Devlog #2 - schematic design

After confirming that the ESP-01S was a viable microcontroller for this project, I continued with designing the schematic for the PCB.

Power Management

ESP-01S is a 3.3V native controller, and the ENS160+AHT21 and the I2C OLED both use 3.3V logic. If I plug my 12V DC adapter directly into the ESP-01S, I’ll fry all my MCU components. Thus, I’m using an LM2596 buck converter to step-down some of the charge to 3.3V.

Air Quality Sensor + MCU

This section contains the sensor components and MCU. The air quality sensors and the OLED both share the same SCL/SDA I2C communication pins, which we established was possible in the previous devlog.

Flip-Flop Switch + MOSFET

Since the pins on the ESP-01S are being used, I decided to try exploring other options to turn my fan ON/OFF. I decided to use a flip-flop switch circuit. This allows me to persistently alter the fan’s ON/OFF state by simply clicking one pushbutton.

The flip-flop switch circuit is a bistable multivibrator. It has two stable states where it can output a persistent signal (either 0 or 1), thus why we say it can represent a single bit of information.

One of the states (transistors) is connected to a MOSFET (STP55NF06L), a heavy-duty voltage-controlled component which handles heavy loads much better than a simple transistor. The MOSFET helps control the fan.

I attached a flyback diode in parallel to the fan. Because a fan is fundamentally wire coiled around magnets (an inductor), when the flip-flop switch suddenly turns off, the magnetic field of the fan collapses, converting back into an extremely high-voltage electric signal that, if not protected against, could instantly fry my MOSFET. The flyback diode allows the current to loop back around through the inductor until it is harmlessly dissipated as heat.

Power-On Circuit

On its own, the flip-flop switch’s initial state is hard to control . It’s based on chance, and small manufacturing differences between identical resistors and capacitors used in the circuit.
To ensure the flip-flop switch always initializes to an OFF state, I added a tiny 10nF capacitor to one of the transistors. This makes one of the transistors slightly slower, thus ensuring the other one will always “win” the race condition.

Next, I’ll be designing the PCB and programming the ESP-01S!