How to have 2 signals triggering a Timer Relay?

Discussion in 'The Projects Forum' started by johntee, Mar 21, 2010.

  1. johntee

    Thread Starter Member

    Jul 20, 2009
    Hi folks. I'm having trouble envisioning how I can get 2 separate desired triggers to feed my Timer Relay without backfeeding to each other (and right now, I can't even figure out how to get it to work at all...)

    Basically the Access Control Unit reads an RFID card, and if it recognizes the card, it completes the circuit to the first Relay for 5-seconds, which does 2 things:
    (1) it powers up the AC transformer, which unlocks the electric door strike for 5 seconds to let the person into the room.
    (2) it sends a DC signal to the Timer Relay, which then powers up the room lights and keeps them on for 10 minutes.

    I'd like to improve the circuit by adding an illuminated momentary pushbutton switch inside the room, so that if the person is still inside when the timer counts down to zero, they can hit the button to restart the timer for the lights to come back on. (They wouldn't be near the door where the Access Control Unit is located, so that's why I need the second switch, inside the room.)

    Seems like I'd need a DPDT pushbutton, but how would I wire it so that the 2 different sources feeding into the Timer's "S" terminal don't backfeed to one another? (One source -- via the pushbutton -- is from the always-live +12V line (I guess on a N.O. side of the pushbutton),
    and the other source -- via the 782XBX relay -- is the only-occasionally-live +12V signal that comes when the Access Control Unit accepts an RFID swipe (I guess on a N.C. side of the pushbutton).
    But I've managed to really confuse myself tonight! :)
    How would I add this new switch, and also prevent damage if both the pushbutton and the Access Control Unit were to send a signal at the same moment?

    Thanks for any help!


    --- 782XBXM4L-12D Magnecraft Electromechanical Relay, "Ice-Cube" Style, 16A DPDT, 12VDC Coil Specs
    --- Time-Delay Relay to feed 120VAC lighting circuit: 822TD10H-UNI Magnecraft Programmable Time Delay Relay, 15A DPDT Specs
  2. Bychon


    Mar 12, 2010
    because the timer relay needs to get hit with dc, just use 2 diodes to keep the 2 signals from meeting each other.
  3. whatsthatsmell

    Active Member

    Oct 9, 2009
    It appears from your diagram that the relay for the light has a built in timer, so all you have to do is trigger that relay to start the light cycle again. Just wire in another switch. You can throw in a diode if you want, but feedback shouldn't be a problem because that circuit is just switches controlling the same 12v source.

    The only thing I didn't show is the 0v (ground) wire from the 12v circuit that you have to run to connect to the light that is in the switch (So you will need a 12v illuminated N.O. momentary switch).