Build Instructions

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• 1

  ESP 8266 pin uses and restrictions

  Not all ESP 8266's GPIOs are born equal, something I have now and then been banging my head at (and burned some ESPs). Some are hard wired for some use, like SPI clock and data, some wired to functionality like flashing and booting. For now Wemos D1 mini's pins, others to be added. Some limitations/features might be missing, to be added when bumped into.

  Dev board Dx, Arduino IDE alias	GPIO #	Notes
  D0	16	- Wire to RST for timed wake from deep sleep - Doesn't support interrupts, PWM, I²C, one-wire
  D1	5	Default I²C SCL, but others except D0 fine for that too.
  D2	4	Default I²C SDA, but others except D0 fine for that too.
  D3	0	- Needs to be pulled up for booting, down for flashing. Wemos has 10k pull-up, convenient for I²C. - Used for booting/flashing, don't hard wire to GND/Vcc. Wemos has hard pull-down for flashing by serial RTS. - Otherwise after boot works as a generic digital I/O
  D4	2	- Built in LED - Wemos has 10k pull-up, convenient for I²C. - Used for booting, doesn't boot up if pulled down while starting. ESP-12 doesn't seem to require a pull-up but recommended to have one. - Otherwise after boot works as a generic digital I/O
  D5	14	Hardware SPI clock, but others can be used for software SPI. Fine for other uses.
  D6	12	Hardware SPI MISO, but others can be used for software SPI. Fine for other uses.
  D7	13	Hardware SPI MOSI, but others can be used for software SPI. Fine for other uses.
  D8	15	- Need to be pulled down for booting up, Wemos for example has 10k pull-down. Not good for I²C, which would require pull-up. - Otherwise after boot works as a generic digital I/O
  A0	A0	ADC input, 0-1 V for generic ESP8266, 0-3.3 V for Wemos D1 mini (which has a 220k/100k resistor voltage divider)
  TX	1	Serial TX, other GPIO use possible
  RX	3	Serial RX, I²S DMA output, other GPIO use possible
  -	10	In ESP-12 modules and boards using them GPIO10 is connected to the internal flash, but in a pinch might be usable for own applications, if quad SPI modes not used. GPIO9 usually doesn't work but might, if you know what you're doing.

  
  When running on batteries, I use ESP-12E/F (12S would be convenient as it has the required pull-up/downs internally, but I've had bad experience waking them from deep sleep due to too high resistance behind RST). This template I print on paper (six fit nicely on an A4) for planning/marking connections:

  Explanation: 

  • RST & EN pulled up (by a ~2.2-12k resistor)
  • For wake-up from deep sleep, add a diode from RST to IO16 - this allows pulling RST low without interfering IO16
  • Pull IO0 up (by a resistor)
  • Recommended to pull IO2 up (by a resistor)
  • Pull IO15 down (by a resistor)
  • The LED is controlled by IO2, inverted
  • IO3 / RX0 is also the I²S out pin, used for WS281x addressable LEDs etc
  • IO4 & 5 are the default I²C SDA & SCL pins, others can be used as well but the defaults often allow running example code etc without modifications.
  • The round pins 1-8 & 9-16 beside the ESP-12 image represent 1/10" / 2.54mm headers, can be used to plan wiring to a SOT-16 adapter, as below, or whatever PCB.

• 2

  Serial communication

  After soon 60 years the RS-232 standard is doing strong, as that's what we're talking with to our most modern IoT thingys (yep I know about TTL vs positive/negative levels, pretty close anyway I'd say). Pretty much any MCU board talks RS-232 to the computer, nowadays generally through a UART inside the MCU board or cable, thus the Serial library is probably the most common denominator that's always available for I/O.

  Initialization

  None needed

  Setup()
  

    Serial.begin(115200); //9600 would be more common, albeit slow, 74880 would print boot loader info as clear text instead of garbage
    while (!Serial) {} //Wait for OK 

  Use

  //Output
      Serial.println("hello"); //With line feed 
      Serial.println((String)"Temperature: " + temp + " humidity: " + humidity); //See Note
      Serial.printf("Temperature: %f humidity: %f\n", temp, humidity);
  //Input
      if (Serial.available() > 0)
        if (Serial.peek() > 42) { //Ignore LF,CR etc
          int userInput1 = Serial.parseInt(); //Adjust number of these as needed. See Note2
          int userInput2 = Serial.parseInt();
        } else {
          Serial.readString(); //Ditch useless data
        }
  

  Note1: The String concatenation method is something you should never do. Unless if lazy. just prototyping and have excess memory. Not that unlikely, if you are reading this.

  Note2: parseInt is a dirty way to do input, quick for testing some parameters but for production you will write a proper input function.

• 3

  Network connectivity - TCP/IP and HTTP

  For pretty much any ESP project I'd want the data to be sent to somewhere, thus networking would be like garlic in cooking - first chop it and then start wondering what are you actually going to cook.

  
  

  Initialization

  #include <ESP8266WiFi.h>
  #include <ESP8266HTTPClient.h>
  const char *WLAN_SSID = "My-network-SID-address";
  const char *WLAN_PASSWORD = "My-network-pw";
  HTTPClient http;
  

  Setup()

  WiFi.mode(WIFI_STA);
  WiFi.begin(WLAN_SSID, WLAN_PASSWORD);
  while (WiFi.status() != WL_CONNECTED) { 
      delay(200);
      Serial.print(".");
  }
  Serial.println((String)"\nConnected: " + WiFi.localIP() + " RSSI: " + WiFi.RSSI());

  Use (HTTP GET call)

  // Helper function
  int doHttpGet(char *url) {
      int httpCode = -1;
      for(int i = 0; i < 20; i++) {
      http.begin(url);
      int httpCode = http.GET();
      http.end();
      Serial.print("URL: ");
      Serial.println(url);
      Serial.print("HTTP return code: ");
      Serial.print(httpCode);
      Serial.print(" ");
      Serial.println(
      http.errorToString(httpCode));
      if(httpCode > 0) //Other retry codes can be added
          break;
          delay(5000);
      }
      return httpCode;
  }
  // Do the request:
  doHttpGet("http://my.url?val1=42&val2=4");3");

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