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3368 posts

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# 180970 28-Sep-2015 12:28
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Another daylight savings event passed. I think it's sole purpose it to mess up my sleep pattern.

NTP, for those who don't know, stands for Network Time Protocol. Pretty much every thing supports it.

A commercial NTP clock are hard to find in New Zealand, and expensive, at $300 or more.

Anyhow my friend came up with almost free wall clock, that automatically adjust the time, by using an unused cell phone.

He had an old Android cell phone, with no phone provider / no SIMM card.

He uses it as a wall clock, by hanging it on the wall, with a USB charger. It uses NTP to sync time to his wireless access point. Obviously it has a battery, so a quick power loss has no affect.

Just about everywhere sells cheap Android tablets. In New Zealand there are a lot for $100.


Though most devices use NTP, an alternate way of getting time is GPS, which essentially are flying atomic clocks.

However GPS time is off by more than 19 seconds.

Also GPS time is set to Coordinated Universal Time (UTC), which doesn't use daylight savings.

Don't believe me?

"GPS cannot tolerate the introduction of leap seconds. Hence, in 1980, when the Department of Defense started keeping time on the GPS satellites, its system time and frequency were set to agree with UTC(USNO MC). At that time, TAI minus UTC was 19 seconds. Since then, UTC has been delayed many leap seconds and GPS time has not. Hence, GPS time is still very close to TAI minus 19 seconds."

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  # 1395706 28-Sep-2015 12:45
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Do you have examples of how he did it?

What tablet?
Which app is he using etc etc?

Does it display weather as well or just the time?

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  # 1395708 28-Sep-2015 12:55
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Geez and I thought repurposing an old iPad as a wall clock/weather HUD was over the top


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  # 1395710 28-Sep-2015 12:56
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Get a cheap tablet. Lots of wall clock app available.

Hang onto the wall... Done!

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  # 1395712 28-Sep-2015 12:58
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NTP also uses UTC.  Each device has to know when local time daylight saving starts and ends.

And of course you can use GPS time, you just need to make the appropriate adjustments to match UTC and then the appropriate adjustments to match your current time zone.

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  # 1395724 28-Sep-2015 13:04
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"GPS, which essentially are flying atomic clocks." - best thing I've read all day :)

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  # 1395803 28-Sep-2015 14:34
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mentalinc: Do you have examples of how he did it?

What tablet?
Which app is he using etc etc?

Does it display weather as well or just the time?

He told me the NTP was already available, so nothing to download there.

I'd guess getting time from GPS would work, but since more people have networking experience NTP seems the more natural choice.

He had to disable the lock screen, obviously.

As mentioned, there's apparently no shortage of clocks, so try several freebies and pick the one you like.

The clock with weather idea is good. I'd guess a news ticker wouldn't be out of the question on a tablet.

I'll ask him what clock he used.

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  # 1395812 28-Sep-2015 14:51
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My phones ignore the NTP server from the DHCP server and go off and get the time themselves from elsewhere. Not sure if it's GSM or NTP but it's out from my domain controller which I have to periodically bring back to correct time.

I have a 3/4 finished arduino and ethernet shield clock that does NTP and outputs to some 7 segment displays. Of course if I was starting it now would use a esp8266 module for wifi clock instead.

Am wondering how quickly the mechanism from a cheap wall clock could step in order to have it go forward /back at daylight saving changeover too.



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  # 1395830 28-Sep-2015 15:07
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Short answer: NTP is one the simple network protocols to get working.

If you're local DHCP doesn't support NTP, try this list in New Zealand, otherwise Google NTP:

The US Naval NTP clocks are also good, since they are related to the GPS clocks.

Long answer, which may be incorrect.

NTP uses a "Daemon" to get the time, and adjust the clock.

How the Daemon works is up to the manufacturer / programmer.

As mentioned, these use UTC, the location information (what country are you in?) and the daylight savings information (every countries adjustments to daylight savings)

If the time reading is wildly different, it's ignored for some period (again up to the manufacturer).

If it's a small change

* less critical device like phones, it's instantly change.

* more critical devices like server, it's slowly change over an extended period

As a sidenote, Windows uses the "slowly change" technique to adjust times


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  # 1395838 28-Sep-2015 15:12
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Now if he can add PIR to detect when someone is around and only then display the clock I would be impressed.

Internal camera is a possibility for movement detection but keeping it powered defeats the purpose of reducing power consumption and increasing safety. Still it would be worth a look.

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  # 1395840 28-Sep-2015 15:16
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GPS time receivers compensate for the 19 seconds and give accurate time, just so that no one thinks a GPS based clock is 19 seconds out.

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