I guess I don't understand why you would want a timer? The compressor has a pressure switch so when tank pressure drops the compressor runs. The pressure switch also prevents the compressor from over charging the tank as well as likely preventing the compressor motor from seeing a greater load than it was intended to see.
Also, in line with OlPhart, what is the mains power? Such as 220/240 VAC US mains? Should the compressor have a contactor already controlling the motor then that could likely be used, however, this goes back to the pressure switch and the reason(s) it exist.
Ron