESP8266 ESP-12 FTDI FT232RL Module - Select 3.3v output An external 3.3v power (because FTDI module is unabled to give 200 ma current) Tutorial :  https://www.fais-le-toi-meme.fr/fr/electronique/tutoriel/programmes-arduino-executes-sur-esp8266-arduino-ide

Mosfet N channel Mosfets can be used to act like a switch For Mostfet N channel Source : Connect to ground Drain : connect to led -> resistor -> +VCC Gate : Connect to +VCC to led ON, Connect to Ground to led OFF See tutorial on Youtube :  https://www.youtube.com/watch?v=gTwq6lxDolM I used STP16NF06L - MOSFET N-Ch 60 Volt 16 Amp Specification Value Supplier Package TO-220-3 Transistor Polarity N Maximum Drain Source Voltage 60 V Maximum Continuous Drain Current 16 A Maximum Gate Source Voltage ±16V Maximum Power Dissipation 45000mW Number of Elements per Chip 1 Peak Forward Voltage 60VDC Output Power 45W Peak Non-Repetitive Surge Current 64A Peak Reverse Current 10uA Channel Mode N-Channel Enhancement Peak Reverse Recovery Time 50ns Channel Type N Peak Reverse Repetitive Voltage 60VDC Configuration Single Maximum Drain Source Resistance 0.09 Ohms@10V Category Power MOSFET Maximum Continuous Forward Current 16A Typical Fall...

ESP-01 Esp01 is a very small esp8266. Less pins and small size. So it is wonder for iot projects But ESP-01 consumption is around 90 mAh. So you can't power it with a battery because lifetime will be only few days. To decrease consumption you have to put esp01 in a sleep mode. Best mode is deep sleep mode which reduce consumption to less than 1 uAh. Power consumption - See datasheet Standby ~ 0.9uA Running ~60-215mA, Average ~ 80mA How to power To power ESP-01 you have to use a 3.3v battery connect pin 1-GND to ground connect pin 8-VCC and to 3.3v Found a 3.3v battery is not easy. A solution is to use 18650 battery 3.7v (or 3x AA) and drop voltage to 3.3v. To drop voltage, you can use a LDO like MCP1700-3302E/TO VIN : 2.3-6v VOUT : 3.3v Goto Deep Sleep mode To use deep sleep mode, it is pretty easy. Just add ESP.deepsleep(5*1000000) in your code to go to sleep for 5 seconds. After 5s ESP will wakeup and restart and run setup() again ...