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Help needed in adding an antenna to a cheap 433.92 mhz key fob to transmit simple sensor data.

Discussion in 'Radio and Communications' started by Purclewan, Jul 10, 2016.

  1. Purclewan

    Purclewan New Member

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    Hi, I wonder if anyone has experience of hacking one of the cheap 433 Mhz keyfobs that can be bought on ebay ( http://www.ebay.co.uk/itm/111882221853) in particular by adding an external antenna.

    I have built a simple but reasonably effective portable data loging system which uses several these key fobs to transmit a simple state change (on/off) from up to four local sensors to a receiver. I chose these because they are very easy to get hold of, are very cheap and can be self contained using the internal 12V battery with a good battery life for field use.

    I (carefully) remove the push button switches and then solder fine wires to the solder pads to give me four sensor inputs.

    Having got my system working reasonably reliably well up to 50/80 metres I would like to extend their transmit reliability and range but cannot figure out where to attach an antenna to the key fob cct board, at 66 my eyesight is simply not up to trying to trace out such a tiny board.
    I have attached three photos of a typical board (they all differ slightly but basic layout is similar) the first with the top of the fob removed and showing the metal rim around the fob. This is not connected to the board but I wondered if there might be some form of capacitive coupling making this an antenna??

    Then there are photos showing front and rear. I noticed the track with holes bored in it looks like it might be an antenna but also what looked like a printed inductor on the front side.

    Any pointers or help would be very much appreciated.

    Purclewan.
    PS the receiver has already had a 17.3 cm antenna upgrade which helped.
     

    Attached Files:

  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    A proper "antenna" is a dipole that for this frequency is 33cm long; not a single 16.5cm piece of wire...

    The left half of the dipole (16.5cm long) should be connected to the pc trace labeled "GND"; the right half of the dipole (also 16.5cm long) should be connected to the trace labeled "Ant". The dipole should be stretched out linearly...

    Fob2a.png
     
  3. Purclewan

    Purclewan New Member

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    Thank you Mike. Appreciate your confirmation that the track is an antenna.

    Would you leave it as is and solder directly to it or cut the track close to what looks like a capacitor (C3?) then solder the new antenna there? I was thinking of the effect on impedance matching if I solder straight on to the track or would this have little practical effect?

    Thanks for reminding of the need for a grounded dipole, this is what I had used on my receiver but it had not occurred to me to do the same for the key fob.

    I am just a tinkerer and experimenter with sometimes daft ideas. However, If I can get this test setup working reliably enough to give me the confidence that what I am aiming for is practical and does what I expect then I will be looking to develop a "proper" version that is slightly more robust.

    Thanks again.

    Purclewan.
     
  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    I would leave the existing trace alone. You could solder the half-dipole wire to the (right) end of the existing trace and shorten it by a couple of cm.

    I am using Flea-bay 315Mhz key-fob transmitters (that have a pull-up telescoping antenna) to operate my drive-thru gate and garage doors. Since my garage is actually an all-steel aircraft hangar, I mounted a 315Mhz dipole antenna on the outside of the building. I can operate the gate and garage doors reliably from about 300m with the transmitter inside a car...

    Long running thread on this topic...

    Another, this one with a picture of my external dipole antenna...
     
    Last edited: Jul 11, 2016
  6. Purclewan

    Purclewan New Member

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    Thanks Mike. Spent several days reading dozens of articles, PDFs, spec sheets and shortened my fingers at least a couple of mm googling all I could find on these key fobs but just got confused. On this forum I learnt more in an hour. Hmm!

    Still, good to know I'm not the only insane tinkerer out there playing with stuff from China.

    Purclewan

    PS VERY jealous of your "shed". I collect old tractors and various cars, motorcycles and other things that move. Never have enough space or storage.
     

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