Transistor in Parallel with a Mechanical Switch?

Discussion in 'General Electronics Chat' started by zygomatic, Sep 5, 2010.

  1. zygomatic

    Thread Starter New Member

    Sep 5, 2010
    Is it kosher to put a mechanical switch in parallel with a transistor


    I would like to add digital control to an existing momentary switch. Would this work to enable digital control and also have the normal switch work? Would I need any protection for the transistor? This is actually for the power locks on my car doors. Thanks all!
  2. kubeek


    Sep 20, 2005
    yes this is absolutely correct.
  3. zygomatic

    Thread Starter New Member

    Sep 5, 2010
    Thanks kubeek, I guess I earned that BSEE after all :)
  4. Ghar

    Active Member

    Mar 8, 2010
    If this switch is part of an existing system there may be some more complication.
    Specifically the referencing could get weird because the transistor must be electrically in parallel with the mechanical switch and your logic drive signal must essentially be in parallel with base-emitter.

    If the schematic is accurate then there is no problem because the emitter is connected to the common.
    If not you may need to look into optoisolators.

    It actually might be a good idea to do that regardless, since it could prevent some of those hard to predict issues with grounding.
  5. Jaguarjoe

    Active Member

    Apr 7, 2010
    If you're switching a solenoid, you'll need a back emf diode across its coil.
  6. zygomatic

    Thread Starter New Member

    Sep 5, 2010
    Ghar, good points.

    I'm not too sure what I've all got going on in the system. I need to get out the DMM and probe around. I know there's a 4-pin connector that plugs into the door switch. The switch is your typical door lock, rocking momentary kind of switch with lock and unlock. I would assume the 4 pins are battery, ground, lock and unlock, but I haven't verified this yet. I like the idea of an opto regardless.

    My goal is to hook up a wireless key fob transmitter with a receiver board that I got for $10. It has 4 buttons on the fob which each control an output pin on the board. It would be good not to have any feedback on that. I might also add a microcontroller in there to interpret the signals so I could add more functionality such as holding a button for X seconds does Y, etc.