# eli5 how does elecronics know the time

41 views
0

How does my ofen know how long a second is if its not connected to the internet?
I set the time once and it just knows the time but how?

In: 1

Your oven has a computer chip in it. That chip can keep track of time once you tell it what time it is as long as it has power because it’s programmed to do so. If it loses power it “forgets” and needs to be reset.

Generally there’s two ways to do digital time keeping:

The simple way, that most cheap aopliances (used to?) use is simply the frequency of the electric grid. The chip on board counts how often the AC cycles, and because the AC frequency is always kept constant, 60 AC cycles always mean a second has passed.

The second is to use a quartz crystal. These are set up to produce electrical signals at a very specific frequency when you agitate them with a small current (their resonant frequency). That frequency is generally fine tuned when making the quartz crystal to be exactly 32.768 kHz. That might seem random, but it’s chosen because it’s an exact power of two, meaning if you cut the frequency in half, which is very easy to do electronically, and then cut in half again, and again, and again… you eventually arrive at **exactly** 1Hz, or one signal per second, that you can then use to track time.

Quartz watches use this resulting 1Hz signal to drive the Second hand

We picked out a moment in time to count as the origin of the timeline, the 0 moment that we start counting from. We call this moment the ***Epoch*** and in most computing systems its set to 0:00:00 Jan 1, 1970.

In order to know what moment in time something occurred we count the seconds before or after the epoch. Anything that happened before the epoch is a negative number. Anything that happened after the epoch is a positive number. The current time is +1676394617 seconds since the epoch.

Internet connected computers reference multiple official clocks to stay synchronized about how many seconds have passed since the epoch. They form a consensus by comparing the time measured by various high precision clocks around the world. This consensus is shared through a system called Network Time Protocol. This is the global standard for timekeeping on the internet.

There are lots of ways that high precision clocks can count time, but one of the most important ones is by counting the amount of radiation emissions from a radioactive Cesium atom. Physicists have determined that these emissions happen on an extremely regular interval that is effectively unchanging in its rate over time. This makes it like a natural metronome, if you want to think of it that way. It ticks at a fixed rate and we can count time by counting the ticks.

And how does a quartz crystal keep time?

How does a guitar-string or tuning-fork keep its pitch?

Mechanical resonance.

Basically the same thing; a quartz crystal is cut into a shape that mechanically resonates (some are literally cut into an ultra-miniature tuning-fork shape), and uses a bit of electronics to sustain the oscillation forever.

Quartz crystal and other piezo-ceramics have a property that means they change shape very slightly when you apply a voltage across them (you deposit a thin silver plating on two sides) and conversely if you squeeze or strain them they produce a small voltage. This allows electronics to monitor, and feed back and sustain the oscillation.