I want to make an auto setting clock...

March 15, 2018 10:48 PM

But I want a 24 hour dial. I've found atomic clock movements and I've found 24 hour movements, but no atomic 24 hour movement. I've seen digital ones, but I want to make a dial clock. Any Ideas?
posted by Marky to Technology (7 answers total)

Best answer: If you grab yourself one of those 24 hour movements and investigate its innards with an oscilloscope, you're almost certain to find a circuit trace with a 1 pulse per second square wave on it for driving the second hand's pulse motor.

Disconnect that from the movement's internal driver, wire it to a GPIO pin on a $10 Raspberry Pi Zero W instead, run some Linux distro or other with NTP installed and connected to your house's wifi, and you've got yourself an atomic-referenced clock that doesn't rely on your being able to get signal directly from WWVB and could even be programmed to do something semi-reasonable during daylight saving transitions.
posted by flabdablet at 12:01 AM on March 16, 2018 [3 favorites]

That Raspberry Pi Zero W also has an HDMI port on it, so if you're not completely attached to using a real mechanical clock movement you could create an arbitrarily pretty analog clock on any old flat-panel monitor you have lying around with at least DVI-D input on it.
posted by flabdablet at 12:13 AM on March 16, 2018

The logic in the 12 hour WWVB clock I used to have would have worked fine attached to a 24 hour movement, assuming the pinouts could be matched. It completely ignored the hour marker and just ensured the second hand was accurately disciplined. It relied on the user to get the initial time setting correct.

The only quirk would be that, in DST following mode it falls back by advancing 11 hours in a few minutes, so you'd end up off by 12 hours using that with a 24 hour display, but it would only be an issue in the spring.
posted by wierdo at 12:25 AM on March 16, 2018

Here's a manufacturer that actually sells (at vast expense) the very thing you want to make.
posted by flabdablet at 5:28 AM on March 16, 2018 [1 favorite]

This answer is really half-baked, but couldn't you find a way to pack a 2:1 gear ratio between the movement and the hands?
posted by paper chromatographologist at 5:52 AM on March 16, 2018

Best answer: It's only the hour hand that would need the 2:1 reduction, and this would require switching out the wheels and pinions that perform the 12:1 minute hand to hour hand reduction for a set that does 24:1 instead.

Take a look at this diagram of the wheel train you'd typically find inside an electronic analog clock movement; this one is for a 12 hour movement.

The pair of wheels that does the 12:1 reduction from minute hand to hour hand is set up as a 4:1 reduction from the minute hand pinion to an outboard wheel, then a 3:1 reduction from the outboard pinion to the hour hand wheel. The outboard wheel is also engaged by a pinion connected to the time setting knob, so you wouldn't want to mess with that; to increase the overall reduction to 24:1, then, you'd need to change the relationship between hour hand wheel and outboard pinion from 3:1 to 6:1 without changing the spacing between their centres.

If a and b are the radii of the hour hand wheel and outboard pinion respectively for the 3:1 reduction, and c and d likewise for the 6:1 reduction, and s is the spacing between their centres, then we have

a = 3b
a + b = s

c = 6d
c + d = s

Substituting for b and s, then a and s, to solve for c then d, we get

a + a/3 = c + c/6
4/3 * a = 7/6 * c
c = 4/3 * 6/7 * a
c = 8/7 * a

3b + b = 6d + d
4b = 7d
d = 4/7 * b

So the hour hand wheel needs to be increased to 8/7 times its existing size, and the outboard pinion decreased to 4/7 times its existing size. Bearing in mind that the outboard pinion will be part of a single moulding with the outboard wheel, and that the teeth on these things will probably need to be finer than present-day consumer-grade 3D printers will support, I think you'd find sourcing a whole 24 hour movement a whole lot easier than sourcing the required replacement gears.
posted by flabdablet at 7:14 AM on March 16, 2018

Best answer: Perhaps if you were super mechanically adept, you could butcher a second 12 hour movement and assemble the wheels and pinions you need to convert the first one by dissecting out the 6:1 wheel and pinion from the donor movement's 60:1 second hand to minute hand reduction train.
posted by flabdablet at 7:19 AM on March 16, 2018