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

Thread Starter

NFA Fabrication

Joined Aug 12, 2012
112
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:



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?
 

MrChips

Joined Oct 2, 2009
30,707
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.
 
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Thread Starter

NFA Fabrication

Joined Aug 12, 2012
112
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!
 

Thread Starter

NFA Fabrication

Joined Aug 12, 2012
112
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.
 

Alec_t

Joined Sep 17, 2013
14,280
I just figured that the 555 would naturally pull the gate low in its "Off" state
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.
 

Thread Starter

NFA Fabrication

Joined Aug 12, 2012
112
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.
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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