Need some help with a LED chaser board

Discussion in 'The Projects Forum' started by Otaku, Apr 24, 2009.

  1. Otaku

    Thread Starter Active Member

    Nov 19, 2008
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    Hi all,
    I have one of these LED chaser boards:

    http://www.allelectronics.com/make-a-store/item/AEC/LED-CHASER-KIT/-/1.html

    and I find that it's behavior is inconsistent. When I power it up, it will usually light LED2 (it never starts on LED1), and occasionally two or more LEDs will light. After a couple of cycles it will fall into a 1 - 10 sequence, but I need it to do this right from the start. There's not a lot of documentation on the circuit aside from bare-bones assembly instructions.

    I did make some modifications: the 4.7uF cap on the 555 timer was replaced with 100uF to slow down the cycling (positive leg attaches to Pin 2 of the 555, the negative to Pin 15 of the 4017 and is also grounded). I also have attached a 100uF cap and 1K resistor to each LED to allow a slow fade when the LED is powered down. A 1N4148 diode is attached to the positive lead of the LED to prevent the cap discharging to the positive bus. Has anyone seen this kind of behavior in this type of circuit? If so, I'd like to know what I could do to fix it. Thanks in advance for all advice.

    Gary
     
    Last edited: Apr 24, 2009
  2. Audioguru

    New Member

    Dec 20, 2007
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    The 4017 starts at a random count when power is applied unless it is reset.
    Pin 15 is the reset pin. Connect a 0.1uF capacitor from pin 15 to the positive supply. Connect a 100k to 1M resistor from pin 15 to ground. Then when power is applied, the capacitor is discharged and makes pin 15 high which resets the 4017 until the capacitor charges then pin 15 goes low and the 4017 begins at count zero.
     
  3. Otaku

    Thread Starter Active Member

    Nov 19, 2008
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    Thanks, Audioguru, that's very clear. Pin 15 of the 4017 is currently connected to pin 2 of the 555 through a 100uF cap and also to ground. Do you think that attaching that 0.1uF cap to pin 15 and the positive supply will cause any problems?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, that will cause problems.

    You will need to cut the trace between pin 2 and pin 15 before you connect the capacitor and resistor as Audioguru instructed. A careful application of a sharp knife like an X-Acto hobby knife will do the trick.

    Note that the 0.1uF capacitor, if polarized, should have it's negative terminal connected to pin 15.
     
  5. Otaku

    Thread Starter Active Member

    Nov 19, 2008
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    Sounds good, SgtWookie. Thanks to all for the help!
     
  6. Otaku

    Thread Starter Active Member

    Nov 19, 2008
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    One more question: should I jumper pin 2 of the 555 to the negative bus in front of the cut location? The trace from pin 2 (555) to pin 15 (4017) grounds both pin 1 and pin 2 of the 555. If I don't place that jumper, I think the oscillator won't run.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Usually, in astable mode pins 2 and 6 of the 555 are connected together. Pin 1 of the 555 must be connected to ground (or what you're calling the negative buss).

    Pin 1 of the 555 must be grounded. Are you certain that the 555 pin 2 wasn't connected to pin 6 and the timing capacitor instead?
     
  8. Wendy

    Moderator

    Mar 24, 2008
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  9. SgtWookie

    Expert

    Jul 17, 2007
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    Bill, that's a really great resource when people are starting from scratch; but in this case our OP is just trying to make a small "tweak" to an existing circuit.

    I'm afraid that all the information you've included in that thread may confuse them. :confused:

    At this point, probably the most helpful thing would be to have either an image of the circuit board traces, or at least a schematic of how it's currently wired up.
     
  10. Otaku

    Thread Starter Active Member

    Nov 19, 2008
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    I should explain -I've been building and using 555 timers for a few years. This is the my first exposure to a 4017 decade counter and I'm still learning how it can be interfaced with the 555. Also, the config of this oscillator is a bit different than others that I've built. I probably shouldn't have asked that last set of questions, the answers really are obvious when I look at the board. I really appreciate the advice - I hadn't found an article on the 4017 that addressed the reset issue and what happens if you don't do it. I'll let you know how it turns out.
     
    Last edited: Apr 25, 2009
  11. SgtWookie

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    Jul 17, 2007
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    Ahh, ok. :)

    Do check out Bill's link after you get your circuit working. Lots of nice ideas in there.

