Two part question re: noise on AC wiring

Thread Starter


Joined Oct 25, 2020

I'm severely hard of hearing (stone-deaf without my hearing aid), so a standard alarm clock is useless for me. I have a special alarm clock that has a 120-volt outlet operated by a relay so that it can operate some kind of device in sync with the normal piezo buzzer.

Some years ago, I was shopping in a surplus furniture store and chanced across the parts to one of those 'Magic Fingers' massage bed units. The coin box was tossed (it was of simple, yet sturdy construction and the timer had already been cannibalized), but the all-important motor unit was complete, rewired with a fresh cord, and bolted onto a 2x4 that goes in between my mattress and boxspring. The motor is merely a typical light-duty shaded pole motor with off-center weight to provide the shaking motion (and it DOES work just fine).

Question 1:

It seems after some years, switching an inductive load with the relay causes it to start sparking (judging by the internal condition of the original relay after I cut it open), a year or so before the relay completely fails. I'm on my second relay in this clock.

The last time I started having trouble was when I was still in an apartment, and it took a while to realize that my alarm clock was spitting out enough noise to turn the touch-operated floor lamp in my living room to a random setting.

Now that I have my own place, I have now gotten out of bed twice to find my garage door wide open, and I strongly suspect line noise confused the opener into thinking it had received an 'Open sesame!' signal.

What kind of noise filter should I be looking at, and where should I install it?

Question 2:

As previously mentioned, switching an inductive load at a fairly steady rate (1Hz, 50% duty cycle would be pretty close) clearly wreaks havoc on the contacts in a standard electromechanical relay. How can I minimize the inevitable sparking and make the relay last longer?