Need help building a NCTC timed/delayed relay

Discussion in 'The Projects Forum' started by shainiac, Mar 26, 2013.

  1. shainiac

    Thread Starter New Member

    Mar 26, 2013
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    I'm a mechanical engr student and a bit out of my element, so please bare with me. For a project, I am trying to build a circuit that will allow a NC relay to stay open for an amount of time if a circuit is briefly completed in a relay via a momentary switch.
    Basically, I want to interfere the NC relay by quickly pressing a button.
    It can operate off of either 12V or 5V DC, there will be power supplies for both in the circuit.
    Current through the relay will be less than 0.5A, but could be used to operate a high current relay if it's an issue.
    I would like to have adjustment of the time delay.
    I think this is referred to as a NCTC relay, but searching didn't bring me any results.
    Can anyone help me with a parts list/schematic?
    Attached is a rough picture of what I have in my head. Please excuse my awesome MS paint skills.

    [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
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    This relay will hold for about a second if you use 4000 microfarads of capacitance across the switch, but, "an amount of time" might be anything from a millisecond to an hour and a half. Personally, I don't have time to design an adjustable, all purpose circuit for a complete guess as to what you want.
     
  3. shainiac

    Thread Starter New Member

    Mar 26, 2013
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    Sorry, I should have mentioned that. The time window will be approximately 10 seconds. I found a circuit similar to what I need on Amazon, but it is located in China and shipping could be as long as a month. This circuit had what looks like a pot that can be turned to change the duration. Here is a link to the Amazon sale.

    http://www.amazon.com/Amico-Indicator-Triggered-Timer-Module/dp/B00A160NDC
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Are you trying to cause a delay in the relay operating or releasing?
     
  5. shainiac

    Thread Starter New Member

    Mar 26, 2013
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    I am trying to delay the release of the relay. I want to be able to quickly press the momentary switch to open the relay and for it to remain open after switch has been released.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Most relays are SPDT (also called C-form,) so when a relay operates, the wiper moves from the NC (Normally Closed) contact to the NO (Normally Open) contact. Thus, it's better to simply talk about operating and releasing the relay rather than opening and closing it.

    I think you simply want to press a button, which operates a relay, and have the relay stay operated for about 10 seconds. If that is correct, then the best way to do that is with a 555 monostable circuit, also called a one-shot. Is that what you want?
     
  7. shainiac

    Thread Starter New Member

    Mar 26, 2013
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    That's exactly it.
     
  8. shainiac

    Thread Starter New Member

    Mar 26, 2013
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    To take the guess work out of it, would something like this work for me?
    eBay Link
     
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Here is a schematic for a 10 second one-shot. When you press the pushbutton, the 555 will output a high on pin 3, and pin 3 will stay high for 10 seconds. You could connect a suitable relay to the output, and it will stay operated for as long as pin 3 is high.
     
  10. shainiac

    Thread Starter New Member

    Mar 26, 2013
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    Thank you! I will try and find those parts locally and build one on a bread board in my uni's lab this week. Can you tell me which capacitor is responsibly for the duration of the delay? Also, does increasing the capacitance increase or decrease the delay?
     
  11. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Assuming that the relay is rated to carry the current you need, that looks like just what you need.
     
  12. shainiac

    Thread Starter New Member

    Mar 26, 2013
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    Thanks again
     
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    See note 2 on the schematic: R1 and C1 control the duration of the high at pin 3. If you do decide to build it, remember to put a counter emf diode across the coil of the relay to avoid damage to the 555.
     
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