Power on/off relay

Discussion in 'General Electronics Chat' started by The Hound, Jan 25, 2010.

  1. The Hound

    Thread Starter New Member

    Jan 25, 2010
    Hi I have done the exact samething as Sangee.
    I built the constant to momentary circuit ,to find out that it only works when power is applied and not when power is removed.
    I have a good back ground in electronics though it has been years.
    My problem is I don't understand the examples given.
    Can someone link me to schematic that shows where I pull off the momentary signal or how the two relays connect.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
  3. bloguetronica

    AAC Fanatic!

    Apr 27, 2007
    Hello and welcome to AAC!

    I could help you, but I have a couple of questions I need to ask you in order to do so. What are you trying to achieve? Do you have a schematic?
  4. The Hound

    Thread Starter New Member

    Jan 25, 2010
    Hi guys, thanks for the warm welcome and the thread move.
    Here's what I have.
    I've got a 12v constant source/trigger from a projector.
    What I want to turn on/off is a garage door opener.
    So essentially I am replacing the button on the wall that activates the garage door opener.

    So I built this constant to momentary switch.
    This activated the opener when the trigger/switched power was applied but did not activate the opener when power was removed.
    When power is applied I need a pulse to the opener and when power is removed I need a pulse to the opener.
    (It is hard to search for something when you don't know what it is called.)

    I found this circuit and think it will do what I want it to.
    Can you confirm this for me, thanks.
    These are two SPDT Relays, they said the resistor could be elimanated and I might need to go to a 2.2K capacitor.
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    I think you were moved from :http://forum.allaboutcircuits.com/showthread.php?p=147012#post147012

    The attached is a schematic of my circuit using your pin numbers. No diode and no resistor. Are you using automotive relays? These have a coil resistance of ~90Ω. The on-time of the second relay is determined by the capacitor and the coil's resistance, plus the mechanical delay. 90Ω and 2200μF will have a time constant of ~0.2 seconds. I think the attack and release delay times (I can't find the data) are about 0.1 Sec. So the contacts should be closed ~0.1 sec when the power is switched on or off.

  6. bloguetronica

    AAC Fanatic!

    Apr 27, 2007
    It seems you are using automotive relays, right? I see what you are trying to achieve. You are using the coil in series with a capacitor so the coil is momentarily energized when power is applied. However, in the first circuit, if the power is removed, the capacitor won't discharge. You will need a second relay, but connect the NC contact of that second relay so the capacitor will discharge via the first coil. Use a diode to prevent a short.

    See the diagram:

    The diode is very important, since it will allow the discharging of the capacitor through the relay coil, but it will prevent a short circuit when power is applied.

    Edit: I think Ken's schematic is better (wasn't there when I posted).
    Last edited by a moderator: Jan 26, 2010
  7. The Hound

    Thread Starter New Member

    Jan 25, 2010
    I am using little, inch long, relays from radio shack, they may be called reed relays.
    The schematic does represent an automotive relay though.
    I will build this circiut this weekend and let you know how it goes.
    Thanks for the input.
  8. The Hound

    Thread Starter New Member

    Jan 25, 2010
    Thank you Cume and moffet for the help.
    When Cume put my pin numbers on the schematic I was able to understand both the schematics posted here, thanks.

    Here is what I did incase someone stumbles over this post with the same issue.
    Starting on the left the first relay is a Radio shack SPDT-Micromini 12VDC relay # 275-0241.
    Second is a Radio Shack SPST-12VDC Reed Relay #275-0233
    I used the original two relay schematic that I posted, though all the schematics will work.
    I kept the 1K uf capacitor and got rid of the resistor.
    If you use a diode across the second relay the circiut doesn't work properly.
    I was able to use a diode across the first relay to stop the spike when the coil field drops.

    In all three schematics power to pin 87, was provided by the door opener.
    I wanted to isolate the two sides of this circiut so, I jumped the power from pin 86 to 87, on the first relay.
    In this way the door opener is not physically connected to the projector.
    It works perfectly.
    Thanks again for the help. I hope this helps others too.