That's how all began

"Son, i have to wall clocks, one is nice but it makes funny noise, and the other is ugly but works good. I want you to put the good mechanism to the nice clock and throw away the other".

That is what my mother told me a few hours ago, and that is what i did, but i didn't throw away any the other parts... I used it to feed my curiosity once more. That is how this experiment began.

The good, the bad and the ugly

The original purpose of this experiment was to see how the clock is working. Recently, i had a discussion with a friend of mine. He said that there must be a step motor inside with 60 steps per rotation. Rather impossible for such a low cost mechanism. It was just about time to find it out.

The change (for my mother) was done in a hurry, as more important things were yet to come. Just for the records, the ugly clock had it's mechanism (the good one) fixed with clips, while the beautiful clock had the noisy mechanism fixed with a nut and a drop of hotmelt glue. I removed the good mechanism and used only hotmelt to fix it on the beautiful clock housing. End with this.

And now for the entrails. The mechanism was opened by carefully releasing 3 clips. Everything used the upper housing as the second base, thus when i removed the upper housing, everything was free to be removed. And i removed everything. The "motor" mechanism was then revealed to me.

The housing opened by carefully releasing 3 clips	Everything was free to be removed	This is the "motor" mechanism

So i had the motor in my hand. A coil is placed on one side of a U metal. The edges of this U metal surrounds a round magnet. The first gear is fixed to this magnet:

So now, i only had to find out what kind of signal was powering this coil. Well, actually i was pretty sure by this time that the coil would be powered with alternating pulses, so that the North and South poles would change, causing the magnet to rotate. Standard AC single pole motor. But for the sake of the experiment (and for calming my curiosity) i just had to do it. Here are the results:

Positive and negative rectangular pulses, 16 pulses per second. I was rather surprised by the frequency of the signal. I mean, 2 Hz is what i expected, not 8 Hz! The seconds pointer should not rotate like the normal clocks does, 2 clicks per second. But due to my rush, i did not see the clocks in operation before the surgery. So, either i would have to "imagine" the rotation of the seconds, or i had to re-assemble the entrails and see it in action. You guessed correct, i re-assembled it. Moreover, i soldered 2 wires for the power supply. The following video shows exactly that this clock had a smoother rotation than the normal clocks:

By the time i saw the pointer moving, i remembered that i had seen before similar clocks rotating that way. So, finally, how this Chinese clock works? A permanent magnet rotates inside an alternating magnetic field. The frequency of this alternating field determines the speed of the rotation. The rest is done with mechanical gears. If i was forced to name this motor, i would not call it step-motor. The principle of operation is the same as a simple AC single phase motor, only that it runs with a very low frequency.

What i could not understand, is how this motor rotates always to the right direction. A single phase AC motor is coin-toss whether it will rotate clock wise or counter clock wise. There are some methods to determine the rotation, but this would require a secondary coil, something that does not exist here. So??? How???

NOT a toroidal magnet, although it looks like one

It took me a while to understand what happens. First of all, the magnet. Although it looks like a toroid magnet, it is not. And it could not be a toroid magnet, because such a magnet has no polarization, no North or South pole, only toroidal magnetic lines. But to make sure that my guess is right, i did a very simple test. Holding the magnet, i tried with a metal the magnetic field in some places. I found out that there are 2 places, one across the other, that the magnetic power is stronger. This is the proof that this is not a toroidal magnet. I marked with blue pen these points.

But this does not solve the mystery of the correct rotation. Taking a closer look, right below the U metal of the coil, there is another almost rectangular piece of metal. The magnet goes through the center of this metal. A closer look at this piece, reveals an not-normal corner. This corner is bigger than the other 3. And frankly, this is not accidental. Look at the following video:

I have completely removed the coil. Only this piece of metal is there. I turn the magnet a little bit, and then i let it free. You will notice that no matter where i leave it, one of the 2 points where the poles are (the small blue lines on the gear of the magnet) will always go there where this big corner is! Brilliant! So, what happens is now simple to understand. The magnetic field of the coil has an angle of about 35-40° from this corner. When a pulse arrives, the magnet poles are polarized accordingly. This pulse lasts for a while and then no current goes through the coil at all. When this happens, no magnetic field is acting on the magnet. Thus, the magnet will rotate so that the closest pole (North or South) will be lined up with the bigger corner, because this corner acts more to the magnetic field of the magnet. This will rotate the magnet 35-45 degrees MORE from it's last position. This rotation is the key! When the magnetic field from the coil changes direction, the magnet is already rotated a little bit MORE to the correct direction, thus it is impossible to rotate the other way! Simple, easy and absolutely ingenious!

Do you see the small rectangular piece of metal?	Take a closer look. It has one corner bigger than the others.	No matter where you release the magnet, it's closest pole (marked with blue line) will always line up with the bigger corner	Ingenious!!!

The previous experiment just gave me an idea. I suppose that the crystal that provides the pulses to the clock circuit is the classic 32768 Hz crystal. How can i tell? Well, this crystal is widely used for clock applications because it can be divided by a power of 2 and provide an integer number of frequency, and it is cheap. But, as before, for the sake of the experiment, i had to see it on the oscilloscope.

The probe connected parallel to the crystal	And the result on the monitor...

The fun circuit

The mechanism of the clock, a little bit "changed" to fit my needs

Hmmmmm, 34.2 KHz. I suppose i was wrong. It looks like that not everyone uses the good old 32768 crystal. Anyway, the circuit that this crystal is connected to will do the divisions. The output is driven to the coil. I remove the coil and i put a circuit that i made, a simple transistor amplifier. But i did not drive both the positive and negative pulses. Instead, i had a diode to cut off the negative edges. I drove the output of the transistor to another transistor that performed a simple inverter, and the output of the inverter powered an LED. Voila!

The camera i used is an old Canon and not a video recorder. This video shows the LED to flash slower than 8 Hz, but this is only due to the camera's sampling rate. It flashes much faster. Anyway, this is a nice and easy way to get a precision oscillator out of a... wall clock!

Here is the circuit. LSP1 and LSP2 are the inputs where the clock circuit is connected to the coil. The coil needs to be removed. If not, funny things may happen, or the transistor will be destroyed due to the high voltage of the coil when the current shuts off. The Q2 and R2 can be totally omitted without any problem at all. I just put it there, because the ON-time of the LED was more than the OFF time, and the flash was not visible to the camera. Use only 1 transistor to get the pulses. Use these pulses for any kind of circuit that requires such a frequency. Have fun.

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