Delayed Ground Circuit

Discussion in 'The Projects Forum' started by pswartz, Dec 17, 2015.

  1. pswartz

    Thread Starter New Member

    Dec 17, 2015
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    I need a circuit (ME struggling with EE stuff). The circuit is sort of a delayed shut-off circuit. Remain in a state (open) for 30 seconds after trigger then close.

    Input is 12 VDC. Trigger is a normally open switch (close will trigger). Output needs to be normally open contacts that will close (or both ground) after a set delay (30 seconds) once triggered. Also, need the contacts to ground if power is cut.

    So...open contacts that close 30 seconds after a normally open switch is closed or the power is cut.

    I was initially looking at some circuits for the IC555 as I have an IC555 and numerous sizes of resistors, caps, etc. But after letting the smoke out of a couple of transistors and making some capacitors go bang, I think it is best to get some help...
     
  2. ErnieM

    AAC Fanatic!

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    Google "time delay relay" and see if you find something suitable.
     
  3. AnalogKid

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    "Also, need the contacts to ground if power is cut."

    This part means that the output relay will be on during normal operation and during the turn-off delay period, something to consider if this is a battery-powered project. Can you draw up a timing diagram or something to show the relationships of the various controlled states - when things are on or off, open or closed, etc?

    ak
     
  4. pswartz

    Thread Starter New Member

    Dec 17, 2015
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    Tried Google...That's how I made smoke and popping sounds with components...
    Hoping to find a working diagram with well defined parts that will give me the following output:

    Power Input (Normally Open Switch) Output (2x terminals)
    Off Closed Closed
    Off Open Closed
    On Open Open
    On Closed 30 second delay then Closed (reset after 5<x<30 seconds)

    As I said, I have an IC555. Tried some of the circuits I've found online, but many have not (obviously) been verified. There are many circuits that will change state from closed to open after triggering, but the trigger instantly changes the state and the delay is on the changed state. I need the initial state to continue for the delay, then change.
     
  5. AnalogKid

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    The delay is the easy part. But your overall circuit still is not clear. Are you trying to drive a relay with the 555, or have the 555 switch the main output directly? Your use of the terms open and closed seem ok, but it is not clear what you mean by "ground":

    "Output needs to be normally open contacts that will close (or both ground) after a set delay (30 seconds) once triggered. Also, need the contacts to ground if power is cut."

    Let's take this apart.

    Output needs to be normally open contacts that will close after a set delay (30 seconds) once triggered - So the output circuit that is being switched by the relay (voltage, current, AC, DC?) sits there in the off state (because the contacts are open) for a long time. Then someone closes your trigger switch and nothing happens for 30 seconds. After the delay, the relay contacts close, completing the output circuit. Things now sit forever until the trigger switch is opened. When that happens - what? Also, at the beginning, if power to the circuit is lost before the trigger switch is closed, the output contacts close immediately. Is that it?

    (or both ground) - Both what? The dual contacts of a double-pole relay? And what do you mean by "ground" in this part? Switching a signal or power line to ground? A wiring sketch connecting functional boxes really would help. Schematic were invented because English sucks at this.

    ak
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    pswartz: if you could, please look for the "code" tag button on the post window and repost your table. Code tags leave formatting (white space) normally removed from posts.

    I just can't see what your intent is, and on my iPad I can't even edit your text.
     
  7. AnalogKid

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    To help bridge the language barrier, don't use "ground" as verb. Ground is a physical thing, an object with defined electrical characteristics. Instead of saying something will ground after a delay, say something like this: Circuit number 2 will be connected to ground through the normally closed contacts of relay 17 when the moon is full on a Tuesday, and connected to a tesla coil through the normally open contacts of the same relay when it rains in Spain.

    ak
     
  8. pswartz

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    Dec 17, 2015
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  9. AnalogKid

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    Excellent. Minutes after the input contacts are closed and the output contacts have closed after the delay, what happens if the input contacts are opened?
    1. Output contacts stay closed.
    2. Output contacts open immediately.
    3. Other.

    ak
     
  10. pswartz

    Thread Starter New Member

    Dec 17, 2015
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    So, after they close have to have the 30 second delay then outputs close regardless of the input state (uninterruptible). The outputs need to stay closed for 5-30 seconds then can reset once there (continues to be) power and the input is reset (open).
     
