Hi

How could I generate a single positive pulse of about 3 seconds width just after the timer is connected to a positive voltage terminal... and what is the amplitude of the pulse? is it the same as Vcc or what?
I searched for a simple circuit for a single positive pulse and I didn't found any...

 

Have you read this?

555 Monostable

 

Thank you Bill. I tested that circuit and it worked fine on the two LEDs. I connected the output pin(3) to a relay as a voltage supply (I want the relay to switch for about 3 seconds), it worked but it didn't switched off as the green LED do when the relay is disconnected. I tried to connect pin 3 to a base of an npn transistor (BC548) and used this transistor as a switch to operate the relay but it did the same... What should I do??
Thanks again.

 

Here is a common interfacing scheme. If it switches the wrong way, change the polarity of the transistor, and connect everything from R2 back upside down.

This particular circuit will turn on when power is applied, basically a delayed power up. I'm not sure, but from your description it may be what you're asking for.

 

there gabs of info on the 555 timer for a one shot add a cap to pin 2 series with a vcc pull up once fired this one shot cannot be reset except by pin 4. pin 2 needs 50 ua to go even your person can generate that. and addind a PNP OR NPN to drive a relay is really not necessary it can source or sink >200 ma.

 

It worked fine.. Thank you very much Bill! and thaks italo too