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Quartz Wristwatch Low Battery Indicator - How it Works

I've known for many years that when a quartz analog wrist watch's battery nears the end of its life many watches will cause the second hand to start making 2 or 3 second "jumps", but circling around the dial once in a minute so the minute and hour hands will still keep proper time. This "jumping" second hand warns the watch's owner to replace the watch battery soon. If the owner doesn't know why that is happening someone in the know will tell him if he asks.

I'm trying to find a technical explanation of what design feature is used to make this work, but so far I haven't found one. Searching with Google seems to only get me only a zillion questions from people who don't know what the skipping second hand on their watch means and are asking what's wrong with their watch.

My guess is that (Say for "Two second jumps") the circuitry in the watch monitors the battery voltage and when it falls to a level when it's time to replace the battery the circuitry stops sending one pulse every second to the stepper motor driving the geared hand system and changes to sending two closely spaced pulses every two seconds. When the stepper motor receives these two pulses it makes the second hand "jump" two seconds and the gear driven minute and hour hands also correctly move their appropriate amounts.

My thoughts are similar for three second "low battery jumps" if that's what the watch manufacturer decided to use.

If my assumption is correct then taking a high speed video of a watch second hand "jumping" two seconds at a time would show that it was actually making two closely spaced one second moves and then pausing two seconds before repeating those moves.

I'm hoping someone in the know can point me to a reference confirming this. I've already tried asking this question on the "alt.horology" newsgroup without success.

Thanks Guys,

Jeff

--
Jeffry Wisnia 
(W1BSV + Brass Rat '57 EE) 
The speed of light is 1.8*10^12 furlongs per fortnight.
Reply to
Jeff Wisnia
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I always assumed it worked that way. It certainly doesn't shift gears in the gear train. It probably uses a little less power to spin the stepper with momentum from the first pulse.

The stepper is just a single magnet, on a shaft, inside a hole in a plate that extends into the core of a coil. I wonder if it's possible to sense when it has turned 180 degrees and time the second pulse exactly so it doesn't lose momentum. Could that be sensed just from the coil current?

Reply to
Tom Del Rosso

I've taken digital clocks apart to see just how they are constructed and th ere is a single magnet in the pivot gear. The coil pulses one direction to align the magnet between the poll pieces, then reverses to turn the magnet 180 degrees. I expect these two pulses are close enough together than the magnet still has momentum from the first half of the turn to continue on s o the reversal always pulls the magnet in the same direction. I don't reca ll the waveform or maybe it was a bit too delicate for me and I couldn't pr ob the coil. I don't know how the magnet is always pulled in the same dire ction from rest. Perhaps there are mechanical aspects that assure the magn et orientation is such that it is always attracted to turn in the same dire ction on the first pulse. I expect

But the upshot of this is those 1 second jumps are continuous, even though they are powered by two pulses. So depending on the timing of the pulses, the two second jump may be noticeable as two jumps or it might look like ju st one.

Rick C.

Reply to
gnuarm.deletethisbit

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there is a single magnet in the pivot gear. The coil pulses one direction to align the magnet between the poll pieces, then reverses to turn the magn et 180 degrees. I expect these two pulses are close enough together than t he magnet still has momentum from the first half of the turn to continue on so the reversal always pulls the magnet in the same direction. I don't re call the waveform or maybe it was a bit too delicate for me and I couldn't prob the coil. I don't know how the magnet is always pulled in the same di rection from rest. Perhaps there are mechanical aspects that assure the ma gnet orientation is such that it is always attracted to turn in the same di rection on the first pulse. I expect

h they are powered by two pulses. So depending on the timing of the pulses , the two second jump may be noticeable as two jumps or it might look like just one.

I watched a video about this and that clock used one pulse and a single 180 degree rotation to move the second hand one second. I might be rememberin g this wrong, but I am pretty sure that is how my clock worked.

Rick C.

Reply to
gnuarm.deletethisbit

The coil current isn't what you'd want to look at - the coil is moving in a magnetic field, and the voltage across the coil would be more informative.

In practice, the stepper works like a mass on a spring - where the "spring" is the interaction between the magnetic field being generated by the coil and the magnetic moment of the magnet on the shaft, and the "mass" is the m oment of inertia of magnet and shaft.

If you time the two closely spaced pulses more or less right, you can make the two pulses narrower than you can normally get away with and use the o vershoot from the first pulse and to get the second pulse to move the rotor over two steps using less power than you'd neen if the two pulses were one second apart.

In general, if you want to drive stepper motors faster than their natural f requency (the one at which rotor vibrates around a specific "step position" you have to be careful how you accelerate the rotor through the critical s tepping rate.

On the motors I was working with the resonant frequency was around 100Hz.

--
Bill Sloman, Sydney
Reply to
bill.sloman

About 20 years ago I made a mirror image clock from a cheap AA-powered wall clock. The mechanism worked like this and wondered how to change direction. I noticed that the core that run the magnet rotor was asymmetric and by switching the asymmetry the other way around, I the direction changed.

--
mikko OH2HVJ
Reply to
Mikko OH2HVJ

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