[time-nuts] Small PCB for measuring zero crossings of AC power (possible group buy?)

[Jeremy Elson]

Greetings,

Recently I've been interested in doing long-term measurements of the mains
power frequency (e.g.,
https://febo.com/pipermail/time-nuts_lists.febo.com/2022-March/105278.html).
I've built a small PCB that makes this easier: plug in an AC transformer
(wall wart) that produces between 5 and 12 VAC, and my board converts it to
a square wave that's 3.3 volts when the AC is above zero and 0 volts when
it's below zero. This makes it compatible with even simple timestampers,
including a low cost microcontroller-based timestampers. The input is a
5.5mm barrel connector, common on wall warts, and the output is an SMA
connector.

Here's a scope trace of it in operation:
https://uploads.lectrobox.com/power-freq-board/trace1.png

The yellow trace is the input at the barrel connector (the output of the
12VAC wall wart) and the blue is the output at the SMA connector. Here's a
zoomed-in version showing the transition from low to high:
https://uploads.lectrobox.com/power-freq-board/trace2.png

A couple of photos of the board, and the schematic:

https://uploads.lectrobox.com/power-freq-board/RevB-Top.jpg
https://uploads.lectrobox.com/power-freq-board/RevB-Side.jpg
https://uploads.lectrobox.com/power-freq-board/RevB-Schematic.pdf

I've made a couple of these by hand for my own use but I thought other
time-nuts might want one. If there's enough interest I could have a batch
made by a PCB house and sell them for $25. Let me know if you'd like to buy
one. If there's enough interest, I'll have a batch fabricated later this
month.

-Jeremy