Reddit:Electronics

I found a piece of a laptop with a fingerprint scanner from 2007 in a junk bin. Surprisingly, it works perfectly with Windows 11 and reads my fingerprint without any problems. It requires a 3.3-volt voltage regulator to power it. I 3D-printed the enclosure and came up with a pretty good device.

Thing of beauty! Wanted to clean it before testing it but its so pristine inside :O

If you saw my last post about accidentally frying my CH32V006 dev board into a working state, this is the next chapter of that mess.

Quick recap: I'm building a custom CH32V006 dev board for OpenServoCore, my project to turn cheap MG90S-class servos into smart actuators with a Dynamixel-style single-wire UART. After the 0.84V rail saga, I had a working board. Time to bring up the Rust bootloader (tinyboot) over UART.

Except UART didn't work.

Specifically, TX worked perfectly. I could blast "Hello world" out of the chip all day. But sending anything into the MCU? Total silence. The HAL driver is essentially the same as the V003, which works fine, so I was pretty sure this was hardware, not firmware.

I scoped the RX line while shoving a stream of 0x55 (UUUUUU…) into it from the host. Quick one-liner if you've never used it:

Alternating 1s and 0s, perfect for scoping.

What I expected: a clean 0V to 3.3V square wave. What I got: a 180 mV ripple sitting on top of 3.3V. Min 3.20, max 3.38. The line was being held high so hard that my USB UART adapter could only sag it by a couple hundred millivolts when it tried to send a zero. Touching RX directly to ground snapped it cleanly to 0V, so the wiring was fine. The driver just couldn't drag it all the way down.

Back to the schematic. The RX line passes through a 74LVC2G241 tri-state buffer that handles the half-duplex direction switching. TX_EN low = listen (DATA -> RX), TX_EN high = talk (TX -> DATA). I'd been picturing this buffer as a passive switch, like a piece of wire that conditionally connects two nets.

By now you electronics gods here probably already figured out what's the issue by now, but I didn't...

Anyways, when TX_EN is low, that buffer is actively driving RX with whatever it sees on DATA. And DATA sits at 3.3V via its own 10K pullup when the bus is idle. So the buffer was reading 3.3V on DATA and pushing 3.3V back out of its high-side MOSFET onto RX with ~24 mA of drive and very low R_DS(on). I was fighting a CMOS push-pull output stage with a USB UART chip. The buffer won. Always.

The firmware workaround is to assert TX_EN while reading. That disables the DATA -> RX path and lets RX fall back to its own pullup, which the host can actually drive. Confirmed it live by poking 3.3V onto the TX_EN pad and watching the ripple snap into a clean rail-to-rail square wave. It's such a satisfying flip on the scope.

The real takeaway, however, is thatTX_EN isn't really a transmit enable. From firmware's view it looks like one, but electrically it's a mux select that picks which buffer drives the bus. Calling it "transmit enable" is what put me in this mental hole in the first place.

For Rev B, the actual fix is a hardware jumper that lets RX bypass the buffer for plain UART mode. Why hardware and not just firmware? Because tools like wchisp use the UART to read/write the CH32's Option Bytes outside of any firmware I control. If my UART depends on my firmware to function, a fresh chip or a half-flashed bootloader can lock me out of recovery. Recovery-path peripherals shouldn't depend on firmware to work.

If you want a more details with scope photos, the schematic, a video of the workaround in action, here is the full writeup.

Teardown video does go over the related PCBs at the component level, with plenty of components being discussed and pointed at. Discussion of various board sections. Thought it might be of interest to some folks here.

Ordered these a few weeks ago and honestly expected the usual first-spin disasters. Instead after hand-placing components and a reflow session everything came up clean on first power-on which is a rare event, so I had to share.

The board is built around an ESP32-S3 as the main controller, talking over SPI to an AD74416H (4-channel 24-bit ADC/(IV)DAC combo, honestly the heart of the whole thing) and five ADGS2414D octal SPST switches that form a 40-point MUX fabric. A DS4424 handles fine iDAC trimming of the 3 output rails, a HUSB238 negotiates USB-PD, and a PCA9535 expands the I²C-controlled GPIOs. Four e-fuses sit in the power path for output protection.

The whole point of the design is to expose all of this as an MCP server and as Python API so AI assistants or scripts can autonomously probe, drive, and debug real hardware, measure voltages and currents, sweep outputs, capture ADC waveforms without a human in the loop for each step. Since handing an AI real control over hardware is a bit nervewracking, there are hard guardrails baked into the firmware and Python API, you can define a board profile for the DUT and it physically can't exceed the defined voltage limits or drive the wrong outputs.

There's also an optional RP2040 HAT that adds a 125 MHz logic analyzer and CMSIS-DAP probe.

Surprised it worked without any bodge wires, I'm now starting to polish the tools and firmware for it.

