How to Make a Human Crossbow

Say you have a team of French engineers, a lake in the summer, a wizened old machinist, and some gigantic bungee cords. What would you build? The answer is clear, a human-launching crossbow. (Video, and making-of embedded below.)

the-human-crossbow-how-we-made-it-kim41mdcizymp4-shot0001You can start out watching the promo video because it looks like a lot of fun, but don’t leave without watching the engineering video. What looks like a redneck contraption turns out to be painstakingly built, and probably not entirely a death trap. The [Rad Cow] team even went so far as to purchase metal cart wheels.

Everyone else on the Intertubes would tell you not to do this at home. We say go for it. That is, draw up reasonable plans, work with an obviously competent machinist, and make something silly. It’s not going to be more dangerous than the stuff that [Furze] pulls off.

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What Can Happen When You Do Try This At Home

In somewhat of a countdown format, [John McMaster] looked back over the last few years of projects and documented the incidents he’s suffered (and their causes) in the course of doing cool stuff.

[John] starts us off easy — mis-wiring and consequently blowing up a 400V power supply. He concludes “double-check wiring, especially with high power systems”. Other tips and hazards involve situations in which we seldom find ourselves: “always check CCTV” before entering the experiment chamber of a cyclotron to prevent getting irradiated. Sounds like good advice.

hotplate[John] also does a lot of IC decapping, which can involve both heat and nasty acids. His advice includes being ready for large spills with lots of baking soda on hand, and he points out the need to be much more careful with large batches of acid than with the usual smaller ones. Don’t store acid in unfamiliar bottles — all plastics aren’t created equal — and don’t store any of it in your bedroom.

The incidents are listed from least to most horrible, and second place goes to what was probably a dilute Hydrofluoric acid splash. Keyword: necrosis. First place is a DIY Hydrochloric acid fabrication that involves, naturally, combining pure hydrogen and chlorine gas. What could possibly go wrong?

Anyway, if you’re going to do “this” at home, and we know a bunch of you are: be careful, be protected, and be prepared.

Thanks [J. Peterson] for the tip!

Hacking Diabetes Meters, Towards an Artificial Pancreas

We’ve covered a number of diabetes-related hacks in the past, but this project sets its goals especially high. [Tim] has diabetes and needs to monitor his blood glucose levels and administer insulin accordingly. As a first step, he and a community of other diabetics have been working on Android apps to log the data when combined with a self-made Bluetooth re-transmitter.

But [Tim] is taking his project farther than previous projects we’ve seen and aiming at eventually driving an insulin pump directly from the app. (Although he’s not there yet, and user input is still required.) To that end, he’s looking into the protocols that control the dosage pumps.

We just read about [Tim] in this article in the Guardian which covers the diabetic-hacker movement from a medical perspective — the author currently runs a healthcare innovation institute and is a former British health minister, so he’s not a noob. One passage made us pause a little bit. [Tim] speaks the usual praises of tech democratization through open source and laments “If you try to commercialize [your products], you run up against all sorts of regulatory barriers.” To which the author responds, “This should ring alarm bells. Regulatory barriers are there for a reason.”

We love health hacking, and we’re sure that if we had a medical condition that could be helped by constant monitoring, that we’d absolutely want at least local smart-phone logging of the relevant data. But how far is too far? We just ran an article on the Therac-25 case study in which subtle software race conditions ended up directly killing people. We’d maybe hesitate a bit before we automated the insulin pump, but perhaps we’re just chicken.

The solution suggested by [Lord Ara Darzi] in the Guardian piece is to form collaborations between patients motivated by the DIY spirit, and the engineers (software and hardware) who would bring their expertise, and presumably a modicum of additional safety margin, to the table. We like that a lot. Why don’t we see more of that?

