{"id":52269,"date":"2026-11-01T10:50:48","date_gmt":"2026-11-01T10:50:48","guid":{"rendered":"http:\/\/siratek.net\/index.php\/tutorials\/iot-sensor-node\/"},"modified":"2026-11-01T10:50:48","modified_gmt":"2026-11-01T10:50:48","slug":"iot-sensor-node","status":"publish","type":"siratek_tutorial","link":"https:\/\/siratek.net\/index.php\/tutorials\/iot-sensor-node\/","title":{"rendered":"Project, an IoT Sensor Node"},"content":{"rendered":"

<\/p>\n

What you will build<\/h2>\n

A small box you put in a room. It measures temperature and humidity, publishes them over Wi-Fi every minute, and tells you when it stops.<\/p>\n

Build two and you can compare rooms. Build five and you have a picture of a building. That is what separates this from the weather station: one node is a gadget, several nodes are information<\/strong>, and the design here is about making a node you would be willing to build five of.<\/p>\n

Before you begin<\/h2>\n

Parts<\/h3>\n\n\n\n\n\n\n\n\n
Part<\/th>\nSiratek product<\/th>\nIts own tutorial<\/th>\n<\/tr>\n<\/thead>\n
ESP32 development board<\/td>\n2590<\/td>\nESP32, Your First Wi-Fi Project<\/em><\/td>\n<\/tr>\n
DHT22 temperature and humidity<\/td>\n2870<\/td>\nDHT22, Better Accuracy Than the DHT11<\/em><\/td>\n<\/tr>\n
USB power supply and cable<\/td>\n<\/td>\nPowering Your Project<\/em><\/td>\n<\/tr>\n
Breadboard and jumper wires<\/td>\n<\/td>\nUsing a Breadboard<\/em><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

A DHT11<\/strong> works too and costs less. The DHT22 is worth the difference here because the DHT11 reports whole degrees, and comparing two rooms that differ by half a degree needs the decimal.<\/p>\n

A broker<\/strong> is needed. Sending Sensor Data with MQTT<\/em> covers the three options; this page assumes one on your own network, which is the right answer once you have more than one node.<\/p>\n

Why an ESP32 and not an Uno<\/h3>\n

Everything in this project could be done with an Uno and a Wi-Fi module, and it would be worse in every way. The ESP32 has the radio built in, is 3.3 V throughout so the sensor connects directly, has memory to spare, and costs less than the parts it replaces.<\/p>\n

It is also the only board here that can do the thing that makes a node practical: deep sleep<\/strong>.<\/p>\n

Design decisions, before any code<\/h3>\n

A node that runs for one evening is easy. A node you will build five of and then forget about needs four decisions made deliberately.<\/p>\n

1. It must name itself.<\/strong> Five identical nodes publishing to siratek\/temperature<\/code> are useless. Each needs its own topic, and typing a different name into five sketches is how you end up with two nodes called kitchen<\/code>. The sketch below derives a unique id from the chip\u2019s MAC address and publishes under it, with a friendly name you set once.<\/p>\n

2. It must say when it dies.<\/strong> A node that stops publishing looks exactly like a node reporting an unchanged temperature. MQTT\u2019s last will<\/strong> solves this: the broker publishes offline<\/code> on the node\u2019s behalf when it vanishes. Without it, a failed node is invisible.<\/p>\n

3. It must survive the network going away.<\/strong> Routers reboot. Wi-Fi drops. A node that needs power-cycling after every blip is not a node you will build five of.<\/p>\n

4. It must not need a laptop to diagnose.<\/strong> The onboard LED carries the state, so you can tell from across the room whether a node is connected.<\/p>\n

