How a quartz watch works

How a quartz watch works. The piezoelectric effect in action. In some sense this everyday watch isn't special at all - I mean I bought it for about ten bucks at Target yet once we look inside it's amazing. Let me show you. I'd call it the key machine of the modern industrial age. Precise time keeping made possible things like the Global Positioning System and our telecommunications infrastructure. Now, this metal container holds the heart of this watch. Inside lies a tiny quartz fork. I have one. Now its so tiny that I keep it in this white cap. You can see it right there in the center. Now, although this modern circuity is vital to the watch, it's based on the same principles of the first clocks built in the 17th century: Resonate motion. It's easiest seen in the pendulum clock. This family heirloom - it hung in the living room when I was a kid - uses the motion of a pendulum to keep time. Now, this pendulum oscillates with a regular period that runs a clockwork that translate this motion into movement of the hands. There are many ways to create resonate motion. For example, a tuning fork. This one vibrates four hundred and forty times a second that's an A note when struck. I love that sound. Now, if I slow down the motion of the fork you can see how the tines move back and forth with a regular period. That's resonate motion, like the pendulum, which can be used to measure time. The quartz crystal I showed from inside this watch is a tiny tuning fork. It vibrates at about 30,000 times per second. But how do you get it to vibrate and how do you measure its vibrations or record its vibrations. I mean we cannot get a hammer in there to hit the quartz crystal. The engineers who designed this digital watch used something known as the piezoelectric effect to make this small tuning fork vibrate. You can see this piezoelectric effect most easily with Rochelle Salt. Here at the center lies the crystal. I've attached two electrodes made of tin foil, and strung wire from them to a small bulb. Watch what happens as I strike the crystal with a hammer. As I deform the crystal it generates a current. The reverse also happens: If you place a voltage across the crystal it deforms. This is how the quartz tuning fork in the watch is pinged. A voltage from the battery sets it in motion, and then the watch's circuitry measures the current fluctuations that represent the resonate motions of the tines. Quartz is ideal for digital watches because of its outstanding physical hardness, and mechanical and chemical stability; and that stability makes this watch work nearly anywhere on earth under all but the most extreme conditions. One more interesting thing about these digital watches is how these tiny tuning forks are made. On a production line engineers must make these quartz tuning forks so accurate that they vibrate at 32,768 times a second plus or minus a six-hundredth - about two parts per million in frequency. If that frequency differs by a six-hundredth the watch will be off by more than one minute a year. To see how they tune these forks look at the ends: You see deposits of gold at the ends of the tines. These are added to make the fork's vibration frequency too low. On the production line a laser zaps tiny bits of the gold off until the frequency of vibration is just right. It's a wonder that these magnificent engineered objects are so inexpensive. I'm Bill Hammack the engineer guy.

Channel: engineerguy

Views: 1,105,336

Rating: 4.9656968 out of 5

Keywords: Bill Hammack, quartz crystal, watch, engineering, University of Illinois U, piezoelectric effect

Id: 1pM6uD8nePo

Length: 3min 33sec (213 seconds)

Published: Tue Dec 14 2010

Reddit Comments

[u/Schrockwell]

It's interesting to me that the frequency is 32,768 Hz = 2¹⁵ Hz exactly. The watch probably has a single 16-bit register that is incremented by one every vibration. When the most significant bit changes (either to a 1 or a 0), one second has elapsed.

[u/uxp]

I fix watches for a living. One thing I would like to correct is how he describes how manufacturers time the tuning forks to be precisely 32.768KHz.

Manufacturers generally make the crystals vibrate fast to some degree. This means your watch would normally run maybe .3 seconds a day fast. Which for this type of watch is considerably too fast. Since the crystal is encased in a canister which is vacuum sealed to prevent any friction from air, they can't realistically encase it in a vacuum canister, time it, and then remove it, and repeat the process without incurring large costs. On the side, the logic circuit is programmed to, once every 60 seconds or so, pause briefly. This makes the relative time of the watch slow, and makes it sync more accurately to the real time.

If you are curious on why the tuning fork vibrates at 32.768Hz, it is because it is evenly divisible by two. 2 15 times is 32768. The logic circuit divides by two, 15 times, (32768 = 2¹⁵⁾ which is equivalent to one impulse of the motor, and indicative of an analog second hand's tick.

[u/*polhold00268]

That was awesome. Thanks for posting.

[u/Buzzard]

There's a Secret Life of Machines episode on the Quartz Watch too (as well as clocks in general).

[u/WilliamOfOrange]

Here is the Guy's youtube channel http://www.youtube.com/user/engineerguyvideo

That is my post put up 1 day ago. http://www.reddit.com/r/engineering/comments/elypx/engineeringguyvideos_simple_yet_interesting/

[u/FunkMastaD]

So theres gold in this thing? AAAAAAAAAAWWWWWWWWW YYYYYEEEEAAAAHHHH

[u/Rad1030]

Did anyone else think that was Mark Hamill in the preview frame?

[u/pigvwu]

Could anyone explain how the mechanism of the watch turns the known frequency of the quartz into telling time?

I already knew that watches were based on a constant frequency generated by a chunk of quartz, and I was hoping that this video would tell me how the watch makes use of that.

edit: I suppose I have to rephrase my question to be a bit clearer.

How does the watch take the mechanical action of the vibrating quartz and turn it into a digital signal that the circuitry of the watch can make use of?

Surely it's not just the quartz striking a button 32768 times a second, or at least not any kind of button that I'm thinking of. Also, I understand how the circuitry of a watch could make use of this kind of information, but what I'm wondering is how the quartz vibrating is converted into an input that a computer chip could understand.

[u/[deleted]]

I knew most of this, but had always wondered how they made it so precise. The gold-tipping thing is pure genius.