Coke-Propane Rocket Blasts Off Without Ignition

Everyone’s seen the Diet Coke and Mentos “experiment” that ends in a brown eruption. But have you seen the Coke and Propane experiment insanity that results in a rocket launch? As [Itay] pointed out when he sent us the tip, this doesn’t need to be lit. The simple act of turning the bottle upside down starts a powerful reaction without any ignition.

coke-propane-rocket-thumbOf course it’s the how of this that tickles our brains, but let’s finish the setup. This starts with a bottle of Coke which is about 3/4 full. The head space is displaced by spraying propane into the bottle; propane is heavier than air. All that’s left is to turn the bottle upside down and pray it doesn’t smack anyone in the noggin as it takes off.

In trying to find an explanation for this phenomenon we came across a plausible answer on the Chemistry StackExchange. It points to the Mentos phenomenon combined with the temperature differential caused by the very cold propane. The answering user theorizes that tiny ice crystals form and when the bottle is turned upside down the cold propane and micro crystals rise through the warmer soda acting as a much more rapid catalyst than Mentos alone. Of course this is just a theory so please share your own ideas below.

We thought the folks who microwave stuff outside of a microwave enclosure had their fill of danger but this videos is also one of theirs. It should be no surprise that they also tried the experiment with an ignition source. That video is found after the break and should immediately convince you to never try any of this yourself.

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Fully-Automatic CD Launcher Looks Dangerously Fun

When [JZSlenker] was challenged to find a creative way to destroy a bunch of compact discs that were burned incorrectly, he did not disappoint. He came up with a rather simple but fun contraption that launches the CD’s at high speeds and with a fast rate of fire. He doesn’t share many details about how this machine was built, but the 18 second video makes it pretty obvious how it works.

The CD gun is built mainly from a piece of plywood. This provides a flat base with which to mount the other components. A stack of compact discs is held in place by what appears to be a metal cage that was welded together. An inexpensive angle grinder is used as the propulsion mechanism. The grinding wheel is mounted just in front of the stack of CD’s in a vertical orientation. The wheel must be placed just high enough above the plywood base for a CD to fit in between the wheel and the base. This design is remarkably similar to the Sticker Gun which our own [Brian Benchoff] is building.

Some type of linear actuator is used as the firing mechanism. The actuator is hooked up to a thin piece of metal, cut into an L shape. It almost looks like a reaper tool. When a button is pressed, the actuator fires instantly. This pushes the metal hammer into the CD on the bottom of the stack. The CD is pressed forward into the grinder wheel which then shoots the CD into the air. Based on the below video, it looks like [JZSlenker] is able to fire at a rate of about three CD’s per second with this rig.

This has got to be a super-villain weapon for an upcoming movie, right? Maybe AOL-man?

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High Voltage: How to cook your goose in 62 easy steps

Prepare to learn. [Grenadier] has put together a collection of information about AC electricity that can safely be called a super-post. In 62 parts he covers a myriad of topics, some of them safe, many of them not so much. You may want to spend time reading through everything that he has to offer, but just in case you don’t, step one is a table of contents. In it you’ll find a listing of major points including transformers of every kind imaginable; from microwave ovens, neon signs, bug zappers, x-rays, and televisions. [Grenadier] covers the type of transformers that these items use, where to find them, and how to set up your own experiments. There’s plenty of pictures and several videos where the high-powered sparks fly. We feel like there’s enough here that we can be satisfied with vicarious AC interactions while safely in front of our monitor and far away from the heart-stopping action.

[Thanks Marcus]

High Voltage Etch a Sketch

What do you get when you mix a simple X/Y plotter, a Flyback transformer, and an unhealthy disregard for safety? Possibly the worlds most dangerous jumbo Etch a Sketch! [Kalboon] started off by making an imprecise X/Y movement device, similar to a CNC machine setup, but with less emphasis on precision. This rig is powered by some commonly salvagable materials, including an old scanner, a remote control car, and some hobby servos. We like this approach because most of these materials could be scrounged from a parts bin, surplus sale, or craigslist for little to no actual cost. The flyback transformer comes from an old TV or monitor, though if you have common sense safety concerns, we would recommend just mounting a dry erase marker and a dry erase board to substitute out the high voltage bits. For people wanting a low cost introduction project to making a CNC or Makerbot style build, this isn’t a bad place to start.