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Number Guessing circuit -- I need help in creating this circuit

  1. Oct 31, 2016 #1
    I need help in creating this circuit. It was given as my final year project for my foundation in electrical and electronic engineering by my lecturer. Every component seems to work perfectly fine especially when it is tested in sub circuits. The problem arises when I try to simulate the whole circuit the LEDs which each represent a number from one to sixteen just continuously switch on and off. I have attached the circuit diagram from which I am to recreate the circuit from. The transformer, 230 A.C power source, IC6 , C3, C4, D17 and D18 are not going to be used as a powered bread board will be used as the power source.
    Ps: I have seperated the circuit into three subcircuits which the first one comprises of the 555 and power supply, the second comprises of the 74161 and 7400 and the third comprises of the 74157 and 75154. And all of these circuits work perfectly fine when tested seperately. But when I try to simulate the full circuit the LEDs will not light up as they are supposed to.
    F 0oEh2.jpg upload_2016-10-31_23-8-44.png
  2. jcsd
  3. Oct 31, 2016 #2


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    I am thinking you may have a problem with stability/spurious oscillation because of five factors:-
    1. You are splitting it into three sub assemblies. Grounding then becomes an issue.
    2) The board does not have a ground plane
    3) You have your own separate PSU which might not be good enough
    4) No decoupling capacitors used. When the LEDs turn on there is a big current splash on the +ve rail.
    5) The 555 also applies big splash to the +ve rail.
    Last edited by a moderator: Oct 31, 2016
  4. Oct 31, 2016 #3


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    Welcome to PF.
    The voltage on pin 18 = nG1 of the LED driver 74154 needs to be low. Check that.
    The old 74154 could sink 16mA to turn on a LED. How much current can the 74STD family sink?
    Last edited: Nov 1, 2016
  5. Nov 1, 2016 #4


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    The wipers of S2 and S13 are not connected to anything!

    This causes U4 pin 1 to remain High, always selecting the B-input (pins 3,4,10,11).

    U2 must be always counting and driving the U4 B-inputs.

    For U2 to be counting, U1 must be free-running. Since the left side of your schematic is missing, I can't tell if U2 is intended to free-run.
    Last edited: Nov 1, 2016
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