The 555 timer can be triggered by another 555 in several ways.
This setup is the one I like for tracers. Connect it a transmitter (mp3 to FM) and you have a cool radiotag. Use a speaker and a LED and use it to find a downed RC plane.
The first 555 turns power on and off to the second 555 through a transistor. Another way would be by connecting the first timers output to the second timers threshold or control. The only reason I use this setup is because the slight wobbling at the beginning of each beep makes me think of old spy movies :-)