How it works<\/h2>\n
DHT22  ->  ESP32  --Wi-Fi-->  broker  -->  dashboard, phone, logger<\/code><\/pre>\n
The node connects outward. Nothing connects to it, which is why it works behind a router with no configuration, and why several things can watch at once.<\/p>\n
Topics, with <id><\/code> being the node\u2019s own identifier:<\/p>\n\n\n\n\n\n\n\n\n
Topic<\/th>\nCarries<\/th>\nRetained<\/th>\n<\/tr>\n<\/thead>\n
siratek\/<id>\/temperature<\/code><\/td>\nDegrees C, one decimal<\/td>\nNo<\/td>\n<\/tr>\n
siratek\/<id>\/humidity<\/code><\/td>\nPer cent<\/td>\nNo<\/td>\n<\/tr>\n
siratek\/<id>\/status<\/code><\/td>\nonline<\/code> or offline<\/code><\/td>\nYes<\/strong><\/td>\n<\/tr>\n
siratek\/<id>\/name<\/code><\/td>\nThe friendly name<\/td>\nYes<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Retained<\/strong> means the broker keeps the last value and hands it to anything that subscribes later. It is right for the status and the name, which a dashboard needs immediately on connecting, and wrong for readings, where a subscriber should wait for a fresh one rather than being told a value from last Tuesday.<\/p>\n
The LED code<\/h3>\n\n\n\n\n\n\n\n\n
LED<\/th>\nMeaning<\/th>\n<\/tr>\n<\/thead>\n
Fast blink<\/td>\nConnecting to Wi-Fi<\/td>\n<\/tr>\n
Slow blink<\/td>\nWi-Fi up, connecting to the broker<\/td>\n<\/tr>\n
On, with a brief flick each publish<\/td>\nRunning normally<\/td>\n<\/tr>\n
Off<\/td>\nNot running<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Two seconds of looking tells you what a serial monitor would.<\/p>\n
Pin reference<\/h2>\n\n\n\n\n\n\n\n\n
Pin<\/th>\nConnect to<\/th>\nNotes<\/th>\n<\/tr>\n<\/thead>\n
DHT22 +<\/code><\/td>\nESP32 3V3<\/code><\/td>\nThe sensor is happy at 3.3 V<\/td>\n<\/tr>\n
DHT22 out<\/code><\/td>\nESP32 GPIO 4<\/code><\/td>\nA general-purpose pin<\/td>\n<\/tr>\n
DHT22 -<\/code><\/td>\nESP32 GND<\/code><\/td>\n<\/td>\n<\/tr>\n
Onboard LED<\/td>\nGPIO 2<\/code><\/td>\nAlready on the board<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Note what is not<\/strong> here: no level converter, because the ESP32 and the sensor are both 3.3 V. That is one of the quiet advantages of this board.<\/p>\n
\"A
Each node names itself from its MAC address and publishes outward. The retained status topic is what makes a dead node visible.<\/figcaption><\/figure>\n
Build it in stages<\/h2>\n
Stage 1: Wi-Fi alone<\/h3>\n
Run the connection sketch from ESP32, Your First Wi-Fi Project<\/em>. Note the signal strength where the node will actually live, not on your desk. Below about -80 dBm, move it or add an antenna before building anything else.<\/p>\n
Stage 2: the sensor alone<\/h3>\n
Read the DHT22 to the serial monitor. Two seconds between readings; the DHT22 needs it.<\/p>\n
Stage 3: publish<\/h3>\n
Combine them with the MQTT tutorial\u2019s sketch and watch a value arrive in an MQTT client. Stop here and confirm it before adding anything else.<\/p>\n
Stage 4: make it a node<\/h3>\n
Add the identity, the will, the reconnection and the LED. The complete sketch below.<\/p>\n
The complete sketch<\/h2>\n
\/\/ Siratek: an IoT sensor node.\n\/\/ Publishes temperature and humidity, names itself, and says when it dies.\n\n#include <WiFi.h>\n#include <PubSubClient.h>\n#include <DHT.h>\n\nconst char* ssid     = \"your network name\";\nconst char* password = \"your password\";\n\nconst char* brokerHost = \"192.168.1.10\";   \/\/ your own broker\nconst int   brokerPort = 1883;\n\n\/\/ Set this per node. The id below is derived and is always unique.\nconst char* friendlyName = \"kitchen\";\n\nconst int dhtPin = 4;\nconst int ledPin = 2;\n\nDHT dht(dhtPin, DHT22);\nWiFiClient wifi;\nPubSubClient mqtt(wifi);\n\nchar nodeId[20];\nchar topicTemperature[48];\nchar topicHumidity[48];\nchar topicStatus[48];\nchar topicName[48];\n\nconst unsigned long PUBLISH_EVERY = 60000UL;\nconst unsigned long RETRY_EVERY   = 5000UL;\n\nunsigned long lastPublish = 0;\nunsigned long lastAttempt = 0;\nunsigned long lastBlink = 0;\n\nenum Health { CONNECTING_WIFI, CONNECTING_BROKER, RUNNING };\nHealth health = CONNECTING_WIFI;\n\nvoid buildIdentity() {\n  uint64_t mac = ESP.getEfuseMac();\n  snprintf(nodeId, sizeof(nodeId), \"%s-%04X\",\n           