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4017 Decade counter

  1. May 16, 2009 #1
    hey, i've been looking at ways to create a LED chaser using a 555 timer chip and 2 x 4017 decade counter chips(more than 10 LEDs). mind you i've only been working with these chips for an hour. so here's the dilemma, i'm not quite sure if i'm right on connecting the 2 chips. what i did was connect the carry output pin of the first chip to the clock pin of the second chip. then i connected one of the last output pins of the second chip to both reset pins of the decade counters. i have provided a schematic below. is this correct or am i missing something?

    Schematic: http://i20.photobucket.com/albums/b219/twiztedazn/SCHEMATIC1_PAGE1.jpg" [Broken]

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 16, 2009 #2
    it's not a good idea to connect the output of these chips like that, however, you can isolate them with 20 switching diodes (10 for each chip). A diode will conduct current in one direction, so if one pin on, current will be blocked by the diode in the other pin, and vice versa. That way the current will guided only to the LEDs.

    You can employ the same trick using one chip, but with less LEDs

  4. May 16, 2009 #3


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    Your circuit should never have outputs connected together like that. This applies for almost any digital chip. Never connect outputs together.

    With two 4017s you can turn on 20 LEDs but two at a time (One LED must be on for each 4017). The outputs just go to the LEDs and then through a common resistor to ground.
    You do need a series resistor to limit the current, but since they are only coming on two at a time, it can be a common resistor.
    Normally, you would have a resistor for each LED.

    You do not need series diodes (in series with the LEDs) . That Knight Rider circuit had them because they were combining outputs to get special effects with the lights going backwards and forwards.
    Last edited: May 17, 2009
  5. May 17, 2009 #4
    I think that's what he is trying to do.
  6. May 17, 2009 #5


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    Yes, I'm not sure.

    It seemed like if he had two 4017s he wanted to have 20 lights coming on in sequence.

    However, in the simple circuit, it was necessary to have one light of each 4017 on at a time and this could be achieved with one 4017 anyway.

    The Knight Rider circuit is better, anyway.
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