Help with a transistor

Discussion in 'The Projects Forum' started by unclebob, Sep 19, 2012.

  1. unclebob

    Thread Starter New Member

    Aug 3, 2012
    Good Afternoon all,

    Probably a question asked a million times before, but,

    I have a 12, switch 12v (0v) and 0v from my source,

    and have a requirement for a 12v, switch 0v (12v) and 0v in my device.

    where the switched 12v is normally 12v and goes down to 0v when active, and the device requires 0v normally, but 12v when activated.

    Now i know i can use a transistor as a switch to do this, but its been a long time since i played with them - about 12 years! so no idea what i need/ how to calculate my requirements to source the correct transistor.

    The transistor will carry no load as such, the device uses the 12v purely as a signal.

    Can anyone recommend me the correct type of transistor?
  2. MrChips


    Oct 2, 2009
    Hello Uncle Bob, your request is as clear as mud. I had to read your post multiple times then I had to read your other post to understand what you are asking.

    So basically you wish to reverse a 0-12V logic signal.

    All signals will carry a load. So I have to assume that the load is small (less than 1mA).

    This output will not go down to 0V but maybe close enough for your device. I don't know what is your device. If I knew what is your device we might be able to make a better circuit.
  3. unclebob

    Thread Starter New Member

    Aug 3, 2012
    Thanks MrChips,

    The two posts arn't actually related! (well not in my mind but i guess i could use a transistor to also resolve that problem!

    This is for use between my house alarm and a phantomlink (

    My house alarm is positive applied, whereby it will constantly provide 12v supply to the outside bell box when not ringing, and drop down to 0v when it does ring.

    The phantomlink however works the opposite, whereby if less then 5v is applied, it will assume all is normal, and when above 5v is applied, it will send an alert via its programmed method.

    I'm looking to put a transistor in between the alarm and phantomlink to allow me to use the device with the alarm.