Is this Relay setup possible ?

Thread Starter


Joined Aug 15, 2006
Hi all,

I would like to do the follow - which seems easy in principle, but I am stuck on one part:-
I'd like to have a relay close when 12v is applied but then to open again after approx 2-3 secs, and stay open even with the 12v still applied - I'd like to reset the relay to allow it to close again only when the 12v supply has been lost.
I have been reading up on delay relays but from what I understand these would either
a) keep opening and closing the relay (with the time delay) while the supply is active.
b) close the relay and then only open again once supply is lost + time delay

Have I understood this correctly ?
Is it even possible to get the desired setup ?

If it makes things clearer - Its to activate a digital switch to turn something on when another device powers up. Hence if the digital switch is held closed for more than about 3 secs the device will turn back off.


Thread Starter


Joined Aug 15, 2006

Thanks for your suggestion, would you mind explaining how that would work ?
I'm confused as to how this would solve the problem of just activating the relay once for a few seconds, then opening the relay until power is cut and re-applied...
Unless I'm missing something - or have misunderstood you, would that setup not just keep the relay closed when power was cut - until the capactior had discharged ?



Joined Aug 8, 2005
You are describing a one-shot circuit. If you search applications for 555 and 556 timer circuits you should find what you need.

Chris Wright

Joined Jul 26, 2006

How are you coming with your project? It looked like a good exercise for a neophyte like myself, so I've been playing around with it and this is what I've come up with based on a 555 timer. The time delay is set by the relation of R1 and C1; something like: Time (sec) = R1 in ohms times C1 in Farads ( It might be 1.1xR1xC1, someone will set us straight). But a resistor R2 is needed to bleed off C1 to reset C1 after the power is removed, so you just have to play with it. Here's how, go to

and download the applet zip file. It's a slick little animated circuit simulator with a graphical interface that you just draw your circuit and watch it work (or not??). A link to the directions is on this page also.



Joined Oct 28, 2005
I think the function you require is called "ON DELAY". You must select an "ON DELAY" function time delay relay with the option for "NORMALLY CLOSED" and "NORMALLY OPEN". Connect the load you want to run to the "NORMALLY CLOSED" connectors. When power is applied to the time delay relay, the contacts do not transfer until the specified timing cycle is complete, at which time the relay contacts open. When power is disconnected from the time delay relay the relay contacts will be in the "NORMAL" condition, which is closed. Omron makes one, I've got them through WW Grainger---Rich