Wiring diagram

The circuit is simple: sensor on I2C, fan through a switch, common 12V power.

12V PSU ──┬────────────────────────► Fan (+)
          │                          Fan (−) ◄── MOSFET (drain)
          │                                       MOSFET (source) ─► GND
          │                                       MOSFET (gate) ◄─ GPIO4 ESP32
          │
          └─► buck 12→5V ─► ESP32-C3 (5V)

SGP40:  VCC ─► 3V3 ESP32    SDA ─► GPIO8    SCL ─► GPIO9    GND ─► GND

Pins

Signal	ESP32-C3 Super Mini pin
I2C SDA (SGP40)	GPIO8
I2C SCL (SGP40)	GPIO9
MOSFET gate (fan)	GPIO4

You can choose different pins — then change the numbers in the code (chapter 5).

Connection rules

1. Common ground. GND of the power supply, ESP32, MOSFET module, and sensor must be connected. Half of "doesn't work" in DIY projects is forgotten common ground.
2. Sensor only on 3.3V. SGP40 cannot handle 5V on power.
3. Fan only through a switch. GPIO outputs milliamps; a fan takes hundreds. Direct connection will burn the pin. How a MOSFET switch works — Transistors and switches.
4. External protective diode for a computer fan is usually not required: the fan has its own switching electronics inside, and from the outside it looks like an electronic load, not a pure inductance. But when switching the power line with a key (especially with PWM), a shunting diode across the fan is useful as protection of the key from inductive spikes — and if it is already in the key module, that is a bonus.

Check polarity before power-on

Mixed + and − on the 12-volt line will kill the buck module and often the board. Beep with a multimeter before the first power-up.

Verification without firmware

After assembly, before loading the main code:

1. Apply 12V — ESP32 should show up in the system as a USB device when the cable is connected (or a power LED lights up).
2. Briefly short the MOSFET gate to 3.3V through a 1kΩ resistor — the fan should turn on.
3. We will check the I2C sensor from the firmware with a bus scanner in chapter 5.