You must have spent a lot of time into this, thank you so very much 🙏

With a bit of persuasion I managed to disassemble it without breaking it. I desoldered U1 and I can confirm that pin 2 is GND. Also, here are better pictures taken with a magnifying glass. Note that pin 1 and 3 were shorted to pin 2, but it didn’t make sense to me, so I assumed they bridged due to the thermal shock when the component burned, so I scraped around them.

And here’s a link to AliExpress.

I guess there is no easy way to bypass it then, it would probably cheaper to buy a new device than to buy a replacement IC. Also, I guess now I will have to upload a clean version of the corrected schematic, I owe this to you and the other great people that replied.

EDIT: I could probably bypass it entirely, I just need to inject 5V. Here’s the updated schematic.

Fair point. There were no such instructions.

I tried desoldering U1, disconnecting the piezo, and powering it with 3V from the batteries side. It blinks rhythmically, so I assume the MCU (or ASIC) is fine. But if I connect the piezo, power consumption drops to < 1mA and nothing else happens. So yeah, that component’s purpose is probably not to charge the batteries.

I will try to read the markings again with a magnifying glass later today. Unless you or someone else have better suggestions.

If not, why does your device have two power sources and one is not rechargeable.

That’s a good question, and that’s also something else I didn’t think about. I just checked the instructions and it’s not clear. Perhaps it’s an alternative way to power it. I’ll be honest, this is a cheap device. I don’t want to fix it because it’s expensive, I want to fix it because I literally didn’t turn it on even once before blowing.

I drew it as a battery, but that’s actually USB power (5V). That was probably a poor choice from me.

try image search

Oh. My. God! It worked! How on earth could it know that this specific PCB is what it is? That’s black magic! It didn’t find this specific one, but it correctly classified it and it also found similar designs. Seems like that blown component actually is part of the charging circuit, since other similar designs seem to omit it. I’ll try desoldering it and see if the rest still works. Thank you very much for this suggestion, that blew my mind!

Sorry about the pictures, that’s the best I could do with my phone camera. I took some more, they are not much clearer, but there are more angles at least, link to pictures. It certainly is a 6-pin package.

It didn’t occur to me that it could be part of the battery charging circuit, that would be nice as honestly I don’t care about using it as a battery charger (it takes two AA). I am a bit worried about plugging batteries again, as I am afraid I might damage it further. Assuming pins on the left to be 1, 2, 3 from the top and pins on the right to be 4, 5, 6 from the bottom: pin 1, 2, 5 and 6 seem to be shorted, pins 3 and 4 give me 90k resistance.

I didn’t try image search, do you mean on Google Image Search?

Yeah these pictures are not the best, are they? 😬 I took some more, they might be a tiny bit clearer, but I’m afraid that’s the best I can do with my camera, link to pictures. Seems to be AL322 or maybe AL022? Neither of the two returns any results for me, though.

R4 leads to somewhere for sure, but it’s hidden below the IC, so I can’t follow it. To disasemble it any further I would need to break it entirely. I guess that’s an option. I checked the middle right pin again, it only seems to go to C3 as far as I can tell. Pin 1 of U1 doesn’t seem to be marked, at least not anymore, but perhaps it might be guessable from the direction of the text?

I don’t get it 😅