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Beating a Pendulum Clock

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(show and tell)
As I approach 80, my hearing is not what it used to be.  I cannot hear the tick-tock of a small wall clock I built from a kit years ago (1975).  With very little searching, I found a BOB based on the MAX9814 that includes an electret microphone.  Quite cheap on eBay or Amazon (2 for $6.38 incl. tax and shipping).  I used the version from Amazon.    It is branded with the Adafruit logo.   The Adafruit site gives a link to its schematic on GitHub.  It easily picked up the beat of the wall clock and a Timex wrist watch, but the signal needed some cleaning up to get the beat right (i.e., the tick and tock periods are equal).  Since the BOB operates from a single supply, the output has a positive offset by about 1.25V.  Peak to peak voltage is only about 700 mV.

In brief, the MAX9814 BOB output was used as input to an LM393 dual comparator with a single supply of 5V.  Vref was set at about 1.5V to 1.7V (measured Vref = 1.72V).  That gave multiple output pulses corresponding to the beat sounds; however the duration of the sound was much shorter than the period of the beats, which was about 1 second.  A CD14538B dual precision monostable mulitvibrator was used in non-retriggerable mode with a pulse width of about 100ms to suppress the additional beats.

An image from my oscilloscope is shown below (attached):
        Yellow = Output from the CD14538
        Blue = Output from the LM393
        Magenta = Output from the MAX9814 BOB

A schematic is also attached.  I have included connections for the unused sections of the dual devices.  Uncoupling capacitors across VDD and VSS were used with each IC, but are not shown.

Next step now that it is getting cold is to do the whole thing, except the BOB, on a small PIC12F1840 or similar.

That's an interesting project, good job! My comment is that it might be worth to try contact microphone, like a piezo plate. It might give much cleaner sound and better ambient noise rejection (in other words, better SNR). Also, if it is a pendulum clock, maybe IR optical sensors are an option, that would be a rock solid solution, but not very universal and possibly requires too much intervention into clock.

Thanks for the interest. 

There are lots of ways ways to do it, including a piece of paper with marks.  It all comes down to equal swings, like we did in analytical chemistry 60 years ago.

Actually, the next step will give a digital output.  I put this project up here because of another thread on "drip detection."  Lots of ways to do that too, but the author of that thread mentioned a data logger, and the little BOB I used might be of interest to him/her.

then  OCXO, MCU, display and it would be device to calibrate mechanical pendulum clocks  quite easily. too bad didnt know any watch maker ..

I come from a family of watchmakers. The person who supplied preschool-me with clockworks to take apart, was my grandmother's uncle, the last master watchmaker in the family. That uncle, who had learned the trade from his father (my grandmother's grandfather), kept a watchmaker's shop on the side, while teaching electronics as a day job. I wanted to learn the trade, but at that time digital watches were considered a pretty neat idea. So I ended up in electronics.

jpanalt, when I read your project description, I was reminded of a tool to adjust wristwatches. A small velvet cushion on top of a cigarbox-sized metal case hid a microphone. When a watch was placed on that cushion, the microphone would pick up the Unruh's (balance wheel) sound and compare it to an internal electronic oscillator. The drift in phase difference between that tick, which for small watches would be a multiple of 1Hz, and the internal clock, would cause the hand of a small Drehspulinstrument (moving coil instrument) to move left or right from its zero center position. The tension on the timing spring would then be adjusted as required.

Thinking about that, this tool might have been the reason why I never had trouble understanding PLLs. ;)


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