Retain radio power after turning off the car

If anyone's interested, here's a simple circuit for a "retained power" feature that keeps your radio on after you turn your car off until you open your door (in reply to Jason's question). It also lets you get the power from your radio directly from your battery so you don't experience the voltage drop inducted by other current hogs in your car (like the radiator fan).

Just a simple spst 12v relay that can handle the radio current which is pretty low (I'd go with a 3 or 10amp unit).

If you want the "retained power" feature it's a bit more work, get a dpdt relay and use the second set of contacts to continue to supply the rely with current, You'll need a couple diodes too. Well, here's the circuit:

                                 ______________  no connection
    +12v bat   ______________ /
                                 ______________  to radio power

                                 ______________  no connection
           __________________ /
          |                      ______________  to door light switch
          |_____|\|____     2 - 50 volt (or more) 1 amp (or more) diodes
                |/|    |
     acc  ______|\|____|_____
    power       |/|          ) ||    The relay will turn on when the key is
                    12 v     ) ||    turned to the ACC or ON position and
                             ) ||    remain on until the car door is opened
                    coil     ) ||
                             ) ||
      ground    _____________) ||
From: Gary Marten
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