How do battery operated wall clocks work and do the hands have to be a certain weight?

Answer 1

To begin with, I refer the reader to the Wikipedia article "Quartz-clock" for the basics.

There are two main types of battery-powered wall clocks. One is silent and the other goes "tick - tick - tick" with a sound you can hear in the dark while you're trying to sleep. The quiet type has a second hand that moves in a smooth and continuous manner. It is considered old-fashioned and has fallen out of style. The newer and more popular type of battery-powered wall clock uses what is called a "stepped movement". The second-hand abruptly moves forward and then stops at a rate of once per second. Since this type is the more popular type, then this is the type I will explain.

For a stepped-movement clock, a small and very simple type of electric motor abruptly turns on and off at a rate of once per second. I have looked with my eyes and the motor turns a fraction of a revolutin at a rate of once per second. My eyes are somewhat blurry, but it seems to me that the motor turns 180 degrees or 1/2 a revolution per second. It is a simple type of "stepper motor". This is the motor that moves the clock's second hand forward once per second. If you look inside one of these clock movements, then you will see a collection of gears that slow down the rate at which the minute hand and the hour hand move forward. Of course, The reader knows that 60 seconds equals 1 minute and that 60 minutes equals 1 hour.

I'm not sure how much the reader wants to know about the little stepper-motor. Perhaps a simplified description will be adequate. A piece of ferro-magnetic metal, (such as iron or some alloy of iron), is shaped in a sort of horse-shoe shape similar to the letter U. On one of the straight legs of this metal "U" there is a coil of wire. The wire is very thin and there are perhaps several hundred or a few thousand windings, (I'm not about to take one apart and count the number of turns). I measure the diameter of this wire to be about 0.003 inches. The electrical resistance of the entire coil is about 85 ohms. On a different clock, I measured the coil resistance to be 150 ohms. To measure resistance, use a DMM, (Digital Multi-Meter). I did not measure the diameter of this second coil wire. It is probably also 0.003 inches.

Incidentally, this is one of the simple electrical repairs that you can do to your clock movement. Sometimes, one of these two very small, (i.e. fragile), wires breaks its connection to the PCB, (Printed Circuit Board). All you need to do is to resolder the connection and the clock will work. You may need to undo one winding of wire to get the length required. If you measure a coil resistance of 2 or 3 Meg-ohms, then you have a broken wire. (1 Meg equals 1 million. An "M" will light up on the right side of the DMM display.) The place to measure the coil resistance is, of course, at the two solder pads on the PCB where the 2 coil wires attach.

In my opinion, the usual cause for a broken coil wire is battery acid leaking onto the connection. For this reason, it is very important that the clock movement be mounted in such a way that the battery is on the bottom and not on the top. Eventually, most if not all batteries grow old and start to leak liquid battery acid. If the battery is mounted on the bottom, then gravity will prevent the liquid acid from flowing uphill and getting into the clock mechanism.

So far, our little stepper motor consists of a "U" shaped piece of metal with a coil of wire around one leg. In the air gap at the open end of the "U" is a small permanent magnet that can be flipped back and forth when a square-wave of voltage is applied to the coil of wire. In an alternating manner, the flow of current in the coil is reversed. A short square-wave pulse takes place at one-second intervals. For one pulse, the current flows forward. For the next pulse, the current flows in reverse. A second later, the current flows forward again. Each time an electric current flows in the coil, an "electrmagnet" is formed which causes a magnetic force field at the air gap which causes the small magnet to flip back and forth. Please note, there is a Wikipedia article called "Electromagnet" that gives a more complete explanation of this.

The small permanent magnet is in the shape of a toroid, (a donut). On one side of this circular magnet is a small plastic gear with teeth on it. This is the gear that drives the second hand and all the other gears of the clock. Part of the original question asks "and do the hands have to be a certain weight?" Well, our little stepper motor only generates a small amount of torque. The clock hands need to be small enough and light enough that our little motor can move them. Since the hour and minute hands move much slower, it is really only the size and weight of the second hand itself that is of primary concern. Some people who have clock problems simply cut the tip off of the second hand and everything works fine. However, before doing this, always first try putting in a new battery. Also, clean off any corroded metal contacts in the battery holder.

One more functional part of the clock needs to be addresed. Where does the one-second timing pulse come from? I'm tired of typing all this in so I'm just going to tell you to read the Wiki article "Quartz Clock" and also "Crystal Oscillator". I hope this explanation was helpful.