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.