Coherers were devices used in some of the very earliest radio experiments in the 19th century. Consisting of a tube filled with metal filings with an electrode at each end, the coherer would begin to conduct when in the presence of radio frequency energy. Physically tapping the device would then loosen the filings again, and the device was once again ready to detect incoming signals. [hombremagnetico] has designed a basic 3D printed version of the device, and has been experimenting with it at home.
It’s a remarkably simple build, with the 3D printed components being a series of three brackets that combine to hold a small piece of plastic tube. This tube is filled with iron filings, and electrodes are inserted from either end. Super glue is used to seal the tube, and the coherer is complete.
The coherer can easily be tested by measuring the resistance between the two electrodes, and firing a piezo igniter near the tube. When the piezo igniter sparks, the coherer rapidly becomes conductive, and can be restored to a non-conductive state, or de-cohered, by tapping the tube.
When you think of an old radio it’s possible you imagine a wooden-cased tube radio receiver as clustered around by a 1940s family anxious for news from the front, or maybe even a hefty 19-inch rack casing for a “boat anchor” ham radio transmitter. But neither of those are really old radios, for that we must go back another few decades to the first radios. Radio as demonstrated by Giulielmo Marconi didn’t use tubes and it certainly didn’t use transistors, instead it used an induction coil and a spark gap. It’s a subject examined in depth by [The Plasma Channel] and [Blueprint], as they come together to build and test a pair of spark gap transmitters.
This is a collaboration between two YouTube channels, so we’ve put videos from both below the break.They both build simple spark gap transmitters and explain the history behind them, as well as running some tests in RF-shielded locations. The transmitters are fairly crude affairs in that while they both use electronic drives for their induction coils they don’t have the resonant tank circuitry that a typical early-20th-century transmitter would have had to improve its efficiency.
They are at pains to remind the viewer that spark gap transmitters have been illegal for nearly a century due to their wideband interference so this is definitely one of those “Don’t do this at home” projects even if it hasn’t stopped others from trying. But it’s still a fascinating introduction to this forgotten technology, and both videos are definitely worth a watch.
We all know the saying: cheap, fast, or good — pick any two. That rule seems to apply across the spectrum of hackerdom, from software projects to hardware builds. But this DIY Tesla coil build might just manage to deliver on all three.
Cheap? [Jay Bowles]’ Tesla coil is based on a handheld bug zapper that you can find for a couple of bucks, or borrow from the top of the fridge in the relatively bug-free winter months. The spark gap is just a couple of screws set into scraps of nylon cutting board — nothing fancy there. Fast? Almost everything needed to build this is stuff lying around the house, and depending on the state of your junk bin you may not even have to order the polypropylene caps [Jay] recommends. Good? That’s a relative term, of course, and if you define it as a coil capable of putting out pumpkin-slaying lightning bolts or playing “Yakkity Sax”, you’ll likely be disappointed. But there’s no denying that this Tesla coil looks good, from its Lexan base to the door-pull top load. And running off a couple of AA batteries, it’s safe to use too.
[Jay] put a lot of care into winding and dressing the secondary coil neatly, and the whole thing would look great as a desktop toy. Not into the winding part? You can always etch a PCB Tesla coil instead.
The Wimshurst machine is one of the oldest and best known electrostatic machines, consisting of its iconic two counter rotating disks and two Leyden jars. Most often you see someone hand cranking it, producing sparks, though we’ve seen it used for much more, including for powering a smoke precipitator for cleaning up smoke and even for powering a laser.
It works through an interesting sequence of events. Most explanations attempt to cram it all into one picture, requiring some major mental gymnastics to visualize. This often means people give up, resigned to assume these work through some mythical mechanics that defy a mortal’s ability to understand.
A Jacob´s ladder is a favorite project of high voltage enthusiasts. It makes a visually attractive and fun display of a high voltage electrical arc climbing a pair of electrodes. [Keystone Science] shows us how to make a Jacob´s ladder that runs on 9 V batteries.
The ladder itself is pretty easy to make. It is nothing more than a pair of stiff wires in a V shape, connected to a high voltage power supply. The more difficult part is the HV power supply. [Keystone Science] explains how to build one using a flyback transformer from an old CRT tv and a few other components. It is a pretty simple circuit and can be powered by a 9 V battery. The ladder works because, when HV is applied to the electrodes, an arc is established at the bottom, where they are nearest each other. The arc is at high temperature so the air rises, and the arc starts to climb the ladder. Since the electrodes are further away from each other as the arc rises, at a certain point the distance is too large to sustain the arc and the process repeats.
This is a nice weekend project if you want to try it. In case you don´t want to make your own HV power supply, you can try another ladder project that uses a commercial one.
I work a lot with high voltages and others frequently replicate my projects, so I often get asked “What voltage is needed?”. That means I need to be able to measure high voltages. Here’s how I do it using a Fluke high voltage probe as well as my own homemade probe. And what if you don’t have a probe? I have a solution for that too.
How Long Is Your Spark?
The simplest way to measure high voltage is by spark length. If your circuit has a spark gap then when a spark occurs, that’s a short-circuit, dumping all your built up charge. When your spark gap is at the maximum distance at which you get a spark then just before the spark happens is when you have your maximum voltage. During the spark the voltage rapidly goes to zero and depending on your circuit it may start building up again. The voltage before the spark occurred is related to the spark length, which is also the spark gap width.
The oscilloscope photo below shows this changing voltage. This method is good for a rough estimate. I’ll talk about doing more precise measurements when I talk about high voltage probes further down.
Taking a break from his book, “How to Gain Enemies and Encourage Hostility,” [FPS Weapons] shows us how to build our own handheld EMP generator which can be used to generate immediate dislike from anyone working on something electronic at the hackerspace.
The device is pretty simple. A DC source, in this case an 18650 lithium battery cell, sends power to an “Ultra High Voltage 1000kV Ignition Coil” (as the eBay listing calls it), when a button is pressed. A spark gap is used to dump a large amount of magic pixies into the coil all at once, which generates a strong enough magnetic pulse to induce an unexpected voltage inside of a piece of digital electronics. This usually manages to fire a reset pin or something equivalent, disrupting the device’s normal operation.
While you’re not likely to actually damage anything in a dramatic way with this little EMP, it can still interrupt an important memory write or radio signal and damage it that way. It’s a great way to get the absolute shock of your life if you’re not careful. Either from the HVDC converter or the FCC fines. Video after the break.