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433MHz door bell - Tx antenna feed

Discussion in 'Radio and Wireless' started by Cirkit, Mar 17, 2018.

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  1. Cirkit


    Oct 28, 2015
    I am trying to work out whether the component indicated by an arrow in the photo in the antenna feed, is a capacitor and what its value is likely to be?

    I have tried measuring it with a few different meters and it reads open circuit in resistance and capacitance mode which makes me think it might be faulty?

    I think the component above connected to the antenna is an inductor and measures around 0.47Ω.

    The door bell is functioning intermittently when the receiver batteries start to get a little low so I'm wondering whether the reduced transmission power may be causing the intermittent operation?

    Attached Files:

  2. davenn

    davenn Moderator

    Sep 5, 2009
    yes most likely a cap probably around 10 - 30 pF range

    It should

    you cap meter may not be able to read that low

    yes, looks like an inductor

    I strongly doubt that that capacitor is the problem

    A better pic would have been good, that one supplied is VERY blurry

  3. kellys_eye


    Jun 25, 2010
    The receiver batteries? Wouldn't the 'fault' be with the receiver part then, rather than the push-button (transmitter)?
    davenn likes this.
  4. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Hmmm. Fuzzy picture seems to go well with the fuzzy logic. Check the batteries on the transmitter for fuzz too.

    Earlier this week, while we were looking for something (I forget what) in our garage, my wife pulled a couple of AA cells out of a box we had packed for our move to Florida last year and asked if the fuzzy cells she held out to me should be thrown away. Well, duh! Fortunately the icky cells were in a plastic baggy, so whatever the leaky fuzzy stuff is, it didn't affect anything. These were standard alkaline cells, so safe to dispose with household trash, which I did.

    As far as @Cirkit's problem is concerned: check the batteries and their contacts to make sure everything is clean, with no traces of corrosion. Make sure the batteries are fresh (check date code) and their open-circuit terminal voltage is up to snuff... if this wireless doorbell was working satisfactorily in the past but is now intermittent, the problem is likely a dirty switch contact, a dirty battery contact, or an almost depleted battery.
    SUBSTATION likes this.
  5. FuZZ1L0G1C


    Mar 25, 2014
    Hi Cirkit.
    If you have a flatbed scanner, or access to one, scanning the board placed directly on the glass gives quite a clear image, as long as there aren't any larger components, as SMD component faces should be at focal point (glass surface).
  6. Cirkit


    Oct 28, 2015
    I'm having problems with the same doorbell again! Better photos of the PCB attached.

    Seems to work fine when the receiver and bell push are fairly close.

    I have tried new batteries in both. I've also resoldered the joints on the bell push PCB and some on the chime receiver.

    How can I test the antenna feed capacitor and how do you calculate what value it should be?

    Attached Files:

  7. Martaine2005


    May 12, 2015
    That PCB antenna trace is probably not helping.
    Try soldering a 12 inch piece of wire where the trace is and then checking to see if it works any better. If it does, you could try making your own coiled/spring antenna or just buy one and try it.
  8. Cirkit


    Oct 28, 2015
    Thanks Martin, do you think it could be the SAW resonator (circular metal can) failing? The problem with the bell push seems to be worse when it's raining.

    The bell push has been fine for many years so I doubt whether the antenna needs improving.
  9. Martaine2005


    May 12, 2015
    I doubt the circuit has a problem.
    However, all homes and neighbours have more and more wireless devices. Garage doors, car locks, home automation, door bells, cameras, CCTV, baby monitors etc etc etc.
    Not to mention toy radio controlled cars, toy walkie talkies etc etc.
    The airwaves are jammed packed in all directions. Sometimes relocating the receiver can help. I would think the signal is being interfered with by one or any of the above items.
    Hence an antenna added to your PCB to see if your situation improves.

    Edit: I don't think the 433MHz crystal is failing.

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