April Fools’ Project Teaches Coworkers Not To Touch Your Stuff


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.

41 thoughts on “April Fools’ Project Teaches Coworkers Not To Touch Your Stuff

  1. Theres an instructable simmilar to this, they used a key switch but it only worked to turn it on and a reed switch was the method of turning it off, neat variation though.

  2. I did something similar except for I called it my curiosity killer.
    I made a plexiglass cube and used bonding solvent to embed stainless steel screen on all four sides that were isolated from each other.
    That amounted a small switch on all four sides and wire them all in series on the normally open contacts. So if any one of the switches was depressed it would open the circuit. I used a small 5 V relay wired as a buzzer and connected opposing sides of the cubes screens to the one side of the coil and the other side of the coil so you would have the reverse inductance of the coil across each side of the cube.
    The practical upshot is if someone picks up the cube off of a flat surface that would energize the coil of the relay and make and break the contact and the reverse inductance of the coil would generate upwards 50 V AC at about 80 to 90 Hz

    Let’s just say you’ll put it down pretty darn quick.

    1. that’s the problem with using a metal enclosure for my project. people are already careful around it as they think it might start shocking them or something.

      I can wire a relay as a buzzer, but the reverse inductance is a new concept to me. care to elaborate on how it is setup?

      1. If you wire a relay as a buzzer then all you need to do is run some wires from the coil connections to the outside of the case somewhere. I would suggest making the case out of two isolated pieces of metal so the shock will travel through just one hand and not across the chest.

        As for what is happening to generate the high voltage I’m not very good with explanations, so I searched around and found this.


        I feel that sums it up much better than I could. But you probably shouldn’t build one of these to leave lying around on your desk, because you’ll end up giving a heart patient a heart attack and sue you into oblivion or worse. So if you do build it and something bad happens, you’ve been warned and I accept no responsibly for your actions.

  3. I added a holding relay to the klaxon my college professor used to call us back into class after a break. He had to go trip the breaker to get it shut off, I actually got extra credit since we were studying electrical wiring and control systems. of course he took it back with I reversed the wiring on the classroom thermostat in the middle of the summer session and the furnace came on instead of the a/c, but that’s another story.

    1. should I decide not to work for my current employer anymore, I want to rig a relay in the same way, to the buzzer on the dock that lets people know a delivery is outside. the button is built into a box on the outside of the building that has regular screws and likely, lots of room inside.

  4. Holy Mother of Meatballs!

    Decades ago — my senior year in high school, in fact — I built something very similar to this except it used a momentary pushbutton as the trigger and a magnetic reed switch to disarm it. It was intended to prank a particular friend who would certainly be unable to resist pressing the button. Since his sixth period math class was right across the hall from my history class, that was the perfect time to leave it on his desk.

    Building the thing was a snap. A Radio Shack project box made a less impressive enclosure than the one shown above, but it was sufficient to keep anyone from easily opening it. A nice, big D-cell pack guaranteed hours of ear-piercing shrieking from a piezoelectric buzzer. Even closed up in the box, it was very, very loud.

    A simple label warned not to touch the button.

    My first morning class was English — I always went there straight after getting off the bus. I’d store the device in my locker for the rest of the day until sixth period.

    The English teacher walked into class that morning carrying two large bags filled with something lumpy. It meant nothing at the time to me, but I did notice it as unusual. She told us that exposure to new things could be a valuable asset for writing, and then asked a question that I shall never forget, because it immediately alerted me to danger, “How many of you know how to juggle?”

    Each bag contained about fifty tennis balls and it was her intention to give a room full of awkward, clumsy teenagers a lesson in creative writing by teaching them to juggle. I looked at the gadget on my desk and knew this would end badly. I couldn’t stuff it into my backpack — my design lacked the magnificent flip-top safety the one from this article possessed, and any pressure on the trigger would set it off. Our desks *did* have a shelf, though. One made of metal struts criss-crossing at about three-and-a-half inch intervals that could hold books and supplies under the top. So I nestled my gadget into the back corner and hoped for the best.

    Calculate the odds. About 25 kids plus one teacher, each armed with three tennis balls. The button was about a half-inch square and secured under a solid wooden surface. The teacher had an excellent grasp of creative writing and no experience teaching juggling. A single, plaintive neuron in my geeky brain insisted that I should pull out a screwdriver and disconnect the power supply. Naturally, the rest of my neurons told it to shut up.

