2 3 seconds to trigger relay

It'd be better if you used a transistor driver instead of directly driving it like that. The problem is that the 50 ohm resistor charges up the capacitor too quickly for your purposes. A larger resistor and/or capacitor would extennd the time delay, but- you have a limit on how large a resistor you can use because you're trying to drive the relay through it, and a limit on the capacitor because it'd get ridiculously large. I'm sure there's post from the past for this purpose with details, if you have any interest in using some electronic bits, I'll look for it-

The mention of electronics and solid state scares off a lot of guys, but this is actually pretty simplistic, and actually would be cheaper than trying to buy humongus caps. If you go with humongus, you might note that the relay will remain on for a time after you try to release it also, if that's any concern. With the solid state driver, you'll be able to drive it with alarm drive outputs without worry about overloading them from the extra drive current.

Can I use that circuit with a negative pulse for input?

I am an idiot: I just checked the capacitors I'm using and they are 470uF. Dualsport: yeah, humongus is an acurate description for a capacitor of that capacity. It's bigger than the relay I'm using! Thanks to you both. Now: can I use a switch negative and constant 12v on that circuit. I suppose that changing the values of R1 and C1 will change the delay duration, right? Will the relay once activated release as soon as the input signal is no longer present? Thanks

This should allow you to trigger with your neg trigger. If your input trigger is already (+)/(-) signal you can delete some of the stuff in the front end. It should delay the onset of the relay being energized and turn it back off as soon as you release the trigger. The other benefit of using a solid state drive instead of directly driving is that the relay turns on and off faster, which is better for the relay contacts. When you have a slow drive such as the capacitor direct drive where it charges slowly to energize the relay, the relay contacts go through a period where they're high resistance for a longer period of time. If it's just a light duty load then this isn't a big deal but if you're switching high current loads, it degrades the relay contacts more than if they switch open and closed quickly as they were designed to do. A relay rating of 30A is based on a fast switching time.

Thanks dualsport... Will try to buy the parts tomorrow... Did you changed the last circuit and removed a couple of diodes on the left?

How's this?

cmorales wrote: Thanks dualsport... Will try to buy the parts tomorrow... Did you changed the last circuit and removed a couple of diodes on the left?

Yes, they're not really necessary because they're built into the IC, so you can do without them to simplify-

cmorales wrote: How's this?

I'm impressed-

The drain and source on the transistor is reversed (that is, your outline and pin definitions are correct, but the wiring is swapped- pin 1 is Drain, pin 2 is Gate, pin 3 is Source), but if you just plug it in 180 degrees from the outline shown, it should be fine (or use a 2N7000, which has the opposite pins )- 

Also, be sure to observe polarity on the diodes and caps, since they're not marked on there.

p.s.  Check mouser.com for the parts if you have any problems finding them at a reasonable price.