How Digital Clocks Work

Circuit Diagram

Here's a circuit diagram for the power supply and time base.

As we saw in the article on electronic gates, the power supply is the most difficult part!


To create the rest of the clock you will need:

  • At least four 7490 or 74LS90 chips
  • At least two 7447 or 74LS47 binary-to-7-segment converters
  • At least 20 resistors for the LEDs in the 7-segment displays (330 ohms would be fine.)
  • Some normal LEDs
  • At least two common-anode (CA) 7-segment LED displays (Jameco part # 17208 is typical.)
  • Breadboards, wire, etc. (See this page for a complete list.)

The number of chips, resistors and LEDs you need depends on how many digits you are interested in implementing. Here we will discuss only seconds, so the "at least" numbers are correct.