Electrical – Energizing relay using short pulse signal

relay

I have DPDT relay which I would like to energize using pulse signal (duration of that pulse should be adjustable) and switch. When switch is on, coil of relay is shortly energized by pulse signal and then deenergized when pulse signal ends, although switch is maybe still closed. When I open switch, nothing happens but when I close it, process repeats.
Any idea how to implement this circuit? Relay model is Omron DPDT 10A 12V DC.
Thanks in advance

Edit:
I looked for solution using 555 timer but it will not work in my case. Here is example of monostable 555 timer:
http://www.electronics-tutorials.ws/waveforms/555_timer.html

If I put switch between pin 2 and ground and close it, output will be high but it will not go low itself If I keep switch closed. I need output to go low even if I keep switch closed for long time.
They say also:

"The Monostable 555 Timer circuit triggers on a negative-going pulse applied to pin 2 and this trigger pulse must be much shorter than the output pulse width allowing time for the timing capacitor to charge and then discharge fully."

Best Answer

You'll need a lesser-used (albeit still extant) monostable for this job instead of the 555. Given the voltage levels you are dealing with, 74xx parts are out of the question, which leaves us with the CD4000 family's monostables, in particular the CD4047 multifunction CMOS timer:

schematic

simulate this circuit – Schematic created using CircuitLab

Notes and caveats: C1 must be a film type capacitor -- ceramics of that value aren't stable in value enough with voltage for timing applications, and the CD4047 requires a non-polarized cap to work. D1 can be swapped for a 12V zener if the relay turnoff is too slow for you. Q1 can be whatever small-ish power NMOS you have in your junk box (a jellybean BJT can be used provided you put an oh 1kOhm resistor on the base, too). The timing of the circuit is currently set for roughly 250ms, and can be changed by adjusting R1 and C1 in accordance with $$T_m = 2.48R_1C_1$$