Help creating a trigger...

Discussion in 'The Projects Forum' started by tester2014, Jul 20, 2014.

  1. tester2014

    Thread Starter New Member

    Jul 19, 2014
    Alright so my proejct is that a replaced a backlight on my lcd monitor with some 12V led strips. I couldn't find a 12V power on my LCD circuit board (didn't play too much with it though and won't [luckily it didn't fry]). Anyways, I am using a 12V power brick to power it.

    I am trying to create a trigger using the led on the monitor (the one that tells the user that its on [blue]). If that blue light is on. Turn on the backlight.

    The LED voltage is 2.75V. The led strip that I want to power is 12V (current about 600-650mA).

    I tried connecting the power button on the monitor to a ne555 and made a trigger switch but it didn't work because the circuit connected to the other circuit (circled in red).

    I also tried doing some mechanical modifications to the frame by making the power button plastic trigger my 'new button' and the original power button' but there wasn't enough room. If there is a way to make the original power button trigger the ne555 without affecting the lcd circuit (i wish the button had 2 switches rather then one) I would be golden.

    I also tried using a 5V relay by 'stealing' the LED voltage and stepping it up to 5V but that didn't work (I don't think it had enough current), as the led powered off and the relay didn't trigger. It did show 5V on my multimeter though.

    So I thought maybe some opto-isolators might work to trigger the ne555? I have some but don't know how they work (popped one by playing around... trashed now). I have 4n35 opto-isolators.

    Any ideas what I can do? I have no transistors... Ordering some but that will take a while.
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    Last edited: Jul 20, 2014
  2. simo_x


    Dec 23, 2010
    You can use an op amp Schmitt trigger comparator (positive feedback).
    Maybe just a comparator if the voltage is not "noisy". With a positive feedback you can better handle the threshold.

    However, 2.75 V is approximately the threshold voltage for a Schmitt trigger logical inverter (i.e. 74 HC) powered at 5 V.

  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    use an opto coupler,put the opto led in series with the blue led, so the opto will switch on when the blue led is on, use the transistor in the opto as your trigger.
  4. tester2014

    Thread Starter New Member

    Jul 19, 2014
    Ah yeah i thought i would have to use a transistor... =( only piece i have never bought yet .

    Not sure how an op amp would work never worked with one (don't have that on me either =(

    Anyways I will use this for future reference. I went with a 'drill a new hole and put in a new switch' as a light switch method. It's more crude I guess, but lets me use the lcd as a lamp too then lol.

  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Here is how i would cut into the blue led
    Last edited: Jul 21, 2014