MadSci Network: Engineering
Query:

Re: how does kinetic powered watches work?

Date: Sat Oct 21 18:43:10 2000
Posted By: Jim Stana, Mechanical Design/Analysis Manager, Lockheed Martin Orlando
Area of science: Engineering
ID: 971848067.Eg
Message:

You pretty well have the right idea, just need a little help in 
understanding the principles.

Electronic watches today all run pretty much on crystals that oscillate 
(vibrate) at high frequencies by being excited by very small electrical 
currents.  The electronics in the watch count the oscillations and use it 
to keep track of the time.

(We sort of do this already when we use a calendar and count the rotations 
of the earth.  Each rotation of the earth counts as one day.  Imagine if 
the moon spun around the earth once every second.  If we were fast and had 
a good memory like a computer, we could keep track of the time by counting 
how many times the moon went by.  Every 60th time would be a minute, etc.  
That's how the clock in your computer works, by the way.)

The beauty of the electronic watches is that they don't need much energy at 
all, so the batteries last for a long time.  I have had them last 2 or more 
years, especially if you don't use the alarm or light.

So, if you don't want to replace the battery ever, you could use the motion 
of your hand to generate electricity and charge up either a capacitor or 
rechargable battery to power your watch indefinitely.

The imbalance wheel generates electricity on the same principle used to 
make electricity in your car.  A shaft with a magnet on it rotates past a 
wire coil.  Any time a magnetic field moves past a wire, it induces a 
current in the wire.  Normally, if you just randomly move a generator shaft 
back and forth, the electricity you generate flows back and forth and you 
get a net effect of nothing.  But if you keep the shaft from running 
backwards, and put an offcenter weight on it, every time you move it around 
the weight will want to fall to the lowest point and move the shaft in the 
allowed direction.  A sudden burst of electricity in the nearby coil will 
charge the battery.  

(Imagine a bicycle resting upside down on it's seat and handlebars, and one 
of those headlight generators mounted to rub on the wheel.  Now put a heavy 
weight at one point of the wheel within the spokes.  If you were to move 
the bike by flipping it over onto its wheels, then laying it on its side, 
then on its back again, etc, the weight would turn the wheel at times, 
sometimes not.  But everytime it did, the generator would put out a short 
burst of electricity.  Now, imagine the bike miniaturized and strapped to 
your wrist.)

Actually, I suspect this is done more electronically by allowing the shaft 
to turn in either direction and using diodes (sort of one way valves for 
electricity) to allow the current to be generated in only the desired 
direction.


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