7400 competition entry: Discrete logic POV clock

in 7400 contest, POV Toy by Ian | 4 comments

Matt’s entry in the Open 7400 Logic competition: Discrete logic POV clock.

This persistence-of-vision clock was built entirely out of discrete logic chips. The design consists of three boards, a LED array and a drive motor. The digit board keeps the time while the display board controls the LEDs.

The digit board

Every second, a pulse increments the value stored in a counter.  When it overflows, the next counter is incremented.  Each counter represents a digit in the time.  A decoder selects one digit at a time and puts that digit’s value on the output bus.

The display board

A pulse from an external source wakes up the display board.  It selects the leftmost digit from the digit board, formats it for a 7-segment-display, and chops up the result into a 5 x 5 pixel matrix. It lights up LEDs for each column in sequence then selects the next digit to the right.

Via the contact form

This entry was posted in 7400 contest, POV Toy and tagged , , .


  1. Arthur Benemann says:

    Cool project man, well done!

  2. Matt Swann says:

    Thanks Arthur! It was a great learning experience.

  3. Shashank Patil says:

    i hope doing it …………in order to respect those IC which gave birth to controllers

  4. Jose Rodriguez says:


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