"Not really at the same time but so fast sequences that it would APPEAR'at the same time' to the user."

You are right, Asok. (Just as an aside : Why did you drop the "H" after "S" in your name ? Numerology ?!)

Mahnoor:

What you said is quite normal and an 8051 can handle much much more than that. And it looks like that you are going to need both interrupts AND a state machine coding.

The seven segment display update will need a interrupt say at about 1ms to 2ms.

Your relay switching can be based on a state machine.

Your input key scanning can once again be based on polling ( with debouncing) or with interrupts.

And after doing all that you may have enough processor free time for many more jobs. Those black boxes ( read MCUs) are quite smart fellows and are very happy to work ALWAYS.

Raghu