Isolated 1-Hz Clock

One of the author’s
physics projects required an accurate 1-Hz (seconds) clock signal.
Unfortunately, precision 10-MHz quartz crystals are expensive, while
another problem was found in the inability of most common or garden 40xx
CMOS logic chips to work at such a high frequency. However, a typical CMOS
counter like the 4017 has such a high input resistance that its clock
input has ‘radio’ properties. The effect is exploited here to convert
the stray magnetic field picked up from a mains transformer into a clock
signal. Here, the signal is induced in a short piece of wire (approx. 5
cm) connected to the clock input of a CD4017 decade counter for
division by 10.

The resulting 5-Hz signal is then divided by 5 by a second 4017 (IC2) to give an output of 1 Hz. LED
D1 flashes to indicate the presence of a sufficiently strong magnetic
field. The pickup wire should be placed close to the mains transformer,
without compromising electrical safety. Always use the greatest distance
at which a clock signal is reliably generated. For 1-Hz output from
60-Hz power systems, use output 6 of IC2 (pin 5).

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