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12V Voltage Sensing Switch (?)

Discussion in 'Electronic Basics' started by Ben Lessani, Mar 7, 2004.

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  1. Ben Lessani

    Ben Lessani Guest

    Hi, I've been dabling with my car central locking, making my attempts
    to integrate the alarm into the system. When I lock the doors remotely
    with the key, the door pins go down (naturally), but when I press an
    additional button on the key, it deadlocks the doors. There is a wire
    (lets call it JOE) which is always at 12V unless the doors are
    deadlocked, when it is at 6V. I am not sure of the current running
    through this circuit, but I know its not powerful enough to switch a
    relay. I was thinking I could use a transistor type circuit to achieve
    what I need. I can use a relay in its opposite state, so when no
    voltage is supplied, the circuit is complete, but when a voltage is
    applied - it opens the circuit. I was thinking I could put a resistor
    from JOE to the BASE of a transistor, to reduce the voltage down to 6V
    as standard, then when the doors are deadlocked, the voltage will drop
    6V, taking the voltage at B to be 0V. I've drawn some diagrams below
    to try and help explain what I'm trying to say. The wire does not have
    enough current to close a relay, so I need a transistor to close the
    relay. But I want to resist the voltage input at the resistor using
    R1, so that it is normally at 6V, this means that when I deadlock the
    doors, and the voltage drops, the input at the transistor is 0V,
    leaving the 'switch' open, leaving the relay open.

    12V
    |
    State 1 (JOE AT 12V) |
    8 |--|-----
    8 | | RELAY CLOSED
    8 |--|-----
    C |
    /
    12V 6V |/
    ----| R1 |----B-|
    |\
    \
    E |
    |
    |
    GND

    ********************************************

    12V
    |
    State 2 (JOE AT 6V) |
    8 |--|-----
    8 | \ RELAY OPEN
    8 |--|-----
    C |
    /
    6V 0V |/
    ----| R1 |----B-|
    |\
    \
    E |
    |
    |
    GND



    Could you please email me with a response at



    Delete the Q's if you want to email me, everyone else is doing it, so
    I thought I might join in the fun :).



    Thanks V. much

    Ben Lessani
     
  2. These are series diodes that are normally used to shift voltage down. Then
    you can use an opto coupler for sensing presence of voltage to switch more
    powerful circuit.
     
  3. Ben Lessani

    Ben Lessani Guest

    I've put all this text in above, but what I'm trying to ask is whether
    this will work, and if it does, what transistor do I need (model
    number please), and what value resistor for R1 do I need?



    Thanks V. Much
    Ben Lessani
     
  4. Ben Lessani

    Ben Lessani Guest

    Whats the laymens term of this.
     
  5. Mikal Hodvik

    Mikal Hodvik Guest

    Ben,

    You're on the right track, but you really need a zener diode in series to
    the base, to get a crisp decision threshold. I suggest an 8.2V zener, with
    the cathode to JOE, and R1 at about 390 ohms. It's wise to shunt base
    leakage current to ground with something like 10K ohms. The transistor could
    be a TIP31, or something smaller if you're not using a power relay. Also add
    a reverse-biased rectifier diode (1N4001, etc.) across the relay coil, to
    prevent damage from inductive kickback!

    Mikal Hodvik
    Decade Engineering
    www.decadenet.com
     
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