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How to hold a relay for 5 seconds

  1. Mar 22, 2010 #1
    Hey everyone

    I got a problem. There is a line that provides 12VDC power in 1 second intervals (On and then Off, yes a signal light). The thing is I want to connect a relay to it but the relay must not turn off and on with the signal light.

    I was thinking of using a capacitor but I don't know the value to buy.

    And I need the simplest way possible.

    Signal On -> Relay On
    Signal Off -> Relay On
    Signal On -> Relay On
    --Switches off Signal light 5 seconds later---
    Signal Off -> Relay Off

    I hope you'll understand what I'm saying
  2. jcsd
  3. Mar 22, 2010 #2


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    5 seconds is a bit long for an RC circuit, you would have to make sure the leakage is very low.
    Take a look at 555 timer circuits unless you really need a solution that costs a cent.
  4. Mar 23, 2010 #3
  5. Mar 23, 2010 #4


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    Staff: Mentor

    Can you use a latching relay?
  6. Mar 24, 2010 #5


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    Gold Member

    I've used the CMOS 555 timers to go out about 10 minutes using tantalum capacitors. The application wasn't that important to require accuracy any better than about +/- 30 seconds. As far as I know, the thing has been running 24-7 since 1995.
  7. Mar 25, 2010 #6


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    Wait, we may be a little presumptuous here... Do you have another source of 12V available? If you don't, the relay won't beable to supply power!
  8. Mar 28, 2010 #7
    I would highly recommend NOT using a latching relay. I recently tried using them for one of my projects and the technical problem of reversing the voltage in the coils to switch/unswitch is definitely not worth it.

    Is power only available when the signal light is on, or does it just flash on it's own? If power is being cut during the light's off stage than the 555 timer would constantly be turned off. Maybe you could use a capacitor to provide constant power to a 555 at .2 Hz?
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