Some suggestions:
optical isolation is a must, even on relay
outputs. For power lines, you can peek into
a computer power supply - VDR, balun inductor, low esr capacitor. Metal shielding of entire device can not harm. 8051 can draw significant currents for short periods. Be sure that power supply
can handle this. 10mFd Tantalum as near as possible serves vell. Input and output inpedance should be low(low res. pulups and downs, capacitors if you dont need a speed.
Zenner or transil also cant harm. Proper grounding! Try to fight with noise on its source - RC snubbers, freewheeling diodes.