Need help modifying 555 output

Discussion in 'The Projects Forum' started by duck36, Apr 28, 2013.

  1. duck36

    Thread Starter New Member

    Apr 17, 2013
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    Hi all!

    Preface (feel free to skip):

    I'm a hobbyist that has been doing electrical/wiring projects for many years and I have recently been approached with tasks that cannot be completed without building circuit boards to control the components. So I am diving head first into this with incredible enthusiasm. I have been studying and researching in most of my spare time (sedra/smith book just came in the other day! :D) and can honestly say that I have met a worthy opponent! The vast majority of what I have learned thus far has been from the experts here on AAC. I am grateful for the help that I've already received and hope to be a contributing member here.

    /Preface :cool:

    A lot of my research has been about the NE555, mostly because of it's versatility and convenience. I am now working on a new project, and have come to you for help.

    (All voltages are DC)

    I want to create a voltage activated switch, which seems simple enough. When the input signal reaches a specific voltage (we'll say 5v...not sure of specifics yet) I want it to generate a 12v single pulse to a switch. The tricky part: It can only send the 12v to the switch one time. The input signal will fluctuate between 0v and 12v (not sure if that is actually the max, as it could be lower, but for now that is what we will assume), but I want the switch to only be activated the first time the input signal reaches 5v. The system will need to reset once power is disconnected from input signal.

    Recap:

    -Input signal starts at 0v and begins to increase

    -Once input signal reaches 5v, a 12v single pulse is sent to a switch

    -When input signal falls back to 5v (or increases to 5v during following fluctuations) no signal is sent to the switch

    -Once power is disconnected and then reconnected to input signal the system resets, allowing it to activate the switch again.

    I have looked into different components to do this, such as comparators, zener diodes, and transistors, but I think an NE555 would be the best bet. I apologize for the very long first post and thanks in advance for the help!!
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    A 555 monostable would be able to generate the pulse you want, and a comparator circuit would be able to trigger the 555 at the chosen trigger voltage.
     
  3. duck36

    Thread Starter New Member

    Apr 17, 2013
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    Right. But wouldn't the comparator allow it to send a signal every time it got to 5v? How would I prevent it from sending the signal more than just the first time?
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I suppose there are lots of ways to accomplish that. You could add a small circuit which disables the 555 by using its own output to ground the reset pin. Another issue that you will need to deal with is that the trigger on the 555 is activated with a negative going pulse. What constraints are you working under? Do you have a hard cost limit? Do you have power restrictions? Does it have to be battery powered? Does the circuit have to be solid state or can you use a relay? Constraints will greatly affect the design.
     
  5. duck36

    Thread Starter New Member

    Apr 17, 2013
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    I'm not concerned with cost too much, as I don't see that getting too out of hand.
    It does have to be battery powered.
    It does not have to be solid state.
    I'm not sure what you mean by the trigger on the 555 is activated with a negative going pulse...
    I don't believe power restrictions are of any concern, given that we stay within reason of a typical 12v switch.

    You'll have to be patient with me, I'm still so new!
     
  6. tracecom

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    Apr 16, 2010
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    We're all new in some ways.

    How long does the 12V pulse need to be?

    The 555 will source up to 200mA, so that should be sufficient, but what kind of switch is it supposed to operate?

    What type of battery is it, and is it going to be recharged? (This affects how much current the circuit is allowed to use.)

    Where is the variable voltage coming from? Is it DC? What is its power source?

    Regarding the trigger for the 555, it has to be a negative going pulse. In other words, the trigger pin (2) is activated by a voltage that is going from high to low (usually ground.)

    The more you can tell about the use, the better suited a solution will be.
     
  7. tracecom

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    It might be feasible to use a flip flop latch on the output of the comparator. This could invert the output and ensure that the 555 only gets one pulse no matter what the comparator does.
     
