relay trouble

Discussion in 'The Projects Forum' started by gergstuart, Dec 31, 2010.

  1. gergstuart

    Thread Starter New Member

    Dec 31, 2010
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    Im trying to use a momentary switch and a relay to switch on an led which is on a completely different circuit. I want to know if I can use only a relay but not a flip flop. It sounds simple but I want the led to remain on after the momentary switch is released. There is a reason for this but its too hard to explain. This is what Im trying to do..... push momentary switch, led lights up, release momentary switch, led remains on and will remain on until reset. I tried using a relay but I cant get my head around it.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    You need a relay with two sets of contacts.

    one switching the LED, the other switching the relay coil voltage. When you press the switch to activate it, the one set of contacts will provide power to the coil, and keep it on. To Reset, put a normally closed switch in the line to the coil from the relay contact. When it is pressed the coil line is opened and the relay releases.
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    yes sir. Don't forget the diode.
     
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    You can also forget the relay altogether and use two inexpensive transistors that basically form the equivalent of a SCR. At Radio Shack; you can buy a 10 pack of NPN's and PNP's for less than you'd pay for one relay. For that matter; you could also use a small SCR. Radio Shack sells them too.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    If you decide to use Alberto's circuit there's an alternative to the N/O (Set) push button and the N/C (Reset) push button switches. You can use two N/O (Normally Open) switches by connecting a N/O Reset switch between GND and the Anode of the SCR. If you do this the circuit will actually have two modes of operation.. If the Led is off when you press the Reset button the Led will light while you're holding the button down and go off (no latch action) when released. If the Led is off when you press the Set button the Led turns on and will remain on when released. When Reset is pressed the Led will go off when 'Reset' is released. I mention this, only, because N/O push buttons are more common and easier to procure.

    BTW: It would appear that Radio Crap Inc. no longer supplies SCRs. A slow death is an awful thing..:(
     
    Last edited: Jan 3, 2011
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    As LED is on a seperate ckt, use a photo-isolated SCR.
     
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