Caan I use arduino as an oscillator and timer?

Hi,

I've never used an arduino before, so please excuse my lack of knowledge.

I need to generate a signal at 135kHz that switches on for five seconds every 20 minutes.

I was gonna do this with a crystal oscillator and a timer switch, but I was thinking it might be better to do it with an Arduino.

Is this feasible? How would I get an Arduino to generate this signal?

Many thanks

Yes this is possible. Change the PWM frequency to 135KHz and just turn a PWM pin on for five seconds. http://www.arduino.cc/en/Tutorial/PWM is a good place to start.