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To avoid noise interference:
1) Alwyas use Metal Oxide Varistors (MOV) across Primary of Transformer.
2) Use PI-Section filter at the output of rectifier instaed of Capacitor Filter (better ripple rejection).
3) use De-Coupling Capacitors (around 0.1uF) near Power Pins of each IC on the PCB.
4) Make sure that no ground loops are present in the alarm network.
5) Do not transmit TTL level (0-5V) signals for more than 5 mts or there are chances that it will catch good amount of noise. Use RS232 for long distance communication.
Well hey I'm really not that smart in electronics at all, but I have repaired a few electric guitars, and I have had this same sort of problem. Even when you aren't playing there is an annoying sort of hum. At first I didn't know how to cure it either, but when I did some research I easily found the problem.
Ok, there is something covering the entirety of the cavity(the part of the guitar carved out to hold electronics and pickups) of the guitar. It is like aluminum foil, but at Home Depot you can buy something better and more proffesional called Aluminum Tape, which does the same thing except better. You cover the cavity with this tape, and it blocks out all the other frequencies, so there is no hum when you aren't even playing. Completely covering your electronics work in this way is called "shielding" and I'm not sure it would work for you but you could try.
So In a nutshell you might want to use this shielding technique on your projects to block out the hum or noise from other things.
:arrow: Remember: I'm not sure of this, It only works for guitars as far as I know.