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a friend of mine came to me with a question and I thought I'd ask you guys for some insight. They were wondering if it would be possible to speed up a regular wall clock so that it would cycle twice as fast as normal. It wouldn't need to be extremely accurate, but if it were accurate to within 10% (ie. one rotation of the second hand would be between 27 and 33 seconds) that would be nice. I think I have a basic idea of how regular quartz clocks work, but I'm really shaky on most of the details, so here were my two thoughts:

  1. Inside a quartz clock, there will probably be a crystal soldered onto a PCB somewhere. If you could find a crystal with the same footprint and pinout that vibrates roughly twice as fast, popping off the old crystal and soldering the new one in would probably work. This would be really easy to do, but I see a few problems. For starters, finding a crystal that meets the listed requirements might be a challenge. Also, I'd imagine that at some point the crystal's vibration gets converted into a square wave. Since the conversion circuitry is based on vibrations, I'm guessing that this all happens within a single package, so you'd need to find a replacement which also has the same drive as the one you're replacing. If the conversion happens externally somehow, you'd still need to make sure that it will work properly with a crystal that's vibrating twice as fast and that its output is linear. I don't think this is the case though, because I'd imagine a "100kHz crystal oscillator" means a crystal with circuitry outputting a 100kHz square/sine wave rather than a crystal actually vibrating at 100kHz.

  2. It might be possible to just yank out the entire crystal/oscillator and replace it with some external driver like a GPIO pin from a microcontroller. Assuming the oscillator's output is relatively slow (it looks like a common clock oscillator speed is 32kHz) and it's driving level isn't super different from a regular GPIO pin, you could just disconnect the oscillator entirely and solder a GPIO pin from something like an Arduino Micro to the pad where the oscillator used to be connected. Then you could drive the GPIO pin with the microcontroller's own oscillator/clock divider and just simulate a crystal. As far as I can tell, the main problem with this approach would be interfacing and making sure that the microcontroller's square wave roughly matches what the old oscillator was putting out

That's all I could really think of. I don't see any humongous, glaring oversights in either line of reasoning, but I thought I would ask you guys if I'm missing an obvious solution or if both of these are totally impossible. I plan on taking apart some analog clocks over the next few days and poking at them to see the characteristics of their oscillators, but I was hoping that somebody here would have a better idea than me of roughly how they work

Thanks

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  • \$\begingroup\$ One thing that might be worth investigating is to see how change of capacitors affects clock operation. Changing them will affect frequency of the crystal. If you're lucky, you might find some combination that starts the oscillator and produces significantly different frequency. Another option, which is much more complex, is to try to filter out fundamental frequency of the crystal and use higher harmonics. Problem with that is that you'd need to make much greater modifications to the circuit. \$\endgroup\$ – AndrejaKo Oct 16 '13 at 10:00
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Usually 32768Hz quartz crystals are used in the clock applications. This standard is very widespread and these crystals are cheap and highly easy to find.

In order to switch the clock to work twice as fast, you will need 65536Hz crystal, that is not standard, not so common and probably not so easy to be purchased. For example my local distributor has 32768Hz crystals on $0.1 each and the next frequency available is 1.8432MHz

Another possible issue is the work of the internal oscillator with the changed crystal. It will probably work, but some capacitors might need to be replaced with other values.

And at the end, the last possible problem is the stepper motor of the clock. Is it capable of stepping twice as fast as usual?

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Looks like 65536 Hz "tuning fork" crystals are easily available. Worth trying that first. There is a good chance that swapping the crystal alone will work fine.

Average current consumption will likely double, of course.

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There are 60 kHz crystals available from Farnell. That's not exactly twice as fast, of course.

Alternatively a small MCU like the MSP430 could generate the drive pulses to the coil directly (assuming you can analyze them on a scope), and let you run the clock at any rate you want.

That would probably mean running it off 3V. Given the impedance of the coil, over-power is unlikely to be a problem but you could reduce the pulse duration to compensate.

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