April Fools’ project teaches coworkers not to touch your stuff

april-fools-siren-box

Take one look at the sticker on top of this project box and it’s pretty clear you’re not supposed to flip the switch protected by the piece of red plastic. But if your coworkers are anything like [The Timmy's] there’s at least a few who will stop by and just can’t keep their hands off. He built this to teach those sorts a lesson. Flip the switch and a very loud siren starts blaring. The thing is, the 107db alarm can’t be turned off by the switch, you must have the key for the switch on the side.

The siren is a self-contained unit that just needs a power source between 6V and 15V. This makes the project quite simple, the only part that [The Timmy] really needed to think about was how to build a mechanical SR Latch (set-reset). The solution is to use a mechanical relay. The toggle switch connects the normally open connector to the common terminal to enable the relay. The key switch breaks the relay’s connection to ground, allowing the magnetic switch to open again.

If you need some help understanding how the relay connections work we’ve embedded an unrelated video after the break.

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Building a wooden air raid siren

siren

A while back, [Matthias] was working on a dust collector for his shop. Being the master woodsmith he is, he decided to build a dust collection system out of wood. Everything worked out in the end, but in creating wooden impellers and blowers, he discovered his creations made a lot of noise. For this project, instead of trying to quiet his blower, he decided to make one as loud as possible in the form of an air raid siren.

The basic idea behind [Matthias]‘ air raid siren is to make two impellers that force air through two stators along the perimeter of the rotor. As the siren spins, the air coming from the impellers is either blocked or passes through the stators, creating an alternating high and low pressure; to be more accurate, it creates a ton of noise. Stack two of these impellers together and you’ve got a two-tone air raid siren made out of wood.

For something that’s spinning very fast, we’re surprised [Matthias] didn’t have more problems with balancing his siren than he did. There are a few useful tricks to be picked up from his tutorial, though: balancing everything on a marble really seemed to help with the build.

As for how loud the siren is, [Matthias] can’t give us a decibel volume. From the video after the break, though, we can tell you it’s really, really loud.

Thanks [Dimitar] for sending this in. [Matthias], it’s alright if you send projects in yourself. You’ve earned the right to say, “I am so cool!”

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Donation meter raises alarm when not plugged with coins

This is something of a mandatory donation meter. If you don’t feed it with coins it sounds a very loud alarm continuously.

[Piet De Vaere] built the device for a free festival in Ghent, Belgium. The intent is to help raise awareness that although free of an admission price, the success of the event depends on donations. It works much like a parking meter. When you feed it coins time is added to the meter. When it runs all the way down that large loudspeaker on the right side of the case sounds the alarm.

In the video after the break [Piet] walks us through a demonstration, followed by a tour of the hardware. The pointer on the meter is a piece of cardboard connected to a servo. An Arduino board controls the servo, adding time in two-minute intervals whenever a coin enters the chute and passes by an optical sensor. There is no distinction between types of coins.

The use of a pizza box as a prototyping board shows that you don’t have to be fancy to build something neat.

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