74LS90 Decade counter

Discussion in 'Homework Help' started by Random3s, Mar 3, 2015.

  1. Random3s

    Thread Starter Member

    Jul 30, 2014
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    Hi all. As part of a project I am building a 74LS90 divide by 10 counter, with a clock input from a 555 timer at 1Hz.

    I have built the circuit just like the circuit diagram in the link below. The Qa input is supposed to toggle each clock cycle, but if I use a logic probe to check this it remains low; same results if I use a led. I've reproduced this on 2 differing IC's.

    http://www.electronics-tutorials.ws/counter/cou29.gif

    I can post a diagram/photo, however my circuit is wired just like the one in the link above, I ve taken care to ensure proper grounding for the reset pins (R1R2S1S2), with clkB connected outputs Qb Qc Qd count upwards as expected, but still nothing from Qa..

    Can anyone assist?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    1. Download the data sheet for the 74LS90, and check the (actual) operation of the device against the tutorial schematic. The tutorial could have it wired wrong. Lots of circuits on the Internet are crap.


    2. Verify that your clock source is really generating a clean clock pulse that meets the TTL input specs, especially Vil and Vih (see the data sheet).
     
  3. Random3s

    Thread Starter Member

    Jul 30, 2014
    38
    0
    I connected a DMM to check the voltage of the 555 timer, when I connected the leads between pin 3 (555 output) and ground the circuit works (Qa output led cycles between high and low). I've checked all ground connections again, but I still cannot find a reason why it behaves like this?
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What is your Vcc voltage?
     
  5. Random3s

    Thread Starter Member

    Jul 30, 2014
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    Its at 5V, through a linear voltage regulator
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    555s barely operate off 5V... Maybe the low out of the 555 is not low enough? Slow it way down by changing the timing capacitor, and measure the low period of the clock with your DMM.
     
  7. Random3s

    Thread Starter Member

    Jul 30, 2014
    38
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    I am already using the largest cap I've got available here (out of a choice of 4). Strangely the circuit started to work when I (re)grounded the 555 timer away from the 7490... on a bread board (with 2 +/- bus bars either side), one side is the regulator, and the ground from the reg is fed over to the - bus on the other side of the bread board, making it a common ground; but when I ground the 555 and the 7490 on the same - bus bar, the circuit won't work. Why is this an issue, any ideas?
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,420
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    Put a 10μF capacitor across the power rails at the 555 timer.

    I would also recommend switching to a CMOS 555 such as LMC555 or TLC555.
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Post your 555 schematic ... and photo of your build. The same with a photo of your mod 10 circuit.
     
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