  11. AnalogKid

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    Input trigger
    30-second delay no matter how long the trigger is closed.
    Output closes
    30-second delay no matter what.
    Output opens
    Indefinite wait
    Power removed -> output closes

    Yes / No?

    What is the output circuit? Voltage, current, AC, DC?
    This determines what kind of relay is needed,
    which determines what kind of timer circuit output is needed,
    which can affect the timer circuit design.

    The timer parts could be nothing more than one CMOS gate package and a some Rs and Cs.

    ak
     
    Last edited: Dec 18, 2015
  12. pswartz

    Thread Starter New Member

    Dec 17, 2015
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    Input trigger
    30-second delay no matter what state trigger is in afterward (uninterruptible 30 second delay to close).
    Output closes
    30-second delay no matter what.
    Output opens
    Indefinite wait (for next trigger)
    Power removed -> output closes
     
  13. AnalogKid

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    Third time:

    What is the output circuit? Voltage, current, AC, DC?
    This determines what kind of relay is needed,
    which determines what kind of timer circuit output is needed,
    which can affect the timer circuit design.

    ak
     
  14. pswartz

    Thread Starter New Member

    Dec 17, 2015
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    Output circuit is simply taking both output pins to ground. The device the output ties into needs open pins during normal operation and during triggered (30 seconds delay) or power interuption, the pins both go to ground. Nothing extravagant. Open or closed. That was why I was looking at the IC555...simple (except for the "I can't figure it out" part).
     
  15. AnalogKid

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    So the output of this circuit is at a signal level, like a control input to a machine, rather than turning on and off a 30 horsepower motor? Because of the requirement of closed contacts with the circuit power off, there has to be a relay involved. The question is how big a relay. Is the relay switching amps or milliamps? Volts or kilovolts? The word ground doesn't tell me anything useful.
    What is the voltage across the output contacts when they are open?
    What is the current through the output contacts when they are closed.

    ak
     
  16. Alec_t

    AAC Fanatic!

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    Do those pins have pull-up resistors to some non-interruptible DC supply voltage (which would enable normally-biased-'on' BJTs/FETs to be used for switching the outputs, thus avoiding the need for a power-hungry relay)?
     
  17. AnalogKid

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    Something not discussed yet is the inherent inaccuracy of large capacitors. Even if you spring for a "precision" +/-10% part, big cap leak, and that leakage current conflicts with the very small current needed for a long time delay. So even with a relatively accurate timer part like a 555, getting decent accuracy at anything over a few seconds means having a calibration adjustment somewhere. Another problem with long RC timers is it takes time for the big timing capacitor to reset, unless you add a diode or some other parts as an active reset circuit.

    Another approach is to use a counter and some logic. The advantages are that a medium frequency clock is much more accurate than a long-period RC timer, and it can be reset instantly. Because everything is bolted down in logic it is less adjustable, but much more stable and repeatable.

    ak
     
  18. pswartz

    Thread Starter New Member

    Dec 17, 2015
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    Looking up the output information - not much there. When the 2 pins are open, the device operates normally. When "triggered" (after the delay), the 2 pins need "contact closure to ground" (according to the documentation). The connection containing the 2 pins also has pin(s) for chassis & signal ground. No detail on V, mV, A or mA... The pins are small...30 ga wire in the connecting cable. Not carrying much power at all... The delay timing of 30 seconds does not need to be very accurate. We need a minimum of 30 seconds currently. So the delay requirement will be likely 35±5 seconds. Generous tolerance should equal simple/inexpensive components, right ?
     
  19. AnalogKid

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    Here you go. When the circuit powers up, R3 holds U1D inputs low/output high, which turns on Q1 and opens the output contacts. SW1 is normally open. When closed as in the schematic, it starts the delay timer, which triggers the activation timer, which turns off Q1 and closes the output contacts. Both timers are true monostables with feedback, so they are immune to retriggering while timing.

    ak
    DelayedGround-1-c.gif
     
  20. pswartz

    Thread Starter New Member

    Dec 17, 2015
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    Thank you. Will likely have some questions as I digest this (and get it prototyped). Appreciate your help.
     
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