Help needed

I am simply a hack at electronics but I need a little help...

I have two cats. One is fat because it eats the other one's food. I have
create a special sensor using simple reed sensor switches, a 9 volt and a
piezo buzzer. There is a magnet attached to the cats collar. It works
quite well, when the cat approaches, the magnet triggers one of the reed
switches to close setting off the buzzer.

The problem is that the buzzer only buzzes for a micro - second as the
magnet passes over the reed switch. I want a simple circuit or relay that
stays on for say 5 seconds. When the reed switch closes, it sends 9 volts
to the piezo...isn't there a simple relay or simple basic circuit that can
hold a relay or switch closed for 5 seconds and then open again?

 

ANY 555 timer will do what you need. Google for 555 timers.
Kim

 

Or adapt this:
http://www.terrypin.dial.pipex.com/Images/555MonoByPU.gif

The 12V supply i sreplaced with your 9V. Change the 10uF cap to 4.7uF
to get a 5 second period. Wire the 555 output to your buzzer.