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Thread: LED Flood Light - Need Help With Flicker

  1. #1
    Join Date
    Dec 2018
    Sebring, FL
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    Question LED Flood Light - Need Help With Flicker

    I know this isn't holiday light related, but since many people have LED experience, I figured I'd ask this question here...

    I have a nice all-in-one dual-head LED security floodlight (white led's only) that puts out a decent 2100 lumens of light. On the box it says "Switch controlled" and it works fine if I have it on a light switch. I wanted to add a motion sensor from a standard 120V floodlight so I modified the casing to mount the motion sensor. I attached the motion sensor wiring as it was in the standard floodlight to the leads that would normally wire into the house/switch power. The leads go into an power adapter that outputs 18-32V 800mA to power the dual-head LED lights. When I hookup the 120V leads and test the sensor, everything works fine. However, when the sensor powers off the lights, the LED lights flicker on and off every second at minimal power.

    I know this happens in cars, usually just a slight light feeding from the power trickling through the line, when you upgrade incans to LED bulbs, or dimmer switches that can't handle led lights properly. I know that on the led floodlight, the motion sensor has a capacitor that is charging just enough to make the LED's discharge it every second. I've seen it stated that a resistor of x wattage and ohms can stop the constant dim or flickering. My issue is I want to make sure I put in the right resistor to test this theory. I'm not an EE but have some basic knowledge (enough to get me in trouble probably). Here are my calculations to find the correct resistor:

    If I took the max output of 32V, then:

    32V / 40 Ohms = 0.8mA
    32V * .08mA = 2.56W

    So a 3W 40 Ohm Resistor should theoretically stop the discharge?

    EDIT: I wired this floodlight to a standard plug and ran it through my Kill-a-Watt. It showed 0.21A and 23.5W usage. I put the multi meter on the leads coming off the converter and it is putting out 12.35V. The 2 LED heads are connected in series to the DC converter (conv+ out to #1 led, - to + on #2 led, - to conv-). Wit this information, maybe a more accurate resistor can be calculated?

    I'm probably wrong, but who knows. I know I'm showing my led and resistor ignorance. I just want to get this floodlight working properly without going out and buying another LED floodlight with motion sensor since I have 3 of these I picked up a while back at a habitat for humanity store (they were brand new, got them for $5 each when retail in store was about $75). Any help or assistance from some of you board designers and EE geniuses would be most welcome. I know how to solder and connect everything, but with stuff like this I want to be sure I'm using the right parts so I don't burn the damn house down! Or just let me know that this won't work at all to fix the problem... but I know there is always a solutionl
    Last edited by AJR214; 02-13-2019 at 04:51 PM.

  2. #2
    Join Date
    May 2016
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    Default Re: LED Flood Light - Need Help With Flicker

    I’m not an EE, but that sounds very similar to a floating pin problem when using a micro controller and a switch or sensor. The fix for a floating pin is to add a pull down (or pull up) resistor to either pull the pin to low or pull it up to high (depending on which way the switch or sensor works). For example, you need to add a pull up resistor with some PIR sensors (“open collectors”) or they will give false movement readings.

    Another possibility is that you have a solder bridge somewhere in your wiring that is causing an intermittent flow of current to the relay. I’d probably check the wiring first.


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