New Circuits With Old Technology

Before the invention of transistors, vacuum tubes ruled the world. The only way to get amplification or switching (or any electrical control of current) back then was to use tubes. But some tube design limitations were obvious even then. For one, they produce an incredible amount of heat during normal operation, which leads to reliability issues. Tubes were difficult to miniaturize. Thankfully transistors solved all of these issues making vacuum tubes obsolete, but if you want to investigate the past a little bit there are still a few tubes on the market.

[kodera2t] was able to get his hands on a few of these, and they seem to be relatively new. This isn’t too surprising; there are some niche applications where tubes are still used. These have some improvements over their ancestors too, operating at only 30V compared to hundreds of volts for some older equipment. [kodera2t] takes us through a few circuits built with these tubes, from a simple subminiature vacuum tube radio to a more complex reflex radio.

Taking a walk through this history is an interesting exercise, and it’s worth seeing the ways that transistor-based circuits differ from tube-based circuits. If you’re interested enough to move on beyond simple radio circuits, though, you can also start building your own audio equipment with vacuum tubes.

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Fun With Negative Resistance: Jellybean Transistors

The concept of negative resistance has always fascinated me. Of course, a true negative resistance is not possible, and what is meant is a negative differential resistance (NDR). But of course knowing the correct term doesn’t do anything to demystify the topic. Negative resistance sounds like an unusual effect, but it turns out to be relatively common, showing up in places like neon lamps and a number of semiconductor structures. Now’s as good a time as any to dig in and learn more about this common principle.

NDR means a portion of a device’s I/V curve where the current falls with increasing applied voltage. The best-known semiconductor device exhibiting negative resistance is the tunnel diode, also known as the Esaki diode after one of the Nobel-Prize-winning discoverers of the quantum tunneling effect responsible for its operation. These diodes can perform at tremendous speeds; the fastest oscilloscope designs relied on them for many years. As the transistor and other technologies improved, however, these diodes were sidelined for many applications, and new-production models aren’t widely available — a sad state for would-be NDR hackers. But, all hope is not lost.

Rummaging through some old notebooks, I rediscovered an NDR design I came up with in 2002 using two common NPN transistors and a handful of resistors; many readers will already have the components necessary to experiment with similar circuits. In this article, we’ll have a look at what you can do with junkbox-class parts, and in a future article we’ll explore the topic with some real tunnel diodes.

So, let’s see what you can do with a couple of jellybean transistors!

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“Vintage” Radio Gets A Modern Makeover

Taking an old piece of gear and cramming it full of modern hardware is a very popular project. In fact, it’s one of the most common things we see here at Hackaday, especially in the Raspberry Pi era. The appeal is obvious: somebody has already done the hard work of designing and building an attractive enclosure, all you need to do is shoehorn your own gear into it. That being said, we know some of our beloved readers get upset when a vintage piece of gear gets sacrificed in the name of progress.

Thankfully, you can put your pitchforks down for this one. The vintage radio [Freshanator] cannibalized to build this Bluetooth speaker is actually a replica made to invoke the classic “cathedral” look. Granted it may still have been older than most of the people reading this right now, but at least it wasn’t actually from the 1930’s.

To start the process, [Freshanator] created a 3D model of the inside of the radio so all the components could be laid out virtually before anything was cut or fabricated. This included the design for the speaker box, which was ultimately 3D printed and then coated with a spray-on “liquid rubber” to seal it up. The upfront effort and time to design like this might be high, but it’s an excellent way to help ensure you don’t run into some roadblock halfway through the build.

Driving the speakers is a TPA3116-based amplifier board with integrated Bluetooth receiver, which has all of its buttons broken out to the front for easy access. [Freshanator] even went the extra mile and designed some labels for the front panel buttons to be made on a vinyl cutter. Unfortunately the cutter lacked the precision to make them small enough to actually go on the buttons, so they ended up getting placed above or next to them as space allowed.

The build was wrapped up with a fan installed at the peak of the front speaker grille to keep things cool. Oh, and there are lights. Because there’s always lights. In this case, some blue LEDs and strategically placed EL wire give the whole build an otherworldly glow.

If you’re interested in a having a frustrating quasi-conversation with your vintage looking audio equipment, you could always cram an Echo Dot in there instead. Though if you go that route, you can just 3D print a classic styled enclosure without incurring the wrath of the purists.

Does WiFi Kill Houseplants?

Spoiler alert: No.

