Old becker with hands free phone

Discussion in 'Electronics' started by Mblinko, Aug 10, 2011.

  1. Mblinko

    Mblinko Hardcore MB Enthusiast

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    Has anyone tried to fit a bluetooth phone kit which assumes a mute wire is available from the back of the head unit? I have a period Becker 754 which has no such wire and i have read that the way to get round it is to put a relay into the accessory power feed so that when the handsfree kit tries to mute it cuts the power to the head unit. However, what relay should i use and how should it be wired up?
     
  2. Ted

    Ted MB Club Veteran

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    Hi Mblinko
    A vcouple of caveats.
    IF the BT kit supplies enough power to drive a relay, and IF the Becker is ok with having the power interrupted (it prolly is) and IF the BT kit has its own amp/speaker (as the becker will be cut off)

    Then a standard low current relay with contacts NC (or changeoover relay) from somewhere like Maplins should do the trick.

    Mute wire from BT kit to one winding of the relay coil, other coil connection to earth.
    Break the accessory feed to the Becker and connect one end of the wire to one side of the NC contact, and the other end of the wire to the other NC contact.


    Should work......
     
  3. Diesel Benz

    Diesel Benz Hardcore MB Enthusiast

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    How old is that radio? Are you sure it has no mute-input, it could be there even if there was no wire attached.

    Does the radio have a switched and a permanent power feed? If it has, it should be easier to switch it off from the switched power wire.

    Sounds like your intention is to install a specific HF kit speaker to the car. Why not install a relay to the radio speaker lines, after market HF kits do this often. Then you can use the car speakers for HF and at the same time you would "mute" the radio.
     
  4. OP
    OP
    Mblinko

    Mblinko Hardcore MB Enthusiast

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    Thanks, but there is such a choice of relays out there i'm a bit puzzled by things like what is the right voltage, gauge and the difference between NC and changeover types. The relays in the speaker lines sound most appropriate.
     
  5. bolide

    bolide Hardcore MB Enthusiast

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    You need two double-pole changeover relays. Wire the speakers through the NC (normally closed) contacts with the switched contact feeding the speakers. Wire the phone system to the other contact. Wire the phone trigger output via a 1 amp fuse to the two relay coils & earth the other side of the coils. Job done

    amp ------- \ (normally closed)
    aaaaaaaaaaa------------------------------- speaker
    phone -----

    phone trigger ----- relay coil ------- earth


    Nick Froome
    the independent Mercedes Estate specialists
     
    1 person likes this.

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