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December 27, 2013

Repairing a Motion Sensing Light

Filed under: Uncategorized — admin @ 2:48 pm

This repair almost falls into the “most repairs are easy” group but it took me a bit of extra trial and error to find the problem this time. Our home in Campbell River that we moved into in 2012 had a motion detector light at the front entrance that had never worked correctly. The sensitivity to motion was much too low, and the sensitivity adjustment made no sense; the sensitivity was poor and useless, since motion more than a few feet away was not detected, and the sensitivity was at a maximum at the middle of the adjustment range, and at a roughly equal minimum at the supposed max and min settings. Usual caution: before you try repairing such a light yourself, disconnect the AC power! Be safe.

This unit is probably a fairly typical motion sensor outdoor/porch light. It has an infrared LED transmitter and an IR field sensor, plus some amp and timer circuitry built around an LM324N quad op amp IC. It could easily be taken apart to get at the circuitry inside by removing 4 weatherproofing plugs on the back of the case and the Philips screws that had been hidden underneath them.

After dis-assembly, and a lot of cleanup of bugs etc. that had accumulated on the thing, and after trying a few other things that didn’t fix the problem, I eventually noticed something odd. The clue of the odd sensitivity adjustment setting result led me to look closely at the adjustment potentiometer, and the soldering looked odd near it on the PCB. On closer inspection, it was clear that the potentiometer was shorted across its two fixed terminals due to a lump of solder that must have come from a shoddy soldering job at the factory. The red line in the photo shows where the problem short was.

After fixing the excess solder causing the short circuit with a soldering iron and desoldering bulb, and checking with an ohmmeter, I tested the unit, and sure enough, problem solved. Lots of sensitivity now for detecting the neighbourhood bears who might come knocking on our door…

The unit was of brand name “Venes”, model MS 900, apparently manufactured in 1993 from the date code on it. Kind of makes sense because the house was built in ’94; this was probably the unit that was put up when the house was built. It probably has not worked correctly for 20 years, until now!
Season’s Greetings, and Happy Repairing!

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