Intruder Alarm McDonald’s Toy Hacking

[malikaii] needed to set up some kind of tripwire style alarm system for his office. His bosses kept sneaking in to find him slacking. So, like any loyal hacker, instead of just working harder he built an alarm system. After a failed attempt to recreate an IR alarm circuit he found on the web and built from old appliance parts, he found the Hack a Day article about harvesting McDonald’s toys. The end result was a fully functional IR detecting alarm for the office doorway. This is pretty simple really, the best kind of hack.

McLVDT: A Straw-based Sensor

We saw [Kevin’s] home-built Linear Variable Differential Transformer in a YouTube video last week and wanted to know more. We’re in luck, he agreed to share all the details as well as a bunch of information on these sensors. An LVDT is used to measure distance along a straight path. Unlike a linear optical encoder, this method uses measurements of inductance between two electrical coils to judge the distance.

[Kevin] used some magnet wire wrapped around two straws of different diameter to fabricate his sensor. A signal generator is connected to the primary coil and the resulting signal induced in the secondary coil is measured to reveal the change in physical position. Check out the video after the break to see the results.

It’s not hard to get your hands on a McDonald’s straw (hence the name ‘Mc’LVDT), a smaller inner straw, and a few feet of magnet wire. This will be a fun one to try when those dark winter days start to get to you.

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