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InterCom / Door Phone

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Non89, Nov 11, 2010.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A cell phone (you call it a handphone) has a radio output power of up to 2W. When it is close to an unshielded electronic circuit then the circuit recifies the signal and produces high currents which can destroy it. A tiny piece of wire makes a good antenna at the microwaves frequencies from a cell phone.

    Shielding a circuit from microwaves at 2W needs a certain education about the extremely high frequencies.
    The circuit must be made on a pcb with a compact parts layout. A breadboard will cause all kinds of trouble.
    The pcb must be installed in a metal case connected to 0V.
    The mic must have shielded audio cable. Maybe the mic needs a shield around it.
    The volume control must also be shielded.
    The speakers might need to be shielded and use shielded audio cables.
     
  2. Non89

    Non89 Member

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    but when i speak closer, the IC temperature also increased....

    i going to start PCB work at next monday ....
    the pcb necessary installed in metal case ? how about wooden case ?
    i need to buy audio cable or 2wire shielded cable to connect the speaker with IC?

    i not understand how to shielded the mic and volume controller .....
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Are you a dragon and you breathe fire?? Then cool down with some beer.

    A wooden case will not block the powerful radio signal from the cell phone.
    Maybe you can use aluminum foil as a shield inside the wooden case. Connect the foil to 0V.
    Maybe. Try the circuit with just a twisted pair of wires that might be a good enough shield.

    Metal mesh woven with small spaces and covering the mic will pass audio but block radio waves when connected to 0V.
    The wires feeding the volume control might need to be a shielded audio cable. Connect the metal case of the volume control to 0V.
    Since the radio waves from a cell phone are tiny microwaves then the wires to 0V must be very short.
     
    Last edited: Mar 10, 2011
  4. dave

    Dave New Member

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  5. Non89

    Non89 Member

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    the aluminum foil need to fully cover the PCB?

    the twisted pair of wires need to remove the covered plastic 1st before twist togather?

    the volume control you mean is pin4 from IC?
     
  6. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    No, just completely surrounding it (line the box with aluminum foil).

    No, they should look like this:
    [​IMG]

    No, I believe he means the metal case of the potentiometer.

    Sorry to jump in on your explanations, AG. Is this what you meant?
     
    Last edited: Mar 10, 2011
  7. Non89

    Non89 Member

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    that not just the meaning?
     
    Last edited: Mar 10, 2011
  8. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    You do not want to literally cover the PCB with aluminum foil directly. That will probably cause shorts that could damage your components. You do want it to be completely surrounded by sheets of foil, but not touching them.
     
  9. Non89

    Non89 Member

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    mean the foil no need seal off the PCB or box ?
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Cover the inside of the wooden box with aluminum foil. Also cover where wires enter the box and leave. Connect the foil to 0V with a very short wire then it will be a very good shield to radio waves.

    DO NOT connect the shield to pin 4 of the IC that is the volume control pin.

    Aren't you using the volume control shown on the schematic of the intercom? It does not use the pin 4 volume control on the TDA7052A because it was designed for the TDA7052 that never had an internal volume control.
     
  11. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    The foil needs to be completely lining the inside (or outside) or the box, but cannot be touching the PCB. Does this make sense?
     
  12. Non89

    Non89 Member

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    ya... i completely lining the inside the box and seal off the PCB not just the same ? it wont touching the PCB
     
    Last edited: Mar 10, 2011
  13. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Okay, then that should work just fine.
     
  14. Non89

    Non89 Member

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    but still need wait audioguru reply , made it after PCB done and function well
     
    Last edited: Mar 10, 2011
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The pcb should be mounted in the center of the wooden box that is lined with aluminum foil. The foil is connected with a short wire to 0V so it is a shield. Do not let any conductor on the pcb to touch the foil.
     
  16. Non89

    Non89 Member

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    link the aluminum to the battery 0V ?
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    0V on the circuit board is the ground common connection for the entire circuit. You want all shields to connect to it. The 0V from the battery is also connected to the 0V on the circuit board.
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Why do we have 4 months (about 122 days) and 156 posts about this extremely simple circuit??
     
  19. Non89

    Non89 Member

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    here is my PCB design , and the DTDP switch connection correct or not ?

    once i trying the circuit , the circuit not function and the temperature of TDA7052 in master circuit continue increasing .....
     

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  20. Non89

    Non89 Member

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    i will double check my component and try it again if the PCB design have no problem
     
  21. Boncuk

    Boncuk New Member

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    This is a typical example of "How not to design a PCB".

    You're using the wrong layer for the traces. Using the bottom layer (16) for traces and the tPlace layer (21) for components you'll obtain a "look through the board" image. Using the top layer for traces you'll have to solder components from the component side, which is impossible since the solder pads will be obstructed by the part itself.

    Using a double sided board with plated through holes the board is easier to populate and solder.

    Further there are lots of acute and odd angles in the traces which will most likely lead to unsatisfactory etching results.

    Last not least there are components overlapping each other from their size. How are you going to place those properly?

    Why didn't you refer to my PCB design? I guess the PCBs are also smaller than yours.


    Boncuk
     
    Last edited: Mar 23, 2011

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