Build a circuit which gives a signal for 5 seconds once in every two hours. What components would you recommend? Answered

Gotta jump on the bandwagon and say microcontroller.  It's the easiest, smallest, most accurate and possibly cheapest way to go about it.

I would use an Arduino, along with the datetime library.  If you hooked up a 10k potentiometer, you could use the resistance value and map it to a delay function.  Arduino.cc has helpful information on projects like this.  Also, try searching for projects on home-made intervalometers for cameras that would normally be used for time-lapses.  They accomplish what you are trying to achieve.

Steveastrouk has a good idea with a standard microconroller, however if you don't know how to program one an Arduino might be a better alternative (USB interface). The Arduino duemilanove has a crystal oscillator built in, so you would have very accurate time keeping.  Not to mention, it runs off of a standard 9v battery because of its built in power supply.

What are you trying to do?

You _could_ use a 555 timer IC in monostable mode. It might require some trial and error to get the right period, it will depend on how accurate you want it to be.

You'd probably be better off buying a microcontroller, you'd be able to get it much more accurate.

You'll struggle with the classic 555 for a long-period timer like that. The 555 will drift fairly wildly with temperature, and you'll have problems sourcing a good enough capacitor.

The best way would be with a microcontroller - its almost trivially easy like that - like a PIC