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LED 101 - light when a plug is inserted

Discussion in 'Electronic Basics' started by anonymous, Oct 7, 2004.

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  1. anonymous

    anonymous Guest

    Here's what Im hoping to accomplish:

    I have a +9V lead and a ground going to a plug. I want to light the LED when
    the plug is inserted. The load on the plug will vary from 0 to ~100 mA.

    Is this (relatively) straightforward or am I thinking foolishly?
  2. anonymous

    anonymous Guest


    my first thought was to run a line parallel to the 9V connected to the (+)
    plug lead with a 350Ohm resistor. I don't think this would work as all the
    current would go through the open line. What if I put a 90K resistor on the
    open line? Seems like this would give me ~100mA to my plug and ~20mA to my

    am I thinking on the right track?
  3. John Fields

    John Fields Guest

  4. anonymous

    anonymous Guest

    I must be too ignorant to understand your explanation properly. It sounds
    like this would always be on regardless of the state of the plug (putting
    the LED + resistor across the load) -

    On further consideration I think I have a problem in that the current draw
    for each possible output device is unknown and variable. I can't rely on
    100mA of current being pulled through the circuit. Some of these devices
    only need 5-10mA. This would give me LEDs of variable brightness. Based on
    my naive understanding it almost sounds like I need a digital circuit that
    would "pull high" when any current flows through the line and then all the
    way off when current stops.
  5. Randy Day

    Randy Day Guest

    What John means is put the LED in parallel with the
    circuit being powered; am I correct in assuming you
    want an LED mounted, say, in the *outlet* - that
    lights when something is plugged into it?

    If you have a spare connection on the plug and
    socket you plan to use, you can do it simply:

    Socket Plug
    | |
    +V-------| ---|------o------ +V to circuit
    | | |
    o----------| ---|------o
    | | |
    | -V-----o-| ---|------------- -V to circuit
    | | | |
    |-----| |
    .-. |
    R | | |
    | | |
    '-' |
    | |
    | |
    LED V |
    - |
    | |
  6. anonymous

    anonymous Guest

    What John means is put the LED in parallel with the

    I can't change the wiring of the plugged in device. It would simply complete
    the connection from +9 to ground with a varying load / current draw.
  7. Randy Day

    Randy Day Guest

    anonymous wrote:

    Can you arrange it so a small SPST switch gets
    pressed when you plug something in?

    Socket or "module" Plug
    .------. .-----.
    | | | |
    +V-o-------- | -------------
    | | | | |
    o--|--o| | | |
    | |==== | |
    o--|--o| | | |
    | | | | |
    -V---------- | -------------
    || '------' '-----'
    || R
    || ___
    | |
    | V LED
    | -
    | |
    (created by AACircuit v1.28 beta 10/06/04

    Can you create a small module to plug in between the
    present socket and plug to do this?

    If you're looking for something to sense a voltage
    drop or current surge when a load is plugged in,
    you're looking for something quite complex and likely
    to be unreliable.
  8. anonymous

    anonymous Guest

    Can you create a small module to plug in between the
    Ack. Ok. Ill put this one down.

    TY for the input.
  9. John Fields

    John Fields Guest

    I assumed (silly me...) that the load was in the device into which the
    plug would be inserted. However, it now seems like the load is going
    to be portable and connected across the plug.

    Probably the easiest/cheapest way to do what you want to do would be
    to put a small resistor in series with the return to -V then use a
    voltage comparator to sense the drop and turn on the LED.
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