Azhar,

The Diode would help in a "low line" situation but what is the likelyhood that you did not use a "zero-ohm" resister (which would not protect you from "spikes"), but instead either empirically derived or calculated a value which would allow the resulting simple low pass filter to reduce your susceptability to periodic noise or short "spikes" (a zener being the best for larger spikes)?

regards,
p