555 and 4017 timer circuit. One or two 555's?

Discussion in 'General Electronics Chat' started by Len Whistler, Dec 10, 2010.

  1. Len Whistler

    Thread Starter Member

    Dec 10, 2010
    44
    3
    Do I need a second 555 timer? I'm building a 555 / 4017 timer circuit to:


    • Flash LED #1 3 times in 3 seconds.
    • Pause for 30 seconds.
    • Flash LED #1 3 times in 3 seconds.
    • Pause for 30 seconds.
    • Flash LED #1 3 times in 3 seconds.
    • Pause for 30 seconds.
    • ......
    • .......
    • .......

    The flashing and pause times will be controlled with a potentiometer to adjust the times. So far I have a simple 555 / 4017 circuit flashing the LED 3 times, but I can't figure out how to pause it for 30 seconds using just one 555 timer.

    I assume I have to hook up a second 555 timer to pause the first 555 timer for 30 seconds, is this correct? I hope my question is clear.
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    Use a single 555 with a timin cycle of 3sec on (with blinking LED) and 30 sec off.
     
    Last edited: Dec 10, 2010
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    LEDs, 555s, Flashers, and Light Chasers

    Pay special attention to chapter 11

    Making Patterns

    Yes, you can do this with a second 555. However, when a 555 flasher starts the duration of the 1st flash is significantly longer. There are some good ways around this if it is a problem.

    What is your power supply voltage?

    If you are flashing only one LED why are you bothering with a 4017? This can be done with only 2 555's, I've done something similar for police lights.

    When I have a little more sit down time I'll sketch a simple schematic how I would do it.
     
    Last edited: Dec 10, 2010
  4. Len Whistler

    Thread Starter Member

    Dec 10, 2010
    44
    3
    Thanks for the posts. Power supply is four 1.2 volt AA Nmh 2500mAh batteries (5.5 fully charged)

    I figured the 4017 could be left out but I decided to use it to give me more error room in adjusting the pulse of it's controlling 555. I have the LED hooked up to pins 3, 2 and 4 on the 4017, and any additional pulses just go to the unused pins 7, 10, 1 etc. 3 flashes in 3 seconds is very important as I'm hooking this up to my camera. 2 flashes or 4 flashes will put everything else out of sync. Using pins 3,2 and 4 for output with the 4017 appears to be an easy hack to get those 3 flashes without any fine tuning of the 555 timer. Perhaps my thinking is wrong on this.


    Re: "However, when a 555 flasher starts the duration of the 1st flash is significantly longer. There are some good ways around this if it is a problem."

    I'm not sure if this would be a problem until I test the results with my camera. I did even notice the 1st flashes being longer so there is a good chance that this will not be a problem.

    It will take me a few days to go through your tutorials. Thanks again for all the help
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
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  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You didn't mention the color of the LED (it matters) so I assumed red with approximately 20ma. For a white LED at 20ma R9 would need to be 36Ω or 39Ω.

    [​IMG]

    R2 calculated to 23KΩ and R6 calculated out to 72KΩ. I suspect the values aren't that critical though. Q2, R7, and R8 will mostly eliminate the 1st pulse delay I was talking about, but you may be able to eliminate the parts entirely.

    The phantom capacitors may or may not be necessary. If the flasher is erratic add 0.1µF for them, and add a 220µF bypass cap. Odds are the 0.1µF will not be needed, but I would use a 220µF cap on the power supply even though it is not shown. It may sound odd, but it will also extend battery life a lot.

    I designed this for very low power, however relatively speaking the 555's are current hogs. You can extend the battery life by using CMOS 555s such as Radio Shack TLC555s. If you do this you will need to add a drive circuit for the LED though using another transistor and resistor. If you want to go this route I will be glad to modify the schematic.

    Q1 and Q2 can be any low power switching transistor. Radio Shack sells packs of them very reasonably, or you can use 2N2222A. Again, it is not critical.

    I have never build this specific circuit, but I am extremely confident it will work to spec.
     
    Last edited: Dec 11, 2010
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