Puzzle
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Ponder This Challenge - June 2010 - A bug in a Boolean circuit

Ponder This Challenge:

The following circuit computes a known Boolean function; but it contains a single gate error.

View the circuit image

What is the function? Which gate is wrong, and what should it be? Pay attention to the inverters (the small hollow circles on the right end of some of the gates)

Update: June 3rd: The bundle of wires on the right side is not a mistake - this circuit has several Boolean outputs.

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Solution

  • The circuit computes the 4-to-7 function of seven-segments display.
    The picture below, sent to us by one of the solvers, shows a clock built with seven-segments output. The clock has been functioning continuously since 1975.

    Gate #20 is wrong. Changing it to NOR provides a solution that is valid for only the decimal digits. If we change it to NXOR we get a valid seven-segments output for all hexadecimal digits.

Solvers

  • Michael Brand (06/01/2010 10:19 PM EDT)
  • Ian Barnard (06/03/2010 04:56 AM EDT)
  • Matthew Paterson (06/03/2010 09:07 AM EDT)
  • Dmitry Litvinenko (06/04/2010 01:09 AM EDT)
  • Ben Holland (06/04/2010 03:54 PM EDT)
  • Ross Millikan (06/04/2010 05:52 PM EDT)
  • Phil Muhm (06/06/2010 11:30 AM EDT)
  • Guler Amos (06/06/2010 12:39 PM EDT)
  • Patrick Fussell (06/06/2010 05:47 PM EDT)
  • Andreas Stiller (06/06/2010 11:05 PM EDT)
  • Daniel Bitin (06/07/2010 09:55 AM EDT)
  • Thomas Rohr (06/08/2010 08:31 AM EDT)
  • Hubert Kahlert (06/08/2010 10:39 AM EDT)
  • John T. Robinson (06/09/2010 02:39 AM EDT)
  • Suman Saraf (06/09/2010 05:02 PM EDT)
  • Sharad Agrawal (06/10/2010 03:55 AM EDT)
  • Beni Bachmann (06/10/2010 06:40 AM EDT)
  • Matthias Graefenhan (06/10/2010 11:07 AM EDT)
  • Mark Mixer (06/10/2010 08:28 PM EDT)
  • William Heller (06/10/2010 10:27 PM EDT)
  • Sai Krishna Komaravelli (06/13/2010 05:27 AM EDT)
  • Kai Guttmann (06/14/2010 07:30 PM EDT)
  • Don Ryan (06/15/2010 01:51 AM EDT)
  • AthenaHealth Problem Solving Group (06/15/2010 02:21 PM EDT)
  • Rani Hod (06/16/2010 10:58 AM EDT)
  • Maheedhar Jalasutram (06/17/2010 02:27 PM EDT)
  • Fabio Filatrella & Luca Bottoglia (06/18/2010 12:30 PM EDT)
  • S. Gunasekaran (06/19/2010 06:12 AM EDT)
  • Raúl Mora Pastor (06/19/2010 11:56 AM EDT)
  • John Duffield (06/22/2010 06:03 AM EDT)
  • Chuck Carroll (06/22/2010 01:30 PM EDT)
  • Rajendran Thirupugalsamy (06/23/2010 03:19 AM EDT)
  • Dmitry Teytelman & Leo Broukhis (06/23/2010 02:14 PM EDT)
  • Srivani Narra (06/24/2010 01:06 PM EDT)
  • Albert Stadler (06/25/2010 01:25 PM EDT)
  • Motty Porat (06/25/2010 01:40 PM EDT)
  • Joseph DeVincentis (06/28/2010 11:51 AM EDT)
  • Michael D Moffitt (06/30/2010 04:40 PM EDT)

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