two 555s triggering each other

Discussion in 'The Projects Forum' started by gerases, Nov 14, 2012.

  1. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Hi,

    I'm playing with 555s and here's my current problem. Here's a schematic of a 555 in the monostable mode.

    http://examples.oreilly.com/9780596153755-files/mkel_04/mkel_04_015.pdf

    The idea here is that you press S1, the LED comes on for about 5 seconds and turns off. The S2 is for interrupting that process any time.

    I would like to add to this schematic one just like it (let's call it IC2) except for the S1 and S2. So that when pin 3 of IC1 goes low, IC2's pin 2 detects it and turns on its own LED. When the pin 3 of IC2 goes low, it is somehow detected by IC1 and the process continues indefinitely. In other words, I want them to trigger each other.

    I tried putting a jumper wire from 3 of IC1 to 2 of IC2, but that only lights up the LED of IC2 once and the process stops because there's constant low voltage on pin 3 of IC1, which doesn't let IC2 to turn off.

    Thanks for any suggestions.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  3. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Well, the author in the book talks about chaining 555's and attaching the last one to the first one. This way a "running LED" effect could be created.
     
  4. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    I agree with Dodgydave - you seem to be describing an astable operation which could be done with a single 555 in astable mode. This will send a constant low-high-low signal to your LED causing it to turn on and off repeatedly at a frequency of your choosing.

    Hooking two up as you described will give you better control over the on and off timing though. If, for instance, you want the LED on for 2 seconds and off for 20 seconds, then your method is probably the route to go.

    If you really wish to make what you have work, you'll want to set up the inputs as edge-triggers. http://www.kpsec.freeuk.com/555timer.htm#edgetrigger

    This will only trigger each 555 initially and will only look at changes. What is happening now is the input to IC2 is seeing a constant low signal which will force the output to be high once the timing cycle is complete. Adding an edge trigger to both inputs should correct this.

    You may need to add a power-on trigger and reset (shown just above in the link above) to the ICs so one starts and one is off when power is first applied.
     
  5. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Cool. Thanks for the reply.

    And I would the edge trigger between pin 3 of IC1 and pin 2 of IC2, correct? I.e., by connecting one end of the trigger (V+) to the 3rd pin of IC1, the ground of the trigger to the ground of the circuit and the capacitor to pin 2 of IC2?
     
    Last edited: Nov 14, 2012
  6. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    You originally mentioned hooking up an LED to the output of each timer, presumably you want one on while the other is off at all times and they alternate?

    Bill has a rather simple and elegant solution to this: http://forum.allaboutcircuits.com/blog.php?b=378

    Look at Chapter 4 and Chapter 5. One timer and the two LEDs blink alternately.

    If you look at the link I sent before, make the edge-trigger as shown with two 10kΩ resistors and a 0.1uF capacitor connected to pin 2 for each IC.

    Now connect pin 3 from one IC to the left side of the capacitor connected to pin 2 of the other IC. Repeat for both.
     
  7. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Correct.

    I'll study it later but I want to try to implement your solution first. Thanks for your help -- a lot.
     
  8. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    If you don't mind me asking, how does the edge trigger work? I can't figure it out just by looking at it. I understand that the capacitor blocks DC, but I don't understand:

    1) Where and how the voltage drop occurs.

    2) Does the capacitor ever discharge? Or it doesn't matter and it will still respond to changes?

    3) If I understand your description of how I should connect the trigger correctly, I shouldn't connect the capacitor's left pole to the ground of power supply at all. Why is that?
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Page 168 of Make:Electronics has an error. Here is a quote from the errata sheet.

    Figure 4-29 shows a schematic for a circuit that is supposed to flash four LEDs in sequence. The problem is that the when the 555 flips to negative output, which triggers the next 555, it will sustain the negative signal, causing the next 555 to stay triggered. For the schematic to work properly, it needs to instead send a pulse. To do this, there should be a capacitor in series between the output of one 555 timer, and the input of the next 555 that it is controlling. I learned this trick from experiment 18, and then was finally able to get the figure 4-29 circuit to work properly. Since this pulse trick isn't explained until later in the book, it would also be worth noting this in a footnote under the schematic. Ironically, this circuit is the one shown breadboarded on the front cover, and the missing capacitors are present in that picture.
    Note from the Author or Editor:
    The original circuit worked for me, and for my reviewers, but the behavior of 555s from different manufacturers may vary. Therefore I believe the suggested modification is a good one. This will entail a revised schematic and figure caption.
     
  10. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Wow, what a find! Thanks, tracecom! That's exactly what I'm experiencing... I will try to modify my circuit tonight now that I have the info about experiment 18 :)
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
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    You're welcome. It's interesting that the author is denying his mistake, but accepting the correction.

    Here's the link to the complete errata list; it might save you some future grief.

    http://oreilly.com/catalog/errata.csp?isbn=9780596153748

    The long list doesn't speak well for the book.
     
  12. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Yeah, weird. But I'm glad I've learned something new. Thanks!
     
  13. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
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    My understanding of capacitors is limited at best, so if someone corrects me, I'd listen to them.

    In studying the same site's info on capacitors, here is how I understand it.

    The resistor connected to pin 2 pulls it high so the 555 is normally off when there is no input signal. Both resistors keep the potential (voltage) across capacitor effectively at zero.

    When a low signal goes to the left side of the capacitor, the capacitor discharges quickly which is seen as a low spike to the pin 2. The capacitor recharges rapidly while the signal is low. Once recharged, pin 2 sees a high signal although the input signal may remain low indefinitely.

    Again, this is my very simplistic understanding of it and I can't say I understand to my satisfaction but I hope it helps a little.

    Correct. The capacitor will discharge when connected to ground, which will happen when the output from the other 555 goes low. If you connect the capacitor to the actual ground, then when the output of the other 555 goes low, it will simply ground out and will not cause a change in potential to occur at the capacitor.
     
  14. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    I'll need to think about this quite a bit. I'll be back if I have questions. Regardless though, thanks!!!
     
  15. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    There are other better ways to create running lights. What you are after is a light chaser.

    LEDs, 555s, Flashers, and Light Chasers

    If you are interested I know of some other web sites that do it differently.
     
  16. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Thanks, Bill. I will definitely look into light chasers. My interest in this particular circuit is strictly academic (zero practical value). I just want to do it this way for kicks :)
     
  17. gerases

    Thread Starter Member

    Oct 29, 2012
    177
    2
    Just wanted to confirm that a 0.1 nF capacitor did solve the problem quite nicely and I had alternately flashing LEDs last night! Cool.

    One nagging question though. In the astable mode, when the power is just turned on, why does a 555 turn on? Pin 3 is connected to pin 6, I do get it, but there seems to be 0V on pin 6 at the beginning, no? I'm looking at this abstraction of the internals of a 555 and I don't see where the low voltage for pin 2 is generated -- since both the poles of the capacitor on pin 6 are connected to the ground initially.

    http://examples.oreilly.com/9780596153755-files/mkel_04/mkel_04_023.pdf
     
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