Your First Sensor
Extend your connected sketch to publish a simulated temperature reading, and see it appear on a live graph. (⏱ ~10 min)
mqtt connected... in the Serial Monitor. This page picks up from there.Add a publish loop
Open your sketch from the previous page. You're going to add two things — a timestamp variable at the top, and a publish block inside the loop. The full updated code looks like this:
#include "credentials.h"
#include "device_id.h"
#include "wifi_tools.h"
#include "mqtt.h"
char DEVICE_ID[9];
unsigned long last_publish = 0;
void setup() {
Serial.begin(115200);
Serial.println("\n\n booting...\n");
device_id.get_or_set(DEVICE_ID);
wifi_tools.begin(WIFI_SSID, WIFI_PASS);
mqtt.setup(MQTT_HOST, MQTT_PORT, DEVICE_ID, MQTT_USER, MQTT_PASS);
}
void loop() {
if (wifi_tools.is_connected) {
mqtt.maintain();
if (mqtt.is_connected && millis() - last_publish > 5000) {
float temperature = 68.0 + random(-30, 30) / 10.0;
mqtt.publish("~/~/numb/temperature/temp", temperature);
last_publish = millis();
}
} else {
wifi_tools.reconnect();
mqtt.report_disconnect();
}
}Arduino IDE users: the SDK includes use angle brackets (<mqtt.h>) rather than quotes — same as in your original sketch.
Understand the publish line
The whole point of this page is one line:
mqtt.publish("~/~/numb/temperature/temp", temperature);The ~/~/ prefix is a shorthand the SDK rewrites to username/deviceID/ before sending, so the actual topic on the broker is larry/a8f3k2m1/numb/temperature/temp.
The platform sees numb in the third segment, so it knows this is a number to graph. It creates a graph called "temperature" with a series called "temp" — no schema registration required.
ALLOW_INSECURE_MQTT as a build flag. On PlatformIO, uncomment the matching line in platformio.ini. On the Arduino IDE, add #define ALLOW_INSECURE_MQTT at the very top of your sketch, before any SDK includes. Then change MQTT_PORT to 1883. Don't ship this to production.Flash and verify
Upload the sketch and open the Serial Monitor at 115200 baud. You should see:
booting...
existing deviceID: a8f3k2m1
wifi connecting...
wifi connected...
mqtt connecting...
mqtt connected...
sending: larry/a8f3k2m1/numb/temperature/temp 68.30
sending: larry/a8f3k2m1/numb/temperature/temp 67.50
sending: larry/a8f3k2m1/numb/temperature/temp 69.10sending: lines appearing every 5 seconds, your device is connected and publishing data to the OhioIoT broker.See it on the dashboard
Open app.ohioiot.com and go to the Data screen. Within a few seconds you should see a graph labeled "temperature" with a line called "temp" that updates in real time.
Go to the Devices screen. You'll see a device card with your 8-character device ID. You can click on it to see details.
What just happened
Your ESP32 published a message to the MQTT broker on the topic larry/a8f3k2m1/numb/temperature/temp with a numeric payload. The platform's processor saw the numb type in the third segment, recognized it as a number to aggregate, and routed it through the data pipeline. A few seconds later, the aggregated value was pushed to the dashboard via a WebSocket, and your graph updated.
You didn't register the device. You didn't create a graph. You didn't define a schema. You published to a topic in the right shape, and the platform did the rest.
Try next
Now that you have a working device, try these:
Add a second series to the same graph. Add another publish line — something like mqtt.publish("~/~/numb/temperature/humidity", 45.0). The "temperature" graph will now have two series: "temp" and "humidity".
Add a boolean. Publish mqtt.publish("~/~/bool/climate/heating", "true") to create a boolean graph that tracks on/off state.
Report device status. Publish mqtt.publish("~/~/status/firmware", "v1.0.0") and check the Devices screen — the firmware field will appear on your device card.
Subscribe to a topic. Read the Subscribing guide to listen for incoming messages and react to commands.