30 seconds waiting circuit

Discussion in 'General Electronics Chat' started by orwikcons, Jun 25, 2008.

  1. orwikcons

    Thread Starter Member

    May 2, 2008
    22
    0
    I have asked about the circuit which will, when plugged on 12V DC, turn on the relay, and after 0.5 seconds turn it off, although 12V is still on. You suggested me a parallel connection of capacitor 470 uF and 5kOhms resistor before the relay. It worked. But now I need it to take 30-60 seconds before turn off the relay. If I am right I would need very strong (also very big) capacitor, so can it be done in some other, smarter way?

    [​IMG]
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You can use the 555 timer as a monostable to make your delays or you can use a smaller capacitor in combination with a transistor to make a simple delay. The transistor draws a small current on its base so you need a smaller capacitor than driving the relay directly with a capacitor. If you want you can add a diode in series with the transistor's base to increase your threshold voltage (the voltage the transistor will turn off after the capacitor has been discharged).
     
  3. orwikcons

    Thread Starter Member

    May 2, 2008
    22
    0
    When I used a transistor, I put 1kOhm resistor to the base, and +12V. What is the minimal impendance with which transistor can survive 12V? When I put the capacitor on the base, what impendance resistor should I put? Thanks!
     
    Last edited: Jun 25, 2008
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Which transistor are you using?

    It would be nice if you post a schematic of what have you done.
     
  5. orwikcons

    Thread Starter Member

    May 2, 2008
    22
    0
    I made this circuit:

    [​IMG]

    and it never turned off the relay. So I put the potentiometter instead of 2.5kOhms resistor, and from 0 to 22 kOhms nothing changed, it holds the relay on permanently.

    For example this circuit works:

    [​IMG]

    It holds the relay on for 0.5 seconds, and then turns it off. The capacitor discharges almost instantly, so I can turn it on again.

    What should I change in first circuit to make it works?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Have a look at the attached schematic.

    I'd posted a slightly different version a bit ago, but then realized that C1 would never discharge. The item marked "V1" represents a slow on-off voltage; it switches from 12.6 to zero volts. Your circuit will not be quite like that; the diode D1 will instead discharge C1 through whatever else the load is.
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You have to connect the left side of the 2.5K resistor to ground and also disconnect it from the capacitor.
     
  8. loosewire

    AAC Fanatic!

    Apr 25, 2008
    1,584
    435
    Check out the 12 volt,com,google click on 3 or 4 window's.
    More 12 volt information ,car alarm's,electric car window.
    Check it out and reply. LOOSEWIRE
     
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