555 Timer Help?

Discussion in 'General Electronics Chat' started by boltcutter, Dec 5, 2007.

  1. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    I have an existing circuit that includes a monostable 555 to set a delay. I've included that portion on the right side of the attached schematic.

    I wish to modify the circuit such that it will trigger only once before being manually reset. I've included a 555 in bistable mode on the left side of the attached schematic, which is the first way I've come up with to accomplish this.

    I have no clue how to connect them, so I've drawn some gibberish in between and outlined it in red. Any counsel would be greatly appreciated.

    Note, I'm not 100% wedded to the use of a second 555. If some other method is more appropriate, by all means suggest it!
     
  2. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    OK, slightly less wrong now. I'm still not at all sure about this design. It does look like this circuit could have application to hobbyguy48's problem too though?
     
  3. GS3

    Senior Member

    Sep 21, 2007
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    I don't understand what you are trying to do. Can you explain?
     
  4. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    I can try again. The right third of the schematic (everything right of the red area) is part of an existing circuit. It works fine, except it can trigger over and over again. I need it to not trigger again until I've manually reset it, so I've (in theory) added the bistable 555 (on the left) and some stuff (in red) to glue it to the existing circuit. Pin 2 on IC1 will eventually go where Pin 2 on IC2 went in the original circuit.
     
  5. GS3

    Senior Member

    Sep 21, 2007
    408
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    I still don't get it. Where is the original trigger of IC2? if it is in parallel with pin 2 of IC2 then nothing you add is going to do anything. Or do you mean that the buttong which is shown as "trigger" is moved from IC2 to IC1.
    I don't understand this. what triggers IC2?
     
  6. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    It was originally triggered by a pulse coming from an op amp that's part of a much larger circuit. It's now triggered by IC1, which in turn is triggered by "trigger" (which, as you guessed, represents the rest of the original circuit). I hope to pull the 555 (here called IC2) out of that circuit, and add this stuff as a daughterboard.

    But just consider it complete as it is here. Have I connected IC1 to IC2 in a way that remotely makes sense?
     
  7. GS3

    Senior Member

    Sep 21, 2007
    408
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    If I am understanding correctly then all you need is an S-R bistable which can be made with a couple of gates. Off the top of my head I think this should work.
     
  8. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    I think you're right, but I'm not familiar with the symbol (SR flip flop?). Can you turn that into a readily obtainable part, with pins diagrammed? (this was the advantage of a second 555)

    Oh, and I'd also like to preserve LED1, in order to remind the operator that it's necessary to reset the circuit.

    Put differently, I'm trying to implement a long duration debounce on IC2's input. What we've talked about so far involves a human pressing a reset switch to end the debounce period, but an automatic debouncer with a period of, say, 2 seconds, would work as well.
     
  9. GS3

    Senior Member

    Sep 21, 2007
    408
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    If all you want is some debouncing it can be done much more simply. The attached circuit is quite standard for debouncing and you can increase RC if you want longer debouncing. No need for additional ICs.
     
  10. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    I can't thank you enough for your help, but I can quite easily ask for more. Funny how the world works that way sometimes.

    I'm not completely wedded to the idea of an automatic debouncer, and would still find use for either a full diagram of the SR bistable with an indicator LED added, or a cleanup of my naive 555 solution!
     
  11. GS3

    Senior Member

    Sep 21, 2007
    408
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  12. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    Thanks. Do you suppose you could add an indicator LED, or clean up the bistable 555 version? (The 555 does also have the size advantage here, being an 8 pin DIP it's easier to stash in the original enclosure than an 14 pin 4011...)

    Also, the trigger pulse coming from the original circuit already arrives through a .01uF cap. I assume I'd have to remove (short) that in order to implement the debouncer you showed?
     
  13. GS3

    Senior Member

    Sep 21, 2007
    408
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    I am not sure your first 555 would work as intended (SR latch). I'd have to check. The 555 is intended as a timer. It can be configured as an SR latch but I'd have to check.

    If you want an SR latch and do not want to use gates it can be done with a 555 or with discrete transistors or an opamp or a relay or a mechanical switch or... almost anything.

    A mechanical switch which is stable on both positions will do it.
     
  14. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    If you can assume the 555 will work as a latch (it stands to reason it should - it's just timing an infinitely long delay), how would you clean up the bit in the middle of the original circuit?
     
  15. GS3

    Senior Member

    Sep 21, 2007
    408
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    No, I do not think it will work as an SR latch. The "reset" pin does not reset the input RS latch but the output.

    The attached should work as a SR latch. It should also work if you tie pins 2 and 6, leaving the rest the same. Try understanding how they both work and how they differ.

    Sorry I can't give this more time. Work keeps getting in the way. :)
     
  16. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    This is all really great from an academic standpoint, and I appreciate your time. I can wait.

    Whatever latch I end up with I will have to tie it to my 555 and would like to have the LED you can see in my original convoluted diagram preserved. If you can draw, for instance, a 4011 based latch that includes the LED (on while the latch is on), that'd be great. If you can pretend that the original 555 latch works and fix up the middle, that'd be great too. Perhaps greater.
     
  17. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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  18. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    It looks familiar, but if you check out post 2 in this thread you'll find a schematic with a scribbly mess that connects a similar latch to a monostable timer. I'm hoping someone can counsel me in particular on the scribbly mess!
     
  19. GS3

    Senior Member

    Sep 21, 2007
    408
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    That's the same configuration which I say may or may not work. 555 chips are not all the same internally. If the reset does not reset the main SR latch then it may not work. On the other hand, come to think about it I think it should work in most cases. What I posted should always work. (In theory, I have not tried any of this ;) )
     
  20. boltcutter

    Thread Starter Member

    Dec 5, 2007
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    Not wanting to beat a dead horse here, but if anyone can advise me on the middle part of this circuit (the bit outlined in red), that'd be great. The bistable (left) part should deliver a pulse to trigger the monostable part (right), then latch with the LED lit until reset. Then everything should be ready to go again. Do I need all these components? Are they all in the right places?

    [​IMG]
     
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