Devlog #6: Getting throttled

Attempting to complete my PCBs, and I’m going through it, fr.

Fried transistors + attempted fixes

After receiving my PCBs, I got to work to solder my components. I had ordered PCBA for any components I didn’t have, such as inductors, the barrel jack connection, etc., but I was planning to save money by hand-soldering components I DID already have, such as transistors and my entire MCU system. This also gave me the added benefit of testing these components beforehand, and it allowed me to go through several iterations of code on the ESP-01S.

The first issue I encountered with hand-soldering was with soldering the tiny, microscopic transistor holes (it was my first time doing so). The copper pads for my PN2222 breadboard transistors are super close together, and with my shaky hands, I accidentally whiffed, put too much solder, and left a solder connection between all three pads. Whoops!

I eventually managed to fix it with a solder sucker.
But after soldering, I decided to test the transistors with a multimeter to check if they were damaged, and both transistors were duds. I probably spent too long trying to fix it with the soldering iron and ended up frying them both.

After this, I attempted to salvage the project by desoldering some of the more expensive components from the board and using it on the second backup PCBA I ordered from JLCPCB. I managed to salvage the OLED screen, but while desoldering the ESP-01S, I broke off several pins, essentially leaving the chip unusable. I decided not to salvage the bigger chips (e.g. ESP-01S, ENS160+AHT21), and decided to just reorder some components instead for the second attempt.

New attempt: flip-flop switch fix + MCU not working

I managed to solder all components to the second PCB cleanly, without any hiccups. However, on trying my project out, I encountered two issues:

1. The flip flop switch turns on the fan, but it doesn’t turn it off.
2. The 3.3V MCU system (including the OLED, ESP-01S, and ENS160+AHT21 sensor) doesn’t seem to turn on.

The first issue was relatively easy to fix. Turns out, I had forgotten to add a pull-up resistor near the bottom of the flip-flop switch, and thus my steering capacitors had no path to VCC to “refill” (reset) their charge after the initial button press. I quickly soldered a pull-up resistor to VCC to fix it.

The second issue stumped me for a while. I found out that I had forgotten to connect pin 4 (EN) of the ESP-01S to 3V3, so I quickly soldered a connection, but it still wasn’t working. Eventually, I took a look with my multimeter and found the problem: my 3V3 logic chips were actually running on 1.23V for some reason.

It turns out, JLCPCB had sent me PCBA’s with the WRONG BUCK CONVERTER. I had ordered an TDSEMIC LM2596S-3.3, which would normally work perfectly, but JLCPCB had sent me PCBA’s with TDSEMIC LM2596S-ADJ instead.

I’ll have to either contact JLCPCB for a refund, or attempt to salvage the PCB myself by modding it. I’ve burnt enough allowance on this ;-;

Below: desoldering fail, attempted fixes (2), close-up of LM2596S-ADJ vs what I actually ordered (https://www.lcsc.com/product-detail/C42421958.html?s_z=n_q_C42421958&globalKeyword=C42421958)

Devlog #5

CAD Modeling

With the code and PCB done, I needed a frame to hold all of the hardware together. Therefore, I spent a couple hours designing two cases in Fusion360 - one to house the PCB, and one to house the fan and the carbon filter I was using.

PCB Case

This PCB case is a little different than the ones I’ve designed so far. For one, the MOSFET and the capacitor that arrived on the PCB assembly I ordered from JLCPCB were much larger than I’d imagined, so I needed to design holes in my casing to accommodate for their size. Additionally, the ENS160 + AHT21 sensor needs generous access to air in order to provide accurate readings. I couldn’t encase them in the casing entirely, so I needed several unusual holes in my design so as not to interfere with the placements on my PCB board.

I ended up designing a snap-fit housing with two parts - top and bottom (my first time designing a snap-fit joint!). The top part has several holes to ensure enough spacing for PCB components such as the OLED screen and keyswitch. It’s not air tight, but I really only need it to keep the PCB isolated and make things more presentable.

Fan Holder Case

I also needed to model a CAD case for the desktop fan to hold it and the active carbon filter in place. The fan case itself is actually a very simple design - three separate “panels” held in place with 4 screws and heat inserts.

Devlog #4

Finalizing MCU program

I used the breadboard prototype I had from Devlog #1 to finalize a working version of the ENS160 + AHT21 sensor program on the ESP-01S. I use the Adafruit libraries for these components to extract their readings, and then display it on a scrolling background on my 0.91 inch OLED SSD1306.

This program is the final step before I solder the remaining components onto my PCB, which conveniently arrived as I was on vacation.

It displays the following information:
Line 1: Temp (deg C), Temp (deg F), Humidity (relative %)
Line 2: Air Quality Index (AQI), Total Volatile Organic Compounds (ppb - TVOC), eCO2 (ppm - estimated CO2)

Devlog #3 - PCB design

Once I had the design finalized from the schematic, it was straightforward enough to convert it into a PCB. I spent some time rearranging components to try and use as little space as possible.

I had to leave some extra space for the ENS160+AHT21 air quality sensor, as it doesn’t work without consistent exposure to air.

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!

Devlog #1 - breadboard prototype

I’ve been soldering a lot more recently thanks to starting a variety of electronics projects, and I’m starting to feel the need to get a fume extractor and some sort of air quality sensor to monitor my workspace.
After some consideration, I decided making my own fume extractor would be a fun project idea, and also a great way to use up some electronics components that I have lying around.

Components

To make this project, I ordered some cheap $6 12V cooling fans meant for PC setups. For the microcontroller, I’ll be using an ESP-01S, an ENS160+AHT21 (which can measure air quality and VOCs), and an I2C 0.91 inch OLED display, all components I already have. I spent some time desoldering the OLED display from an older PCB project to reuse it in this one.

Breadboarding

The ESP-01S is an incredibly small controller, with only two pins (GPIO0 and GPIO2) available as digital pins (apart from RX and TX, but I plan to use those for debugging). Since both the OLED and the ENS160+AHT21 are I2C components that communicate with Serial Data (SDA) and Serial Clock (SCL), theoretically these components could share GPIO0 and GPIO2 as SDA and SCL pins if they had different device addresses.
However, to be sure, I breadboarded and flashed a quick program to output the addresses and confirm that I had three separate, unique identifiers.

Now that I have this information, I’m ready to design the PCB next.