I made a motion detection worklight, Still has 1 issue...

Discussion in 'The Projects Forum' started by NFA Fabrication, Oct 28, 2014.

  1. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    I made a 10W LED work light to go over my toolbox at work that is triggered by a PIR motion detection sensor. I used a 555 timer (Set up as re-triggerable) to make the light turn on with motion sensing, but turn off after a bit if no motion is sensed. It basically works as designed, but if it has been hooked up to my "12V" source for a while, the LED will glow at about 5-10% when it should be in the "Off" mode., it will still turn on fully when motion triggered. I think my IRF 540N Mosfet is not fully turning off, and I don't know why.

    I am a novice electronics hobbyist, and designed the circuit myself, so there may just be a blatant mistake I am over looking. Here is my diagram:

    [​IMG]

    The LED and driver combo I have used a bunch with no issues, and obviously power is getting to it to make it glow a bit. Any ideas?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Try adding a 1k to 10k-ohm resistor from the gate of the MOSFET to GND.

    If that doesn't work, try a 1k-ohm resistor from the 555 pin-3 to the gate followed by 1k-ohm from the gate to GND.
     
    Last edited: Oct 28, 2014
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  3. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    By the time you replied, I had tried a 1K from the Gate of the Mosfet to ground, with no change. So I added the 1K in line from pin 3 on the 555 to the gate as you suggested, and this solved my problem! I just figured that the 555 would naturally pull the gate low in its "Off" state, so I am having a hard time understanding why the resistors were needed? I get that they make it easier to pull the gate low turning off the mosfet easier, I just thought the 555 would be able to do it on its own. Does that make any sense? Thanks again MrChips!
     
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  4. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    Well, I had it hooked up for a while today, and it is still doing it, so I am beginning to think I have a bad IRF540N, I think I am going to replace it and see if the problem persists.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,804
    1,105
    It should do. Perhaps the pull-down transistor in the 555 is damaged? 470uF being discharged repeatedly into pin 7 without any current-limiting resistor may have upset it.
     
  6. NFA Fabrication

    Thread Starter Member

    Aug 12, 2012
    104
    3
    I socketed the 555 just in case, and it does the exact same thing with a new one installed (I didn't even think about the repercussions of using such a large capacitor, I am fairly new to all this). I built another complete identical system using all of the same components/diagram since, and it is not having any issues, so I am pretty sure I have a bad component. I meant to bring the light back home from work to try and swap out the IRF540N, but I forgot, I will try Monday.
     
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