To come to that conclusion, which runs counter to the combined wisdom of several recent YouTube videos, [Andrew McNeil] ran a pretty neat little experiment. [Andrew] has a not inconsiderable amount of expertise in this area, as an RF engineer and prolific maker of many homebrew WiFi antennas, some of which we’ve featured on these pages before. His experiment centered on cress seeds sprouting in compost. Two identical containers were prepared, with one bathed from above in RF energy from three separate 2.4 GHz transmitters. Each transmitter was coupled to an amplifier and a PCB bi-quad antenna to radiate about 300 mW in slightly different parts of the WiFi spectrum. Both setups were placed in separate rooms in east-facing windows, and each was swapped between rooms every other day, to average out microenvironmental effects.

After only a few days, the cress sprouted in both pots and continued to grow. There was no apparent inhibition of the RF-blasted sprouts – in fact, they appeared a bit lusher than the pristine pot. [Andrew] points out that it’s not real science until it’s quantified, so his next step is to repeat the experiment and take careful biomass measurements. He’s also planning to ramp up the power on the next round as well.

We’d like to think this will put the “WiFi killed my houseplants” nonsense to rest – WiFi can even help keep your plants alive, after all. But somehow we doubt that the debate will die anytime soon.

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Tiny Amplifier With ATtiny

Small microcontrollers can pack quite a punch. With the right code optimizations and proper use of the available limited memory, even small microcontrollers can do things they were never intended to. Even within the realm of intended use, however, there are still lots of impressive uses for these tiny cheap processors like [Lukasz]’s audio amplifier which uses one of the smallest ATtiny packages around in the video embedded below.

Since the ATtiny is small, the amplifier is only capable of 8-bit resolution but thanks to internal clock settings and the fast PWM mode he can get a sampling rate of 37.5 kHz. Most commercial amplifiers shoot for 42 kHz or higher, so this is actually quite close for the limited hardware. The fact that it is a class D amplifier also helps, since it relies on switching and filtering to achieve amplification. This allows the amplifier to have a greater efficiency than an analog amplifier, with less need for heat sinks or oversized components.

All of the code that [Lukasz] used is available on the project site if you’ve ever been curious about switching amplifiers. He built this more as a curiosity in order to see what kind of quality he could get out of such a small microcontroller. It sounds pretty good to us too! If you’re more into analog amplifiers, though, we have you covered there as well.

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Daphne Oram And The Birth Of Electronic Music

For most of human history, musical instruments were strictly mechanical devices. The musician either plucked something, blew into or across something, or banged on something to produce the sounds the occasion called for. All musical instruments, the human voice included, worked by vibrating air more or less directly as a result of these mechanical manipulations.

But if one thing can be said of musicians at any point in history, it’s that they’ll use anything and everything to create just the right sound. The dawn of the electronic age presented opportunities galore for musicians by giving them new tools to create sounds that nobody had ever dreamed of before. No longer would musicians be constrained by the limitations of traditional instruments; sounds could now be synthesized, recorded, modified, filtered, and amplified to create something completely new.

Few composers took to the new opportunities offered by electronics like Daphne Oram. From earliest days, Daphne lived at the intersection of music and electronics, and her passion for pursuing “the sound” lead to one of the earliest and hackiest synthesizers, and a totally unique way of making music.

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VinylVideo Is Literally Video On Vinyl

It’s often said that the music etched into a vinyl record takes on a transcendent quality that you simply can’t find in a digital recording, but does that still apply when you add motion picture? The collaboration of [Sengmüller and Diamant] sure think so, because they are offering a new experience for the turntable with the introduction of their VinylVideo pre-amplifier. No tape reels here, this project shows the extend of what is possible through analog video.

Vinyl Video Amp

While all record players capable of playing back 7 in. 45 RPM are compatible with the system, the VinylVideo records themselves specially cut in order to generate the video signal. Each of the custom records has room for a 4-minute music video on the A-side, and the single on the B-side. Videos play back in black & white, sub-standard definition with mono audio, and run around 12 frames per second. The pre-amp takes in the analog signal from regular audio cables via RCA jacks or 3.5mm headphone jack, and then a Raspberry Pi model A+ handles the analog-to-digital conversion. Video out options include HDMI and composite video via a 3.5mm TRSS jack.

The current VinylVideo pre-amp is actually a refinement of the original project from the mid ’90s where it was a part of folk art exhibits. The legacy website (circa 1999) is still live, so you can give it a visit. However, for the most authentic experience you may want to fire-up a virtual machine with Netscape Navigator and Real Player installed.

For a more in-depth look at the VinylVideo in action there is a great video below from [Techmoan]:

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