Half a croissant, on a plate, with a sign in front of it saying '50c'
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Solar powered pointless loud bang device

Because.
  (+7)
(+7)
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THe unit consists of a sturdy tripod base about 150 mm high. A photovoltaic panel is mounted on one of the legs in a weather proof enclosure feeding a simple DC-DC convertor capable of producing high voltage . In the centre of the tripod is a threaded receptacle into which the neck of a standard plastic drink bottle can be screwed. There is also an upstand pipe about 300mm long which conects to the receptacle via a U-bend and a small metal orifice block. Two stainless steel electrodes protrude from the centre of the cap; at the top, they are curved toward one another, and sharply pointed.

To use, take an empty drink bottle and fill it to the brim with water. A few soda crystals will help the operation of the device but are not essential. Screw the bottle into the base, then invert the whole assembly. Stand the unit in a sunny spot with the solar panel facing the direction of maximum solar radiation. Then just leave it.

As the PV panel absorbs radiation, the electrical energy will electrolyse the water and thin streams of hydrogen and oxgen will rise. The bottle will slowly fill with a stoic H2/O2 mixture, and water will be expelled from the upstand tube keeping the bottle very slightly above atmospheric pressure.

Eventually, the gas pressure will force out all the water and the current will electrolyse the last drops of moisture around the electrodes. At this point, a suitably sized capacitor connected across the electrodes will start to charge from the convertor. When the breakdown voltage is reached, a spark will strike between the electrode points and the gas mixture will detonate*. The overpressure will be unable to escape rapidly via the tiny hole in the orifice block and thus the bottle itself will fail catastrophically, making an extremely loud noise.

*Neither burn nor deflagrate; an actual detonation.

8th of 7, Apr 02 2019

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       Thank you
pocmloc, Apr 02 2019
  

       This device is susceptible to improvements.   

       For one thing, the device should be capable of repeated unattended operation. This will be tricky to accomplish and you're going to have to rely on rainwater. For the containment, instead of a one-use-only bottle, you could use a very robust, wide U-bend, contrived such that the residual water contains the gases sufficiently for detonation, but can be blasted out without permanent damage to the apparatus*. For another, the device should be capable of concealment up a tree or elsewhere. For yet another, some additional electronics should ensure that it detonates between 3am and 5am, perhaps only every 10 or 12 days on average (and, within these parameters, also at random).   

       Suitably placed, and in a location where echoes from large structures can make the source of the sound difficult to place (especially at 4am), such a device could cause considerable concern to large numbers of people.   

       [*on the downside, this turns your prank into what is legally known as a pipe bomb]
MaxwellBuchanan, Apr 02 2019
  

       I worked for a cola distribution company once and we used to see how many wraps of cellophane around the CO2 tank outlets would produce the loudest boom when threaded with an empty 2 liter bottle on it... and then of course the optimal amount of valve turn-age to scare the maximum amount of coworkers. <side-story>
I also once worked at a balloon printing factory and got to witness what happens to a weather balloon when filled with air for as long as my hands could hold at twenty-something and then get to watch it bounce around the inside of a massive warehouse for like thirty or forty seconds when I couldn't hold it anymore. You can't buy that shit.
<wait... one more>
  

       Ooh... and if you cut the ass end off an empty CO2 cartridge and ream it out with a chain-saw file wide enough to accept the heads of strike-anywhere wooden match heads, stuff a sparkler up its butt and aim it just right, it'll take most tanks off their treads. Pretty sure looking at the damage anyway. Damn thing went through three cross-braced 2x10's and was picking up speed.   

       well ... that was cool ...   

       Science! Yay!   

       Things that make you go BOOM!
UnaBubba, Apr 03 2019
  

       I work on tanks. It would take a lot more than a CO2 cartridge to take one out.   

       For me, the original fun was a few strips of aluminum foil inside a sealed 2-liter bottle with some HCl toilet bowl cleaner.   

       That was a very loud boom.
RayfordSteele, Apr 03 2019
  

       [+], and I've only read the title so far.
notexactly, Apr 03 2019
  
      
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