Found via rtl-sdr.com, includes adjustment factor which depends on the diameter of the conductors.

Greaseweazle allows versatile floppy drive control over USB. Read and write your vintage floppy disks from your PC/Linux/Mac computer. This USB device lets you to connect an old floppy disk drive to your modern computer to read/write floppy disks from any system with Shugart standard drives (Amiga, Atarí, Commodore, Amstrad, Sinclair, PC, CNC machines, musical instruments, industrial equipment, and more.)

By extracting the raw flux transitions from a drive, any disk format can be captured and analyzed. The Greaseweazle also supports writing to floppy disks, from a range of image file formats including those commonly used for online preservation (ADF, IPF, DSK, IMG, HFE, …).

Greaseweazle V4 is the latest version, updated for mass production and with the following features:

Reads and writes 3″, 3.5″, 5.25″, 8″ disks (with suitable drive and cable)
Buffered outputs, for communicating with older 5.25″ and 8″ disk drives
Integrated power connector for directly powering most 3.5″ disk drives
Write-enable jumper can be removed for safer preservation of precious vintage disks
Supports floppy-modded 5.25″ drives
Supports Disk-Change detection as used by Rob Smith’s integration into the WinUAE Amiga emulator
3 user-definable outputs (eg. 8″ interface REDWC signal)
100% factory tested, and tested again by me before shipping

Found via https://www.tuhs.org/pipermail/tuhs/2023-January/027510.html

Hardware to emulate the SPI memory in PCs to speed up the development of free BIOS firmware.
Found on the Oberon mailing list.
See also the articles about retrocomputing and security, e.g. https://trmm.net/Cosign/

PTP time on a Raspberry Pi CM 4. Note that the Rpi 5 also has PTP support.
See also https://www.jeffgeerling.com/blog/2022/ptp-and-ieee-1588-hardware-timestamping-on-raspberry-pi-cm4
https://store.timebeat.app/collections/timecard-mini
He mentioned another time-related HAT on his video https://m.youtube.com/watch?v=RvnG-ywF6_s

https://www.jeffgeerling.com/blog/2023/build-your-own-private-wireguard-vpn-pivpn is interesting too.

The websites of the manufacturer Holux are no longer accessible. Holux users will not find any information, downloads (drivers, software etc.) or support at holux.com.
A former technician of Holux Germany and a former Holux specialist dealer are designing this free project. Holux users continue to receive detailed information about Holux GPS devices and support with questions or problems.

Can this function be used to run radioclk http://www.buzzard.me.uk/jonathan/radioclock.html from a GPI pin instead of DCD, DSR, CTS?

With UPS and RTC features, PiSugar2 makes your rpi ready for fast prototyping and product development.

Keep accurate time on a Linux machine based on radio time signals such as DCF77, MSF or WWVB. Found via https://packages.debian.org/stable/net/radioclk
It uses the modem control input signals to monitor the signal from the radio receiver. For a Raspberry pi, where I want to use this, the articles
"Raspberry Pi RTS / CTS Flow Control" https://ethertubes.com/raspberry-pi-rts-cts-flow-control/ and
"Raspberry Pi 3 Hardware Flow Control"  http://www.deater.net/weave/vmwprod/hardware/pi-rts/ will be useful.

Shielded loop antenna for VLF, found on http://www.n4iqt.com/easy_loop_antenna/Notes.txt

Found on https://tinydevices.org/forum/viewtopic.php?t=52