Need some help with a simple circuit

Discussion in 'The Projects Forum' started by whitemike, Dec 5, 2011.

  1. whitemike

    Thread Starter New Member

    Dec 5, 2011
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    0
    Hello everybody,
    I'm a beginner and just starting to read up and experiment with things.

    Basically,
    I'm creating a circuit to light an LED when a certain switch is NOT connected.

    I'm using a 12v source, and an LED equipped for such voltage. The problem I'm having is what to use to achieve this goal? A JFET? A relay? Can I use two standard FETs?

    Again, switch OFF = light ON. Switch ON = light OFF.

    Thanks for any and all help.

    Mike
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    It depends on whether the switch is controlling the ground side or the positive side of a load.

    If it's switching the ground side, you could use an N-ch enhancement mode MOSFET like a 2N7000. You could also use an NPN transistor.

    If it's switching the positive side, you could use a P-ch enhancement mode MOSFET like a BS170. You could also use a PNP transistor.

    In either case, if the load is inductive, you would need to use a resistor and a Zener to protect the gate of the MOSFET.
     
  3. whitemike

    Thread Starter New Member

    Dec 5, 2011
    3
    0
    Thanks for the reply.

    I would prefer the switch control the ground.

    Could I possibly bother you to demonstrate (verbally is fine) how I would go about achieving this goal? Keep in mind the light needs to be OFF when the switch is connected (aka supplying the gate?)

    Thank you!
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
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    What amount of current is the switch controlling?

    You could run an LED and resistor across the switch terminals, so the 12V power would light up the LED whenever the contacts were open,but that would allow 20mA of current through the switched circuit in the "OFF" position. If that is an acceptable condition, it would be the simplest solution.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Here, have a look at the attached.

    On the left, the LED is controlled by a MOSFET. The resistor and Zener diode make certain that if the load is inductive, the MOSFET Vgs limit of ±20v is not exceeded.

    On the right, the LED is controlled by a transistor.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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  7. whitemike

    Thread Starter New Member

    Dec 5, 2011
    3
    0
    Thank you all so much! I'm not just trying to get a working project, I'm trying to learn too, and this was a huge help.

    Thanks again,
    Mike
     
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