jeinzi

Thank you very much :) If you already were aware of the exception for resistors with gold/silver in the fourth band that MrOtingocni mentioned, where have you encountered that before?

Close; as it turns out, it is a snubber resistor :)

Thank you very much for your detailled answer, this resolves my question and I can sleep peacefully again :) The device is also fixed; as I wrote in response to BigDanishGuy, the problem was caused by a connected pressure switch. The circuit board seems to have no issue at all.

I solved it, thanks for your help :)

The relay winding is powered directly by the mains and current can only be interrupted by the main power switch and one of the two pressure switches. As there was electrolyte in the connecting gas line when I got the device, I’m suspecting the switch is damaged and doesn’t have enough hysteresis anymore, causing the rapid switching.

• The heatsink is for a triac doing phase angle control. The second pressure switch is supposed to be set to a lower pressure and if it closes, it inhibits the phase angle control, stopping gas production. The problematic pressure switch seems to be just a redunandancy that removes power from the entire circuit using the relay.
• As I found out, “manually affecting the circuit driving the relay” is just operating the main power switch, and that works without any issues.
• There is no discrete transistor in the circuit :)

My solution was to move the set points of the two pressure switches further apart (there are screws on them for that), so the switch inhibiting the triac triggers first again.

Thanks for your tips :) I already finished drawing the schematic, this resistor was the last part I hadn’t clearly identified. Now I just need to understand it :D

I don’t know how serious your project is and how much reliability and predictability you need; but regarding

I would need a 2-300ohm resistor in series to protect the microcontroller

I will just say: I have yet to kill an AVR.

I haven’t used any gate drivers yet, but I remember an introductory video about MOSFET control that helped me a lot, and at about 7:40 he shows his go-to setup.

And again, I don’t know your requirements, but my default logic level MOSFET is the SI2302. It’s available from a bunch of different manufacturers, has a threshold voltage less than 1V and can conduct several amps continuously.

I have no clue about wireless earbuds in particular, so I won’t be of help with question 1.

But I could give you some general remote help. I don’t know how successful that will be with probably limited tools and documention, but I’m willing to try :D

Is this a thing? Have you encountered resistors with 5 color bands without a multiplier?

With all other resistors on the board, my measurements match the color code. This one is the only exception :/

As stated I measured; the question is how and why the measurement is so far off from the encoded resistance, and why those values are so strange no matter how I read them.

Broken solder joints is a very good idea, thank you. I already found one earlier, but I haven’t resoldered them all yet.

The relay is not performing as intended. The circuit board is controlling an electrolysis reactor which works fine until apparently the pressure switch decides the H2/O2 pressure is high enough and it tries to turn the reactor off. At that point the relay goes brrr, switching on and off very fast, producing a lot of sparks, and it stops when the fuse blows. Deactivating the pressure switch “fixes” the problem, but that is dangerous in itself.

The horizontal bar is commonly used to indicate ground, meaning the part of your circuit you define to be at 0V. So they are all connected, yes. I don’t know anything about a capacitor symbol with a dot though, never seen it until now.

I can highly recommend surgical scalpels to work on PCBs. It also sounds like the shorting of the two pads only has to happen once to flash the firmware, so maybe you could use one of those cheap needle probes to temporarily get contact with the via under the ripped of pad.

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tl;dr: I’m reasonably sure this will work as is.

The following comments involving my personal opinions might be most useful for more complex projects:

While I also try to isolate building blocks in my schematics, I think it’s sometimes beneficial to have some visible connections using wires. Your schematic isn’t very complex, but I still needed to jump around a bit to understand how current flows from the USB connection to the output. I would arrange the building blocks so that current flows from left to right and include one wire that starts at the USB jack, passes by the CH224K and its bypass cap, through the FET to the terminal block, so you can read the current flow like you would a line of text.

Layout:

• Before manufacturing, better before starting the layout, I would include the design rules of your manufacturer under File > Board Setup > Design Rules > Constraints. Currently you haven’t defined minimal clearances, widths etc. Google “[your preferred manufacturer] capabilities”. You might also find existing KiCad templates that you can import.
• I would place the reference designators on the silkscreen so they are visible after assembly, to help with debugging and repairability.
• I would also take care that everything you want to show is legible; currently, your JP-labels overlap U1
• To find enough room on the silkscreen, you could probably reduce your text size. Look up the minimum in your manufacturer capabilities; in addition to putting those values into your design rules, you can also add them in File > Board Setup > Text & Graphics > Defaults > Silk Layers. Apart from that being the new standard, you can then also easily go to Edit > Edit Text & Graphics Properties to set all existing reference designators to those new default values. In my personal experience (with JLCPCB at least), the text also stays legible waayyy below the quoted minimum size.
• I make it a point to include some metadata on all my PCBs; a version number, date, a project title and the name of the designer, so I don’t confuse myself or others when the PCB is found some years later in a random box.
• I like your package size for the resistors and caps, but if you have space issues, you will probably have no issue soldering the smaller 0805 packages by hand either. We regularly have discussions what size can be comfortably soldered by hand without magnification; one of my colleagues insists that even 0603 is “comfortable”.
• EDIT: Your trace from the capacitor to the VDD pin of the IC is a bit long. It will work, but it would be better to place the capacitor as close to the IC as physically possible, so that the area enclosed by the loop “Cap.+ -> IC.VDD -> IC.GND -> Cap.GND -> Cap.+” is minimized. Something like this:

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Regarding both the schematic and layout: run the ERC/DRC and fix all errors and warnings. Most of it is noise, but hidden beneath that, serious issues can hide. Be sure that you don’t miss anything important there.

Another idea that might be out of scope for your project: You could add optional 5.1k pull-down resistors on the CC lines and a solder jumper from VBUS to VOUT. Then you could use the board even without the CH224K and the FET if you only need 5V.