Preparation:

• Node-red installation
• MQTT Broker
• Chatbot plugin: https://flows.nodered.org/node/node-red-contrib-chatbot
• Create a bot, Telegram in this example: https://core.telegram.org/bots
• Arduino IDE
• Solder deep sleep jumper on the underside of the board, see build instruction.

Here is an example code for node-red for Undervoltage alert to Telegram.

You need to add:

*A bot to the sender node,

*Put the text you want to be sent out in the text node,

*Add chatid in conversation node: https://firstwarning.net/vanilla/discussion/4/create-telegram-bot-and-get-bots-token-and-the-groups-chat-id

*Your topic and Broker to the MQTT node.

• [
      {
          "id": "a0b2ef4c.ac08d",
          "type": "mqtt in",
          "z": "4e61ffbd.293758",
          "name": "",
          "topic": "device_topic/voltage",
          "qos": "2",
          "broker": "74f4f305.79b8e4",
          "x": 210,
          "y": 179,
          "wires": [
              [
                  "2838f77a.391648"
              ]
          ]
      },
      {
          "id": "2838f77a.391648",
          "type": "function",
          "z": "4e61ffbd.293758",
          "name": "",
          "func": "\n \nvar voltage = parseInt(msg.payload) / 1000;\n\nvar msg = {payload:voltage}\n\nreturn msg;",
          "outputs": "1",
          "noerr": 0,
          "x": 396,
          "y": 179,
          "wires": [
              [
                  "d899ef9c.f29bf8"
              ]
          ]
      },
      {
          "id": "e695c610.d8b18",
          "type": "chatbot-telegram-send",
          "z": "4e61ffbd.293758",
          "bot": "cfed462d.fcf59",
          "track": false,
          "parseMode": "",
          "outputs": 0,
          "x": 1112,
          "y": 179,
          "wires": []
      },
      {
          "id": "19c9b344.512d35",
          "type": "chatbot-message",
          "z": "4e61ffbd.293758",
          "name": "",
          "message": [
              {
                  "message": "Battery under 3.3V on unit XYZ"
              }
          ],
          "answer": false,
          "track": false,
          "x": 914,
          "y": 178,
          "wires": [
              [
                  "e695c610.d8b18"
              ]
          ]
      },
      {
          "id": "57bb9322.d0eb8c",
          "type": "chatbot-conversation",
          "z": "4e61ffbd.293758",
          "name": "",
          "chatId": "",
          "transport": "telegram",
          "messageId": "",
          "contextMessageId": false,
          "store": "",
          "x": 748,
          "y": 178,
          "wires": [
              [
                  "19c9b344.512d35"
              ]
          ]
      },
      {
          "id": "d899ef9c.f29bf8",
          "type": "falling-edge",
          "z": "4e61ffbd.293758",
          "name": "",
          "threshold": "3.3",
          "x": 559,
          "y": 178,
          "wires": [
              [
                  "57bb9322.d0eb8c"
              ]
          ]
      },
      {
          "id": "74f4f305.79b8e4",
          "type": "mqtt-broker",
          "z": "",
          "broker": "localhost",
          "port": "1883",
          "clientid": "",
          "usetls": false,
          "compatmode": true,
          "keepalive": "60",
          "cleansession": true,
          "willTopic": "",
          "willQos": "0",
          "willPayload": "",
          "birthTopic": "",
          "birthQos": "0",
          "birthPayload": ""
      },
      {
          "id": "cfed462d.fcf59",
          "type": "chatbot-telegram-node",
          "z": "",
          "botname": "",
          "usernames": "",
          "polling": "1000",
          "log": ""
      }
  ]

 
Arduino code:

Basic work: Wifi connect -> MQTT Broker connect -> Measure Voltage -> Send Voltage over MQTT -> deep sleep "sleepTimsS" seconds

#include <ESP8266WiFi.h>
#include <PubSubClient.h> // MQTT

#define sleepTimeS 600 //Time between updates

// CHANGE
const char* ssid = "ssid";
const char* password = "pass";
const char* mqtt_server = "IP";
String device_topic = "your_topic";
float cal_volt = 6.85; // Calibrate this value if the voltage reading is of

String voltage_topic = device_topic+"/voltage";

WiFiClient espClient;
PubSubClient client(espClient);
long lastMsg = 0;
char msg[50];
int value = 0;
  
void setup() {
  pinMode(15, OUTPUT);
  digitalWrite(15, HIGH);
  //Wifi
  WiFi.begin(ssid, password);
  int wifiTryConnectCounter = 0;
  while (WiFi.status() != WL_CONNECTED)
  {
    delay(500);
    wifiTryConnectCounter++;
    if (wifiTryConnectCounter >= 10)
    {
      ESP.deepSleep(sleepTimeS * 1000000);
    }
  }
  
  //MQTT
  client.setServer(mqtt_server, 1883);   
  // Create a random client ID
  String clientId = "ESP8266Client-";
  clientId += String(random(0xffff), HEX);
  // Attempt to connect
  if (client.connect(clientId.c_str())) {
    //Success
  } else {
    ESP.deepSleep(sleepTimeS * 1000000);
  }
  
  //Sensor
  digitalWrite(15, LOW);
  int sensorValue = analogRead(A0);
  digitalWrite(15, HIGH);
  // Convert the analog reading (which goes from 0 - 1023) to a voltage (0 - 5V):
  float voltage = sensorValue * (cal_volt / 1023.0);
  //Send
  snprintf (msg, 75, "%ld", (int)(voltage * 1000));
  client.publish(voltage_topic.c_str(), msg);

  //Sleep and repeat
  client.loop(); // MQTT work
  ESP.deepSleep(sleepTimeS * 1000000);
}

void loop() {
  // Will not run
}