Quartz Crystal Synchronizer for Pendulum Windup Wall Clock





Mechanical Clock Project:

Last year, I found a 31 day pendulum wall clock (dissambled) in a box of
parts at a swap meet and decided to try and put it together and regulate
it with a quartz crystal oscillator. The escapement part that rocks back
and forth and drives the pendulum was missing and had to be made from
a couple razor blade pieces and heavy copper wire. The razor blade escapement
worked well but only allowed the movement to advance as the pendulum swung,
and would not sustain the pendulum motion by itself. But this wasn't a
problem since the quartz crystal divider circuit provides energy to the
pendulum with an electromagnet to keep it swinging with only a 5 second
error per day. The 31 day mainspring was included, but I wanted to use a
weight and drive wheel in place of the spring.


The clock movement is made in Korea by "Sau Jin LTD" "Dae Woo CO LTD"
and has no jewels. It measures 4.5 inches diameter by 1.5 inches
deep plus 1.5 inches for the hands and drive wheel shafts, so the clock
face and hands are about 3 inches from the wall. The pendulum period
is close to 53.4 complete swings per minute. Can't figure out why that
particular period was used.  I have seen similar movements on ebay.

The clock pendulum was made using a 10 inch, 3/16" wooden dowel with a
strong magnet attached to the bottom and a small weight near the top
to adjust the period close to 53 beats per minute. The circuit board
and electromagnet to drive the pendulum are located on a small shelf
(not shown) and positioned so the pendulum magnet swings close to the
stationary electromagnet and receives a small pulse on each swing to
sustain oscillation. The pulse duration is about 5% of the pendulum
period and a LED is used to indicate the pulse output. The clock starts
fairly easily by releasing the pendulum near the magnet when the
LED flash is observed.

Quartz Crystal Synchronizing Circuit:

The synchronizing circuit that produces a short magnetic pulse to keep
the pendulum in near perfect time was made using a crystal oscillator
and binary counters to generate a 60mS pulse at the required rate of
53.4241 PPM. I didn't know the exact rate, but it appeared to be close
to 53.5 cycles per minute by just adjusting the pendulum length and
monitoring the error over a several hour period. The oscillator circuit
uses an old 20KHz quartz crystal, but other low frequency crystals can
be used. A standard watch crystal of 32.768 KHz is probably the best
and easy to obtain. The idea is allow the counters to count to the
desired number and then reset the counters, generate the desired pulse,
and repeat the cycle. In this case, I needed a time of 60/53.4241,
or 1.1231 seconds. At a frequency of 20KHz, this is about 20000 * 1.1231
or 22462. To detect this number, a multiple input NAND gate (CD4068)
is used with inputs connected to the appropriate counter stages. Each
counter stage has a value of twice the one before (divide by 2), so
the values are 1,2,4,8,16,32 and so forth. So the problem was to select
the counter stages that add up to 22462, which works out to:

Stage 3 (Q3, pin 6) of the second counter with a value of 16384
Stage 1 (Q1, pin 9) of the second counter with a value of 4096
Stage 11 (Q11, pin 15) of the first counter with a value of 1024
Stage 10 (Q10, pin 14) of the first counter with a value of 512
Stage 9 (Q9, pin 12) of the first counter with a value of  256
Stage 8 (Q8, pin 13) of the first counter with a value of 128
Stage 7 (Q7, pin 4) of the first counter with a value of  64
Total = 22464

So, the error is about 2 counts out of 20,000 or maybe 4 seconds
per day. Further adjustment can be made by fine tuning the oscillator
frequency by adjusting the value of the 1000pF cap at pin 1 of the
inverter.