Full schematics, firmware, and build instructions: https://github.com/lollokara/BugBuster

Anything radio never gets old to me. Somehow I missed this TA7642 radio on a chip until I came across it via surplus. I thought this might make a neat project for for an EMI sniffer. AM is great at detecting the various noise/trash thrown off by modern digital electronics. I find it a fascinating alternate world that you can explore, given the right tool.

There are various plans around the internet for projects with the TA7642, including an application example in the datasheet. So, coming up with a design is really easy.

Keep in mind that this is a prototype and there's tons of room for improvement.

On the left side of the board is the power/audio section, to the right is the RF.

I opted to exclude a tuning capacitor and go with fixed SMD capacitors on a dip switch. I know it's dirty, but for my application it's fine. Now I can switch-tune to various frequencies to get the best response to EMI and keep the overall footprint smaller.

It runs on a single AAA battery with about 1.3mA of current draw, which means it will run a very long time, some 500+ hours.

What became apparent while I was building it is how tiny you can actually make it. With custom boards and SMD components, this sucker will shrink down a ton. I'll probably revisit this idea sometime in the future.

Happy workbench Wednesday! I’ve been itching to post this since last Thursday haha.

I’m Abacus of FooseyRhode and I specialize in repair of a specific type of computer part called an ASIC Hashboard, and those PCBs are what this setup is built around servicing.

Honorable mentions at my desk that might catch some curiosity

1. The Overhead Cable Trays.

2. The rail mounted, Digital Microscope.

3. The Heatgun Glory Hole.

4. Soldering Iron suspended by pulleys.

5. The Quad-table-top fans (Fan Deck).

1 and 2.) The overhead cable trays are super helpful for the obvious, but also for storing some accessories out of the way.

My primary heatgun for example. With it up there, wrist strain from the heavy heatgun gun hose is practically eliminated.

I also mounted my PC and digital microscope up there. Microscope benefits because table vibrations are gone, and computer is just there for cable management.

3.) See image 3. Looks crazy but it’s very necessary for my work. The PCB I work on are typically single layer PCB secured to a giant aluminum plate. A lone heatgun is not capable of achieving solder flow, and hot plates are extremely impractical for PCB like this. Thus, I apply heat from above, and below.

Getting the damn thing mounted safely was the hardest part. I used a pneumatic vesa monitor mount, but backwards. I hammered the vesa mount into shape and secured it to a desk beam. Then I secured the opposite end of the mount to the heatgun.

4.) The pulleys just keep the soldering iron cable out of my way. Honestly, I’m just resolving a minor inconvenience for myself with this.

5.) My 3D printed Fan Deck! It’s four 120mm fans powered through a potentiometer so I can control the speed. It’s used primarily when I am diagnosing a board.

The boards I work on use around 40-90 amps when operating, but for diagnostic, require only 10-20 amps to test properly. Point being, they heat up very rapidly, and heat affects my diagnostic. The Fan Deck is a means to cool boards down while simultaneously injecting power into them.

And wrote a blog post about it https://atomicsandwich.com/blog/breathing_apparatus

This little project is the mainboard for a noise activated roller-blind and the circuit incorporates 31 through-hole components and 20 SMD components.

Now time to begin testing!

PS: Please excuse the soldering

I have to brag, but first I have to tell on myself a little bit. You should really read the datasheet more thoroughly than I did. On the DRV8304 gate driver in 3PWM mode, the INLx pins need to be connected and pulled high for the phases to be turned on. If the pins are left unconnected or pulled low, the driver will put the phases into coast mode (all MOSFETs disabled). Also DVDD is an output pin, so don't connect it to 3V3.

In this image you can see where I cut the trace from 3v3 to DVDD (between the C and the 5 of the C5 reference). Happily, I was able to scrape some soldermask off the trace before the cut, and then bodged some 34AWG magnet wire onto it and connected it to the INL pins to pull them high. After this (and some fixes to a couple of failed joints on other pins) the device was showing correct outputs on the phases.

This is the first time I've ever bodged a PCB so I'm really excited I was able to get it working. This is just a test board for a more complete project I'm going to do down the line, so i'm not too worried about the longevity of this fix. But it's good to have this skill in the toolkit.

Ordered a SOT323 diode instead of a SOD323, worked out in the end. Just had to make sure not to let pin 2 touch the exposed ground plane

I made this little piano using an ATtiny85 and a some push buttons. All 12 keys are read through a single ADC pin using a resistor-ladder voltage divider. Each button taps a different point in the chain, so the voltage tells the chip which key is down. Functional but quite limited as only one key really works at a time.

This was my first project to learn the ATtiny85 and I'm happy with how it turned out. Sounds pretty rough though.

Tried to use it to light some LED’s though I think the circuit expects a battery voltage to use as feedback as it has very low output current otherwise. Short circuit current was 300mA

Current setup 😅

ESP32 + RFID + SDR + random modules

Not sure if this will fully work yet…

But it’s getting interesting 👀

Any ideas what I should add next?

There must be a T48 UV erasing addon with the EPROM blank check.

270-280nm 800mW diode.