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General: Annoying hum

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  1.  
    • CommentAuthorRex
    • CommentTimeDec 13th 2025
     
    Unfortunately, it is me again!!!

    Have the replacement panel up and running and since if it activates, it is impossible to hear the external sounder in the house and particularly if entry is from the back.

    So this time, I have fitted a sound bomb, if for no other reason that the excessive inside noise is slightly disorienting.

    However, I now have a very faint humming from the sounder and wondering if it is induced by the incoming mains.

    The red line is roughly the cables from the sounder that go on the bell terminal on the board. These two wires go under the plastic housing and enter the interior just below the terminals. The yellow lines are the mains to the transformer.

    I know little about the science of electricity but could the transformer and/or the incoming mains be inducing a micro current in the sounder wires that is causing the sound to hum?

    If so, what is the solution? Reroute the wires so they do not cross the incoming mains or go under the transformer? Reposition the sounder?

    Grateful for any suggestions.

    Compliments of the season to one and all,

    Rex
      IMG_20251213_140105_442 (Medium).jpg
    •  
      CommentAuthordjh
    • CommentTimeDec 13th 2025
     
    Earlier you called the sound a buzz; the difference may matter. 50 Hz hums and buzzes are most frequently caused by some magnetically influenced component in a circuit, such as a transformer. Often by the laminations in the transformer core knocking against one another in response to the varying voltages - that tends to be a buzz.

    If your problem is caused by the two yellow wires crossing the red wire, then one solution would be just to twist the two yellow wires around each other. A 'twisted pair' won't induce interference in the same way. Or as you suggest, rerouting the wires would help.

    But maybe the interference is generated elsewhere and is already on the red wire. If the circuit design was intended to have a mechanical bell or suchlike on the end then a small ripple wouldn't cause a hum in the same way it might with a piezo sounder.
  3.  
    In the photo, the piezo sounder itself is mounted 1 inch from the mains transformer and 2 inches from the mains supply FCU.

    So irrespective of how the DC wiring is routed, the sounder itself can be picking up the mag field from the transformer and the mains cable.

    You can try repositioning the sounder a few feet away from anything mains and see if that helps?
    • CommentAuthorRex
    • CommentTimeDec 13th 2025
     
    Thanks for the reply. I only mount it there because the two wires are not much more than normal alarm wires and they were too short. Had to solder an extension. By mounting it there, the wires are not exposed.

    I will see about repositioning the sounder under the panel and the wires can go straight up to the terminal.
    •  
      CommentAuthordjh
    • CommentTimeDec 13th 2025
     
    Posted By: RexBy mounting it there, the wires are not exposed.
    Just use a screened twisted pair instead of whatever wire you used and then it doesn't matter whether it's exposed and you can mount the piezo sounder further away.
    • CommentAuthorphiledge
    • CommentTimeDec 13th 2025
     
    In the other thread you said the panel is tucked in a cubby hole in the guest bedroom. You don't really want a sound bomb tucked away where it's unlikely to disorientate someone trying to nick your telly. You can run a number of tests via the keypad so operate the soundbomb with the guest bedroom door shut and see if its deafening downstairs. If not I'd put the soundboard somewhere central downstairs and run a cable out to it.

    If it's still humming when you move it away from the AC, alarm suppliers will have a small PCB mounted relay which you can wire to the bell output and wire the soundbomb via the relay so it only ever gets connected when the bell output is live and switches the relay
    • CommentAuthorRex
    • CommentTimeDec 14th 2025
     
    Totally agree regarding the sound bomb being in a poor location. If I knew at the time of building, what I now know, I would have mounted a few sound bombs in various parts of the house with the associated cabling.

    But.... that did not happen and I'm not about to start running cables up walls. Its a TF house with Fermacel so challenging to chase the cable into the wall.

    Disorienting would be nice but my main aim is to ensure that anyone breaking in can hear that they have activated the alarm. As it is, the 3g glazing and Warmcel is extremely good at reducing the exterior alarm to close to inaudible.

    Will reposition the sound to below the panel today so the wires are not adjacent to the mains input.
    • CommentAuthorRex
    • CommentTimeDec 14th 2025
     
    Moved the sounder to below the panel and the induced hum has stopped (probably as expected.)

    Thanks for the suggestions and support from a rather non-electrical, expert DIYer.

    Compliments of the season to one and all,

    Rex

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