    Just FYI: Most LED sequencers using the 4017 (and other 4000 series ICs) are usually sourcing/sinking much more current than the IC was designed for. A practical limit is around 4mA, although they are regularly abused to get 15mA or even more out of them. Just don't expect 4000 series ICs abused in such a manner to either last very long, or especially not to also drive other logic at the same time as sourcing/sinking current from LEDs.
     
  12. Otaku

    Thread Starter Active Member

    Nov 19, 2008
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    Thanks for the FYI. Would it be more appropriate to attach NPN's or MOSFETs to the 4017 outputs to switch power for the LEDs?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Yes.
    You can also use Darlington driver IC's like ULN2003 (seven Darlingtons), ULN2803 (eight Darlingtons) to take care of most of the outputs. These drivers can only sink current though; they can't source it. However, they have built-in current limiting resistors for the input transistors' bases, built-in diodes for when driving inductive loads, and can sink up to 500mA per channel (350mA max continuous recommended). They'll save you space and component counts.
     
  14. Otaku

    Thread Starter Active Member

    Nov 19, 2008
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    Most cool - I think I see where I need to go with this hack. Thanks again!
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Ok. Don't forget that the ULN2x03's will have a Vce of around 1.1v when sinking 350mA. Check out a datasheet.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    This thread is completely usless without seeing the schematic of the very simple chaser kit.
    1) Pin 2 of the 555 is not its output. Pin 3 is its output and sometimes pin 7 is its output.
    2) Pin 15 of a CD4017 is not its input. Pin 15 resets the counter when it is high and the counter runs normally when it is low. The input (the clock) is pin 14 but sometimes the Clock Enable pin 13 is used.

    Texas Instruments has very detailed datasheets for their CD4xxx Cmos ICs. They spec a max dissipation for each output transistor of 100mW which is plenty to drive LEDs.
    They also have graphs of minimum and typical output currents vs supply voltage and voltage drop. For a CD4017 only one output is on at a time so it will not get hot.

    With a 9V supply and 2V red LEDs the output transistors dissipate typically 112mW which is too high without a current-limiting resistor. The current is typically 16mA. The minimum current is 7.5mA.

    With a 9V supply and 3.3V blue LEDs the output transistors dissipate typically 85mW and are fine without a current-limiting resistor. The typical current is 15mA or a minimum of 7mA.
     
  17. Audioguru

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    Dec 20, 2007
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    It looks like pin15 of the CD4017 is part of the 0V trace for many things so it is difficult to cut it because then the 0V trace will need to be spliced.
    I think the kit is connected with output pin 3 of the 555 feeding the clock input pin14 of the CD4017 like this:
     
  18. Otaku

    Thread Starter Active Member

    Nov 19, 2008
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    Audioguru,
    Yes, I understand that pin 2 is not the 555 output. Your enhanced attachment maps the circuit accurately. I'll be testing your 4017 reset hack today, and I'll be sure to jumper pins 1 and 2 of the 555 to ground.
    As long as we're on the subject, I was planning to have this board perform one 0-9 cycle, then stop. I can do this with a separate 555 monostable (supplying power via a relay to the chaser board) and adjust the time to coincide with the 4017's cycle, but the more elegant solution would be to automate the shutoff. Could a NPN be attached to output 9 of the 4017 and be used to ground pin 4 (reset) of the 555 monostable, which would shut off the timer? Or is there a way to have the 4017 perform one 0-9 cycle and then stop until triggered? I haven't checked the 4017 datasheet yet...
    Thanks again for all the help. Much appreciated!
     
    Last edited: Apr 27, 2009
  19. Otaku

    Thread Starter Active Member

    Nov 19, 2008
    128
    2
    Update: I connected the 0.1uF cap and a resistor as advised, but the behavior is unchanged. The circuit still starts on a random LED, sometimes multiple LEDs. I bridged the pin 2 (555) trace to ground upstream of the cut. I tried both a 100K and a 1M resistor on pin 15 of the 4017, but it made no difference. Could the correct value be somewhere between those two?

    Edit: Found the problem. Pin 15 was connected to ground in two places. One via the trace from pin 2 of the 555 and the other directly to the larger ground trace going to the negative power terminal. I cut the second trace and all's well - the sequence starts at output 0 every time. Again, my thanks for the help. Now to get to work on the auto-shutoff!
     
    Last edited: Apr 27, 2009
  20. Audioguru

    New Member

    Dec 20, 2007
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    A 555 will not do anything if its pin2 is connected to ground. Pin2 normally connects to pin6 and the timing capacitor.

    If you want the CD4017 to stop after 9 outputs then connect the 10th output to a transistor inverter that resets the 555's pin4.
     
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