friendlyName, (uint16_t) (mac >> 32));\n\n  snprintf(topicTemperature, sizeof(topicTemperature), \"siratek\/%s\/temperature\", nodeId);\n  snprintf(topicHumidity,    sizeof(topicHumidity),    \"siratek\/%s\/humidity\",    nodeId);\n  snprintf(topicStatus,      sizeof(topicStatus),      \"siratek\/%s\/status\",      nodeId);\n  snprintf(topicName,        sizeof(topicName),        \"siratek\/%s\/name\",        nodeId);\n}\n\nvoid showHealth() {\n  unsigned long interval = 0;\n  switch (health) {\n    case CONNECTING_WIFI:   interval = 150; break;\n    case CONNECTING_BROKER: interval = 600; break;\n    case RUNNING:           digitalWrite(ledPin, HIGH); return;\n  }\n\n  if (millis() - lastBlink >= interval) {\n    lastBlink = millis();\n    digitalWrite(ledPin, !digitalRead(ledPin));\n  }\n}\n\nbool connectBroker() {\n  \/\/ The last three arguments are the will: what the broker publishes\n  \/\/ on our behalf if we disappear without saying goodbye.\n  if (!mqtt.connect(nodeId, NULL, NULL, topicStatus, 0, true, \"offline\")) {\n    Serial.print(F(\"broker connect failed, state \"));\n    Serial.println(mqtt.state());\n    return false;\n  }\n\n  mqtt.publish(topicStatus, \"online\", true);     \/\/ retained\n  mqtt.publish(topicName, friendlyName, true);   \/\/ retained\n  Serial.println(F(\"broker connected\"));\n  return true;\n}\n\nvoid setup() {\n  pinMode(ledPin, OUTPUT);\n  Serial.begin(115200);\n  delay(300);\n\n  buildIdentity();\n  Serial.print(F(\"Siratek node: \"));\n  Serial.println(nodeId);\n\n  dht.begin();\n\n  WiFi.mode(WIFI_STA);\n  WiFi.setAutoReconnect(true);\n  WiFi.begin(ssid, password);\n\n  mqtt.setServer(brokerHost, brokerPort);\n  mqtt.setBufferSize(256);\n\n  lastPublish = millis() - PUBLISH_EVERY;\n}\n\nvoid publishReadings() {\n  float t = dht.readTemperature();\n  float h = dht.readHumidity();\n\n  if (isnan(t) || isnan(h)) {\n    Serial.println(F(\"sensor read failed, not publishing\"));\n    return;\n  }\n\n  char payload[12];\n\n  snprintf(payload, sizeof(payload), \"%.1f\", t);\n  mqtt.publish(topicTemperature, payload);\n\n  snprintf(payload, sizeof(payload), \"%.1f\", h);\n  mqtt.publish(topicHumidity, payload);\n\n  Serial.print(F(\"published \"));\n  Serial.print(t, 1);\n  Serial.print(F(\" C  \"));\n  Serial.print(h, 1);\n  Serial.println(F(\" %\"));\n\n  \/\/ A visible flick, so the LED shows activity without leaving the running state.\n  digitalWrite(ledPin, LOW);\n  delay(30);\n  digitalWrite(ledPin, HIGH);\n}\n\nvoid loop() {\n  if (WiFi.status() != WL_CONNECTED) {\n    health = CONNECTING_WIFI;\n    showHealth();\n    return;                                  \/\/ WiFi reconnects by itself\n  }\n\n  if (!mqtt.connected()) {\n    health = CONNECTING_BROKER;\n    showHealth();\n\n    if (millis() - lastAttempt >= RETRY_EVERY) {\n      lastAttempt = millis();\n      connectBroker();\n    }\n    return;\n  }\n\n  health = RUNNING;\n  showHealth();\n  mqtt.loop();\n\n  if (millis() - lastPublish >= PUBLISH_EVERY) {\n    lastPublish = millis();\n    publishReadings();\n  }\n}<\/code><\/pre>\n
Four details that make this a node rather than a demonstration<\/h3>\n
The id is derived, the name is chosen.<\/strong> kitchen-A3F2<\/code> is unique even if you flash the same sketch to five boards and forget to change the name. The friendly name is published retained, so a dashboard can show \u201ckitchen\u201d while the topics stay unambiguous.<\/p>\n
setAutoReconnect(true)<\/code> and an early return.<\/strong> When Wi-Fi drops, the ESP32 reconnects by itself and the loop simply does nothing useful until it does. No manual reconnection logic, and no blocking.<\/p>\n
Retained on status and name, not on readings.<\/strong> A dashboard connecting at midnight should learn immediately which nodes exist and which are alive. It should not be handed a temperature from hours ago as though it were current.<\/p>\n
A failed sensor read publishes nothing.<\/strong> Publishing nan<\/code> puts a hole in every chart downstream. Saying nothing for one minute is the better failure.<\/p>\n
Testing it<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n
Try this<\/th>\nIt should<\/th>\n<\/tr>\n<\/thead>\n
