Compare four I2C sensors on one ESP dashboard — and watch them drift

Every temperature/humidity sensor claims a tolerance on its datasheet, but the honest test is putting several of them in the same air and reading them at the same instant. This project does exactly that: four popular I2C parts — BME280, BMP280, AHT20 and HDC1080 — on one bus, one ESP32 or ESP8266, and one web dashboard that shows all of them at once.

It's built with RisalDash, an open-source Arduino library that turns a few dash.metric(...) / dash.chart(...) calls into a responsive, live dashboard served straight from the board over WebSocket.

What the numbers said

All four came online together. The temperatures were tight — within about half a degree. The humidity was the interesting part:

The BME280, BMP280 and AHT20 clustered around 30–32% relative humidity. The HDC1080 sat at 39.7% — nearly ten points higher. Neither is obviously "wrong": this is exactly the kind of unit-to-unit offset you only catch by comparing sensors in the same breath. It's also a great argument for calibrating against a reference rather than trusting one bare module. The two pressure sensors (BME280 vs BMP280) agreed to within 1 hPa.

The hardware

• ESP8266 (Wemos D1 mini) or ESP32 — ~$3–5
• BME280 (0x76), BMP280 (0x77), AHT20 (0x38), HDC1080 (0x40) — a few dollars each
• A breadboard and jumper wires

Wiring is as simple as it gets: every sensor shares the same four wires — SDA, SCL, 3V3 and GND. That's the whole point of I2C. On an ESP8266 the bus is SDA = D2 (GPIO4), SCL = D1 (GPIO5); on an ESP32 it's GPIO21 / GPIO22. Each device answers on its own address, so there's no conflict as long as no two parts share one — here all four are distinct.

The code

The dashboard is declarative. Each sensor is a small block — an online LED, a couple of metrics and a trend chart — and RisalDash lays them out and streams the values live:

#include <RisalDash.h>
#include <Wire.h>
#include <Adafruit_BME280.h>
#include <Adafruit_BMP280.h>
#include <Adafruit_AHTX0.h>
#include <ClosedCube_HDC1080.h>

RisalUI dash("Sensor Compare");

void setup() {
  Wire.begin();                       // ESP8266: D2/D1  ·  ESP32: 21/22
  bmeOK = bme.begin(0x76);            // auto-detect: plug it in, it comes online
  bmpOK = bmp.begin(0x77);
  ahtOK = aht.begin();                // 0x38
  hdcOK = i2cPresent(0x40);

  dash.separator("BME280 · 0x76");
  dash.led("BME280 online", &bmeOK);
  dash.metric("BME280 temp", &bmeT, "°C").decimals(1);
  dash.chart("BME280 temp trend", &bmeT, "°C");
  dash.metric("BME280 humidity", &bmeH, "%").decimals(1);
  // ...three more blocks: BMP280, AHT20, HDC1080

  dash.enableOTA();
  dash.apName("SensorCompare");
  dash.begin();                       // captive portal, then joins your Wi-Fi
}

Sensors are auto-detected: begin() returns whether the chip answered, and that same boolean drives the "online" LED on the card. Pull a sensor off the bus and its card simply reads offline — the others keep running. Read the values every two seconds in loop() with isnan() guards, and every open browser updates at once.

Wi-Fi setup — exactly like every RisalDash project

There's nothing to hard-code. On first boot the board can't find a saved network, so it raises a captive-portal access point called SensorCompare. Connect your phone to it, the setup page pops up automatically, you pick your home Wi-Fi and type the password. The ESP saves the credentials, reboots, and from then on joins your network on its own — the dashboard is waiting at its IP on your LAN. Everything is served by the device; nothing touches a cloud, and your readings never leave your network.

Because dash.enableOTA() is on, later firmware goes over the air from the browser — no USB cable after the first flash.

Why bother comparing?

If you're logging a greenhouse, a server closet or a reptile enclosure, a 10% humidity offset is the difference between "fine" and "alarm". Watching several sensors together tells you which one to trust, whether a module is drifting, and how much a fresh part disagrees with the one that's been running for a year. It's the cheapest calibration check there is — and it makes a genuinely satisfying live dashboard.