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Beware of Motion Bricklet Ghosts


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The other night, my girlfriend woke me up in the middle of the night.. "The light just switched on", she whispered, with a serious tone in her voice. I was fully asleep just moments before, so it took a while before my brain absorbed what she said. "The light, the light is on.", she repeated, while I still didn't understand why she would wake me for a stupid light being on. But the light was, as she said, and without any shadow of a doubt, on.


A while back I created a simple "poor man's" burglar alarm by using a Motion bricklet, and a bunch of other things, plus a Raspberry Pi hooked up to a loud speaker for a real, convincing alarm. My stack also switches on a bedside lamp (using a Remote Switch bricklet) if and when, the Motion bricklet detects motion after 22:00, and before 08:00.


So it was past midnight, with both of us fast asleep, and my burglar alarm had triggered, because the light had switched on (the alarm didn't trigger any sound, because I had disconnected the speaker that evening).


"Are you kidding me? Did you switch on that light yourself?", I asked her, a bit grumpy. "No, no, the light just switched on, it woke me..." .. slowly, very slowly, it dawned on me (in the middle of the night) that a burglar could be downstairs while we were in bed, upstairs. I had to get to the bottom of this.


Much against my sense of well-being, and not relishing the idea of a midnight encounter with a burglar, I crept down the stairs, to the lower floor where I could look at my Mac's screen, and the log file of my stack. Sure enough, there it was on the screen: 10 minutes earlier the Motion bricklet had detected motion. My heart sank.. this was not a practical joke by my girlfriend! I had to go downstairs to ground level (where the sensor is placed), and check out the cause of the whole situation. We don't have guns or baseball bats in the house, so I armed myself with a screwdriver and a pair of scissors. And so I descended, prepared and armed to the teeth.




Nobody in the front room. ... Nobody in the kitchen... Nobody in the living room. All lights on... and still nobody.


Relieved that the lethal encounter hadn't happened, I walked back towards the stairs, and past the motion sensor, which is close to where my Raspberry is taped to the electricity meter. I noticed the tape holding the Raspberry had failed on one side, so the Raspberry was hanging from the remaining side of tape.. so I held the Raspberry and re-attached the tape, and went back upstairs to bed.


Then my penny dropped: the failing tape had caused the Raspberry to fall a few centimeters and rotate... it was the Raspberry that caused the Motion bricklet to trigger the alarm!!






Whatever you make with those Motion bricklets, be careful not to suffer any heart attacks because of a semantic mismatch between what your software is programmed to "think" in response to what reality is throwing at your sensor !!


Parts list:

1x Motion

1x Remote Switch

1x Rotary Poti (to set alarm severity level)

1x 7-segment Display (just for a little HH:MM clock)

1x RPI

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Nice story ;-)


I could imagine what you felt in that night, I also had such an experience before.


There is a reason why latest motion detection sensors not only use infrared but also a combination of two or even three technologies like (infrared, microwaves) to reduce false alarms. But I don't know if this would have helped in your case.

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