The Problem
I needed a reliable way to monitor ambient temperature in my server rack and get an immediate visual reading without logging into a dashboard. The device also needed to push data to Home Assistant over MQTT for historical logging.
Hardware
- ESP32 Dev Board × 1
- DHT22 Sensor × 1
- 0.96" OLED (SSD1306) × 1
- 10kΩ Resistor × 1
- Breadboard + Wires × 1
System Architecture
graph TD
A[ESP32 MCU] -->|I2C| B[SSD1306 OLED]
A -->|One-Wire| C[DHT22 Sensor]
A -->|WiFi/MQTT| D[Home Assistant]
D -->|Persistence| E[InfluxDB]
D -->|Visualization| F[Grafana Dashboard]
Wiring Logic
graph LR
subgraph ESP32_Pins
GND[GND]
V33[3.3V]
D21[D21 - SDA]
D22[D22 - SCL]
D4[D4 - DATA]
end
subgraph DHT22
D_VCC[VCC]
D_GND[GND]
D_DATA[DATA]
end
subgraph OLED_SSD1306
O_VCC[VCC]
O_GND[GND]
O_SDA[SDA]
O_SCL[SCL]
end
V33 --> D_VCC
GND --> D_GND
D4 --> D_DATA
V33 --> O_VCC
GND --> O_GND
D21 --> O_SDA
D22 --> O_SCL
The Code
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include "DHT.h"
#define DHTPIN 4
#define DHTTYPE DHT22
DHT dht(DHTPIN, DHTTYPE);
Adafruit_SSD1306 display(128, 64, &Wire, -1);
void setup() {
Serial.begin(115200);
dht.begin();
display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
}Result
The build works perfectly. The OLED refreshes every 2 seconds and the Serial Monitor shows clean output. The device successfully integrated into my Home Assistant instance via MQTT.
> SYSTEM_LOG.EXE
[2026-05-20 14:32:01] SUCCESS: MQTT Connection established with HA_BROKER
[2026-05-20 14:32:05] SUCCESS: DHT22 Sensor calibration complete
[2026-05-20 14:32:10] INFO: Initial temperature reading: 24.5C / 42% RH
[2026-05-20 14:32:15] SUCCESS: System status: OPERATIONAL