Power it up<\/td>\nFast blink, slow blink, then steady with a flick each minute<\/td>\n<\/tr>\n
Subscribe to siratek\/#<\/code><\/td>\nSee the name, the status, then readings<\/td>\n<\/tr>\n
Unplug the node<\/td>\noffline<\/code> appears on the status topic within the keepalive<\/td>\n<\/tr>\n
Plug it back in<\/td>\nonline<\/code> and readings resume with no intervention<\/td>\n<\/tr>\n
Reboot your router<\/td>\nReconnect by itself once the network returns<\/td>\n<\/tr>\n
Unplug the sensor<\/td>\nNode stays online, publishes nothing, says so on serial<\/td>\n<\/tr>\n
Flash a second board<\/td>\nA second id appears, distinct from the first<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
The third is the one worth watching deliberately. Seeing offline<\/code> arrive without the node having sent it is the moment the will message makes sense.<\/p>\n
Making it yours<\/h2>\n
    \n
  • Batteries and deep sleep.<\/strong> The real version of this project. The ESP32 wakes, connects, publishes and sleeps for fifteen minutes, averaging a few milliamps instead of eighty. That changes a node from something plugged in to something you put anywhere. Regulators, Batteries and Solar<\/em> covers the supply; note that reconnecting Wi-Fi on each wake costs a few seconds of full current, so the arithmetic is worth doing.<\/li>\n
  • More sensors per node.<\/strong> A BMP280 on the I2C pins adds pressure for very little. Keep any analog sensor on GPIO 32<\/code> to 39<\/code>, because the others stop working with Wi-Fi on.<\/li>\n
  • A dashboard.<\/strong> Node-RED or Home Assistant subscribing to siratek\/#<\/code> will draw charts and send alerts without you writing any more code.<\/li>\n
  • Accept commands.<\/strong> Subscribe to siratek\/<id>\/command<\/code> and the node can be told to publish immediately or change its interval. Sending Sensor Data with MQTT<\/em> has the callback, including why the payload is not a string.<\/li>\n<\/ul>\n
    Troubleshooting<\/h2>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
    Symptom<\/th>\nLikely cause<\/th>\nCheck or fix<\/th>\n<\/tr>\n<\/thead>\n
    Fast blink for ever<\/td>\nWi-Fi not connecting<\/td>\n2.4 GHz only; check the password<\/td>\n<\/tr>\n
    Slow blink for ever<\/td>\nBroker unreachable<\/td>\nCheck the address; is the broker running?<\/td>\n<\/tr>\n
    Two nodes keep disconnecting each other<\/td>\nSame client id<\/td>\nThe MAC-derived id prevents this<\/td>\n<\/tr>\n
    Readings arrive, status never does<\/td>\nNot published retained<\/td>\nThe true<\/code> argument<\/td>\n<\/tr>\n
    Dashboard shows a dead node as alive<\/td>\nNo will message<\/td>\nSet it on connect()<\/code><\/td>\n<\/tr>\n
    publish()<\/code> returns false<\/td>\nPayload over the buffer<\/td>\nsetBufferSize(256)<\/code><\/td>\n<\/tr>\n
    Charts have gaps<\/td>\nSensor read failures<\/td>\nExpected occasionally; check the wiring<\/td>\n<\/tr>\n
    Charts have nan<\/code><\/td>\nPublishing failed reads<\/td>\nCheck isnan()<\/code> before publishing<\/td>\n<\/tr>\n
    Node resets when transmitting<\/td>\nThin USB cable<\/td>\nShorter cable, better supply<\/td>\n<\/tr>\n
    Works at the desk, not in the room<\/td>\nSignal strength<\/td>\nCheck RSSI<\/code> where it will live<\/td>\n<\/tr>\n
    Analog sensor reads nothing<\/td>\nADC2 pin with Wi-Fi on<\/td>\nUse GPIO 32<\/code> to 39<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    Sources and attribution<\/h2>\n
    All text, wiring guidance and code in this project are original Siratek work.<\/p>\n
      \n
    • Component behaviour is as described in ESP32, Your First Wi-Fi Project<\/em>, Sending Sensor Data with MQTT<\/em> and DHT22, Better Accuracy Than the DHT11<\/em>, including the ADC2 and Wi-Fi conflict, PubSubClient\u2019s buffer size and the DHT22\u2019s two-second interval.<\/li>\n
    • MQTT\u2019s retained messages and last will are defined by the MQTT specification.<\/li>\n<\/ul>\n
      No third-party tutorial text, diagrams, photographs or code has been reproduced here.<\/p>\n
      <\/p>\n<\/body>","protected":false},"excerpt":{"rendered":"
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