  8. duck36

    Thread Starter New Member

    Apr 17, 2013
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    The pulse should be no longer than 1 second

    The "switch" that I referred to will actually be a lock actuator. (I didn't put the circuit together in my head correctly)

    Its a 12v battery that holds about 60aH I think and it will be recharged

    The variable voltage is coming from a sensor. I'm not entirely sure how the sensor works, but there's a gear inside that spins. While the gear is not spinning, the output is 0v. As the speed of the gear increase the output voltage increases.

    If the 555 requires a negative going pulse then would it even be a valid candidate for my application?
     
  9. #12

    Expert

    Nov 30, 2010
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    I was thinking about an SCR because the OP said the circuit resets after power is removed and then reconnected. Of course, a rather sensitive SCR can be made with 2 transistors and the 555 allows very imaginative ways to arrange this kind of trick.

    Translation:
    There are dozens of ways to do this.
     
  10. tracecom

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    A pulse can be inverted with nothing more than an NPN transistor and a couple of resistors, so that's no problem.

    A one-second (or less) pulse is no problem for the 555.

    The battery sounds like it is more than sufficient.

    Do you need to be able to adjust the voltage at which the 555 triggers, or once set, will it remain unchanged?
     
  11. duck36

    Thread Starter New Member

    Apr 17, 2013
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    Once the correct voltage is determined It will remain unchanged. So if the correct voltage is determined to actually be 5v, then 5v it will remain.

    How does an SCR differ from a 555 in regards to the circuit being reset?
     
  12. #12

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  13. tracecom

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    I'll wait for #12 to discuss the SCR.

    A $1.50 microcontroller with an ADC input, a logic-level MOSFET, and a couple of resistors would do the whole job. Of course, it has to be programmed, and would need a lower supply voltage than 12V.

    So, do you want this to be a learning experience, or do you just want the shortest route to a solution?

    ETA: I see #12 has posted some links while I was typing.
     
  14. #12

    Expert

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    Ha ha. Fooled you:D

    Rather than write a veritable book in this thread, I referred duck36 to Wikipedia.
     
  15. duck36

    Thread Starter New Member

    Apr 17, 2013
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    Thank you #12. I have actually already read both of those wiki articles. I understand the difference (for the most part) between the two, I just don't understand what the difference is, specifically when you disconnect the power source. Wouldn't they react similarly?

    I want it to be a learning experience however, I don't have the proper equipment to program a microcontroller. If it could be done without said microcontroller, that would be the preferred method.

    I say this without really knowing cost and equipment needed to do said programming.
     
  16. #12

    Expert

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    When you disconnect the power source, both the SCR and the 555 timer stop working, but so does every other simple component except maybe a battery.

    I am quite puzzled as to why you are concerned about what happens after the trigger has triggered, the timer has timed, the pulse has pulsed, and the lockout has locked out...and then you disconnect the power. What do you need it to do after it has no power?
     
  17. tracecom

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    Apr 16, 2010
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    OK, then the first step would be to draw a schematic for an NE555 monostable multi-vibrator, also known as a one-shot. C1 and R1 values should be selected for a pulse of one second, or whatever you want. Leave some room around the schematic to add some additional circuitry.

    Oh, and when you have mastered the Sedra-Smith book, you can teach EE design. I have a copy, and it's quite a tome.
     
  18. duck36

    Thread Starter New Member

    Apr 17, 2013
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    I suppose I should clarify what I meant then.

    I'm not concerned with what happens after disconnect. I am concerned with the output attempting to send a signal to the lock actuator every time the sensor crosses the 5v threshold. After the initial signal has been sent (the first time the senors output signal reaches 5v) to the lock actuator, I don't want the lock actuator to continue attempting to actuate. Once the sensor is disconnected from the power source and reconnected, that would allow a "new" initial 5v threshold.

    Hopefully that makes more sense.
     
  19. #12

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    I have these in a bag and I'm shaking it. It isn't quite done yet.
     
  20. tracecom

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    Apr 16, 2010
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    Shake away. :)

    C1 = 10 μF
    R1 = 91k
    T = 1.001 S. (give or take component tolerances)
     
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