Seconds section of a digital clock

The seconds section of a digital clock counts 1 Hz pulses from 00 to 59 and sends one pulse per minute to the minutes section. It is a mod-60 counter made of a mod-10 (units) counter followed by a mod-6 (tens) counter, with decoders and displays.

Block diagram

 1 Hz pulses (from ÷50 divider of 50 Hz mains)
      |
      v
 +-----------+  Q3 (falls 9->0)  +-----------+
 | MOD-10    |------------------>| MOD-6     |--> 1 pulse
 | units     |                   | tens      |   per minute
 | (7490)    |                   | (reset at |   to minutes
 +-----------+                   |  0110)    |
   | | | |  BCD                  +-----------+
 +-----------+                     | | |
 | 7447      |                   +-----------+
 | decoder   |                   | 7447      |
 +-----------+                   +-----------+
   |a..g                           |a..g
 [7-seg units]                   [7-seg tens]

Working principle

1. 1 Hz input: the 50 Hz mains is shaped into pulses and divided by 50 (÷10 then ÷5) to give 1 pulse per second.
2. Units counter (mod-10): a decade counter such as 7490 counts 0, 1, …, 9 on successive seconds. On the 10th pulse it returns to 0; at this moment its MSB $Q_3$ goes from 1 to 0.
3. Tens counter (mod-6): the falling edge of the units $Q_3$ clocks the tens counter, so it advances once every 10 seconds: 0, 1, …, 5. It is a 3-bit ripple counter reset at 6 (0110) by a NAND gate of $Q_2$ and $Q_1$ (or a 7492 wired as ÷6).
4. Carry to minutes: when the display shows 59 and the next pulse arrives, units go 9 → 0 and tens go 5 → 6 → 0. The falling of the tens counter's $Q_2$ at this reset gives one pulse per minute, which clocks the minutes units counter.
5. Decoding and display: each counter's BCD outputs drive a BCD-to-7-segment decoder/driver (7447 for common-anode displays). The decoder lights segments a–g to show the digit, so the seconds read 00, 01, …, 59, 00.

Count sequence

The minutes section is built exactly the same way; only the hours section uses different (mod-12 or mod-24) logic.