Question 36.2: What does the circuit of Fig. 36-8 do?

This is one way to create a 60-Hz clock, a square-wave signal used as the basic timing mechanism for inexpensive digital clocks. The transformer steps the line voltage down to 12 Vac. The diode clamps then binds the input to \pm 0.7 V. The inverting comparator produces an output square wave with a frequency of 60 Hz. The output signal is called a clock because its frequency can be used to get seconds, minutes, and hours.

A digital circuit called a frequency divider can divide the 60 Hz by 60 to get a square wave with a period of 1 s. Another divideby- 60 circuit can divide this signal to get a square wave with a period of 1 min. A final divide-by-60 circuit produces a square wave with a period of 1 hr. Using the three square waves (1 s, 1 min, 1 hr) with other digital circuits and seven segment LED indicators, we can display the time of day numerically. The 60-Hz ac power line maintains a frequency precision of better than 0.1% so timing accuracy is very good.