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.