Automatically switch speakers between car radio and 8 track deck?

Discussion in 'The Projects Forum' started by 1964Fastback, Sep 17, 2014.

  1. 1964Fastback

    Thread Starter New Member

    Sep 17, 2014
    3
    0
    Hi All,

    First post here! I've got a project in mind for the old-school stereo system on a car I'm restoring. The stereo that's in it now is an older in dash AM/FM/cassette that I plan to keep there for the time being. I have an under dash 8 track deck that I'd like to use with the same rear speakers. (The in dash radio operates 4 speakers; the 8 track deck only the two rear ones).

    My thought is to use a 5pdt relay for this. Four of the poles would be for the + and - for the rear speakers. The fifth would be for the power line for the in dash radio. The rear speakers would be connected to the center contacts and the outputs from the radio would be to the NC contacts. The power to the radio would be on the center and NC contacts of the 5th pole. So in the unenergized state, the radio would play normally - it would have power and its rear speakers would be hooked up. Its front speakers would always be hooked up because they are wired directly to the radio. The outputs from the tape deck would be conntected to the 4 NO relay contacts.

    When the relay is energized, the rear speakers would be switched to the tape deck and power to the radio would be cut, automatically silencing the front, radio-only speakers. I'm pretty sure all this would work with an A/B switch. Where I need help is with the trigger for the relay. I'd like to be able to "sense" current going to the tape deck and use that to energize the relay. The tape deck normally doesn't draw any current at all when it's off. It's turned on by putting in an 8 track and turned off by pulling the 8 track out. My thought is to somehow use a transistor on the power line to the tape deck, to drive the relay but I don't know the specifics. I'm also wondering about other components that I'm overlooking. E.g. would I need diodes and/or caps somewhere to protect the radio or tape deck from spikes when switching back and forth, etc.?

    All this stuff is pretty low-power, too, by the way. I don't have the specs. in front of me but I would guess speaker output below 30W max, probably closer to 7-10W. No giant amps involved. Again, guessing, I believe the radio and tape deck would draw between 2-5A max.

    Any help on the triggering part of the circuit would be appreciated, as well as component selection/sources.

    Thanks!

    Pat
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,789
    945
    Hack the switch inside the 8 track that activates it when a cartridge is inserted. A low power 12 volt relay could be added to the activated pole on that switch and used in the way you have already described. one small wire routed to were ever you put the relay attached to the coil and the other side of the coil grounded. (appropriate fusing said wire is suggested to prevent an accidental short circuit damaging any of the 8 track components.)
     
  3. 1964Fastback

    Thread Starter New Member

    Sep 17, 2014
    3
    0
    Kermit,

    Thanks for the reply! That's a good idea I hadn't thought of. So 12V would go into and out of the NO points on the small relay, and from there to one side of the big relay's coil, with the other end of the big relay's coil grounded. For the small relay, one side of its coil would be grounded; the other would be coming from a hot-when-on point inside the tape deck. Maybe I could tap in where the power goes to the drive motor on the deck. Besides the fuse, you think I need anything to prevent any spike from the small relay's coil collapsing, back to the deck, or is that just negligible/overkill?

    I'd kind of got the idea from this expired patent: http://www.google.com/patents/US5007104 but couldn't really figure out what was going on with the switch part of the circuit. E.g. the two diodes and a resistor in parallel? And would this be substantially reducing the power going to the tape deck? Plus I wouldn't know what value for the resistor, what type of diodes, transistor, etc.

    Thanks,

    Pat
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,789
    945
    [​IMG]

    Here is my suggestion. I am not switching any power leads to the units involved. Just find the switch inside that activates the 8 track when a cartridge is inserted. Use a voltmeter and locate the pin which has voltage ONLY when the switch is activated. Add a wire at this point by soldering.
    Bring the wire out and attach to the 4 pole relay as shown.

    The rear speakers will be connected to the 8 track and disconnected from the rear outputs on the cassette player. The front speakers will not be affected, so you will have to turn off the cassette player or turn the volume down so that you cannot hear it.

    you could add a 500 mA fuse in the wire to the relay for safety if you desire.
    The relay will be a 4P2T, or a 4 pole double throw.
     
  5. 1964Fastback

    Thread Starter New Member

    Sep 17, 2014
    3
    0
    Okay, got it. I thought you were proposing to use two relays, one with very low current draw off the internal tape deck switch to then drive the bigger relay which would switch the speaker leads and possibly the power to the radio/cassette. Yes, I've noticed that 4P2T are much more common than 5P2T so if I wanted to control power to the radio I'd use a second relay with its coil driven in the same manner as the relay for the speakers.

    I'm still considering the external sensing circuit using a transistor as in the patent but if I don't get anywhere with that I am confident the above circuit will work.

    Thanks,

    Pat
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
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    a ot of the older am fm cassete radios hade a wire to run an antenna up. usually colored yellow, if I remember right, that could be used to activate a relay to trun on power and connect speakers to the external 8 track. the motorized antenna would be activated when the am fm cassette was turned on.
     
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