    Within less than a minute of starting the lesson, a tennis ball shot past me and bounced off the floor, angling up for the *bottom* of my desk. That neuron suddenly managed to convince his neighbors that there really was a risk and time slowed down as that fuzzy sphere of doom — a bright green embodiment of elasticity and chaos that had no business in any civilized English class on the planet — headed straight for my buzzer. Surely it would miss! Surely it couldn’t possibly squeeze through the gap in the wire mesh barely larger than the ball itsel–


    And nearly eighty tennis balls dropped simultaneously to the floor as everyone else in that class instantly forgot them. I leaped to disarm it, but tripped over my chair and sent myself and the desk tumbling. On top of that desk was the disarm magnet.

    When I picked myself up, there was no immediate sign of the magnet. I was forced to stand there in the same sort of humiliation I had planned to inflict on my friend as I dug out a screwdriver and clawed my way into the box as every eye in the class — including the teacher — stared in disbelief at what an absolute wierdo I was.

    I’ve come to recognize the extraordinary contortions Murphy will go through to foil my plans. In hindsight, it should have been obvious what was going to happen that morning as soon as those bags of tennis balls entered the room.

    1. I have to say that your teacher has done better than she expected, that was a well-crafted hilarious story. Not to mention that the truth in it makes it even more funny. Now if you could furnish some plans, I’d like to give my electronic communication teacher a what-for. >;D

      1. yes, I agree! great re-telling of events.

        I’m curious how a magnetic reed switch can be used to shut off the device as any of those switches I have ever seen were normally open, and would close when exposed to a magnetic field, whereas the keyswitch I used in mine was kept in the closed position and would open when turned, removing power from the device long enough to return it to its off state.

        1. That’s exactly what a set/reset latch does – close one contact to ground and it closes the gate. Further closures of that contact do nothing.

          Close the -other- contact though (this would be the reed switch) and the gate would open.

          Use said gate to drive the buzzer.

        2. I’ve never seen an N.C. reed switch either, but I suppose they could exist. Or he could have osed an N.O. wired directly across the relay coils. The dead short would drop the current to the coil, resulting in the switch opening and the whole circuit de-energizing.

          Hey, it’s HACK-a-day, not Design-a-day.

    2. Straight out of an early 60’s Popular Electronics. I remember the letter next month about one left in school and it was found in the trash in pieces the next day.
      Now days; eeeeeee eeeeeee eeeeeee evacuate! Bomb detail.

  5. My knowledge at electronics sucks, but I want to build one of these. I’ll buy the parts on http://todoelectronica.com/ (Not spam, is the only online store that ships components to my country, dont worry about language, there is an english version of the site) and I have the box, the siren and the key switch, but I don’t know what kind of switch and relay I need. Can you help me?

    1. the relay is a DPDT, and the toggle switch can be a SPST– it doesn’t have to be a toggle switch either, it can be an ordinary button (“normally open” type). and here is the link to the wiring diagram that can be found on my page that talks about the device (listed in the article above): http://thetimmy.silvernight.org/pages/sirenbox/booby-trap-siren-schematic.svg

      one of the best things about this build is that it doesn’t use any battery life while waiting to be activated. so in theory, until switched on, it should be ready to go for as long as the shelf life of the battery itself.

      1. Please fix the link. I remember reading the article in Popular Electronics. The version that they built had flashing LED’s if memory serves me correctly.
        I’ve been trying to find the schematic recently in an effort to break my grandson of his habit of pushing/flipping switches.

        1. The link to my schematic? I just checked it– it works fine. This may be your browser not being capable of displaying .SVG files so directly. You can right-click on the link above and choose “Save Link/Target”, and see if any of your image viewing applications are capable of displaying its contents.

          It shows just fine for me when I click on it (firefox).

          1. Thanks. Long story – short version, I had to install “Inkscape” (Freeware) to open the file (I really love Microsoft). If anyone else has a similar problem, do NOT pay for one of those registry repair programs…it will not solve the problem.

    1. Ahh, but with an Ardwhatever, it could also have flashing LED’s, and the LED’s and the siren could increase in frequency with each subsequent button press, and it could also reset itself after a minute or so… and with a Bluetooth shield it could notify the printer to spew paper as well…or notify the PA system (tannoy) to broadcast “Core Meltdown in 15 minutes!” and shorten the announcement by two minutes with each subsequent button press.

  6. Hurting people is not funny. At 107 db permissible exposure time before possible damage to hearing is somewhere around 2 to 3 minutes on average.
    see for instance dangerousdecibels dot org
    (Of course those with less damaged hearing may suffer from less, and may combine with other noise exposure.)

    1. um… don’t hold it up to your ear?

      Loudness drops off pretty quickly with distance. Something like the inverse of the square… I think there’s a law about it, but I searched the Federal Register and came up with nothing.

      1. I guess I should also mention that being enclosed inside the metal project box, the siren isn’t anywhere near 107 dB when switched on. Drilling a few sound holes would help to fix that, but it does well as is to teach a lesson about keeping ones hands to oneself.

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