The Minimin Aims To Be The Simplest Theremin

Hackaday.io user [eagleisinsight] is a high-school hacker whose dreams of becoming a Theremin virtuoso were thwarted by the high cost of a commercial instrument. His response is the Minimin, an affordable Theremin design using a 555 and an ATMega328.

The 555 is configured as an astable oscillator running at about 5MHz and with a loop antenna attached to its timing capacitor. The parasitic capacitance of the musician’s hand against the antenna varies the frequency of the oscillation, as you would expect. In a classic Theremin the signal from the 555 would be mixed with the output from a fixed 5MHz oscillator and the sound would be generated from the difference between the two oscillators, but in [eagleisinsight]’s design the 555 clocks the ATMega328’s timer. The processor can thus read the oscillator frequency and use that value to control a waveform generator.

There is something missing from this Theremin: a second antenna for volume. For now a potentiometer does that job, but [eagleisinsight] is working on a MkII device to correct this omission, along with plans to replace the ATMega with an XMega processor whose DAC can produce a sine wave output and whose USB port can be used to enable the Minimin as a MIDI controller.

As you might expect, we’ve covered numerous Theremins over the years here on Hackaday. You can browse them all, but we’d like to draw your attention to a typical breadboard instrument using a soda can antenna, people using Theremins as Guitar Hero controllers, and Léon Theremin’s terpistone, a full-body instrument.

Rotating Plasma Vortex Speaker

[Anthony Garofalo] has made a fancier plasma speaker. Not content with a simple spark, he uses a plasma vortex. To make the vortex, the spark gap is swapped out for an electrode placed in the centre of a ring magnet. The Lorentz force experienced by the arc causes it to rotate rapidly enough round the arc of the magnet’s centre to appear as a continuous sheet of plasma.

The speaker gets its power from an inverter using a flyback transformer driven through a MOSFET by a 555-based pulse width modulator. You can see the result in the video below the break, it’s very impressive to look at but probably not quite ready to sit in your hi-fi stack. The resulting sound isn’t quite as good as that from a stationary arc, but it looks a lot cooler.

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555 Plasma Speaker

The 555 can do anything. OK, that’s become a bit of a trope in our community, but there is quite a lot of truth behind it: this little timer chip is an astonishingly versatile component.

[Alexander Lang] has added another achievement to the 555’s repertoire, he’s used one in the creation of a plasma speaker. Working at Hackspace Manchester, he’s used the 555 as a pulse-width modulator that drives a flyback transformer through a MOSFET, which feeds a spark gap mounted in a lasercut enclosure. The results maybe aren’t yet hi-fi, but it works, and is very audible.

We’ve been following this project for a while, as he’s updated his progress through several iterations. From initial design idea through PCB and enclosure design, to a first working prototype and some audio refinements, and finally this latest post with the spark gap in its enclosure. He is still refining his speaker, so there is more to come

In the video below the break he demonstrates his pulse width modulator, and tests the device using a keyboard as an input.

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555 Teardown and Analysis

If you are even remotely interested in electronics, chances are the number ‘555’ is immediately recognizable. It is, after all, one of the most popular IC’s ever built, with billions of units sold to date. Designed way back in 1970 by Hans Camenzind, it is still widely available and frequently used for various applications. [Ken Shirriff] does a teardown and analysis of a 555 and gives us a look at the internal structure of this oldie.

A metal can package allowed him to just chop off the top and get access to the die, which was way safer and easier than to etch out the black epoxy of a DIP package. He starts by giving us a quick run down on how the chip works, showing us the two comparators, the output flip-flop and the capacitor discharge circuitry that make up most of the chip. He then puts the die under a metallurgical microscope, and starts identifying the various sections of the chip. Combining pictures of individual elements with cross-sectional diagrams, he identifies the construction of the transistors and resistors, the use of a current mirror to replace bulky resistors, and the differential pair that makes up the comparators.

He wraps it up by providing an interactive map of the die and the schematic, where you can click on various parts and the corresponding component is highlighted along with an explanation of what it does. There’s some interesting trivia about how a redesigned, improved version – the ZSCT1555 – couldn’t survive the popularity and success of the 555. He wraps it up with a useful list of notes and references. While de-capping blog posts are interesting on their own, [Ken] does a great job by giving us a detailed look at the internals.

Thanks [Vikas] for sending in this tip.

Oh Baby, Baby10 – Build a Classic Analog Music Sequencer

Recently I’ve been learning more about classic analog music synthesizers and sequencers. This has led me to the Baby10, a classic and simple analog sequencer design. In this article I’ll introduce its basic operation, and the builds of some awesome hackers based on this design.

Sequencers produce, a sequence of varying voltages. These control voltages (CV) can then be use to control other components. Often this is a simple tone generator. While the concept is simple, it can produce awesome results:

A basic sequencer is a great beginners project. It’s easy to understand the basic operation of the circuit and produces a satisfyingly entertaining result. The Baby 10 was originally published in a column called “Captain’s Analog”, but has now been widely shared online.

baby10
The original Baby10 article.

The circuit uses the 4017, a simple CMOS decade counter. The 4017 takes an input clock signal then sequentially outputs a high pulse on each of 10 output pins. As such, the 4017 does almost everything we need from a sequencer in a single IC! However, we want our sequencer to output a varying voltage which we can then use to generate differing tones.

To accomplish this variable resistors are connected to each of the output pins. A diode in series with the variable resistor stops the outputs fighting against each other (in layman’s terms).

To make the sequencer more visually attractive (and give some feedback) LEDs are often also added to the output of the 4017. A complete Baby 10 sequencer is shown in the schematic below. The original circuit used 1N917s, these are no longer available but the part has been replaced by the 1N4148.

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A Game Pad For The Apple II

[Quinn Dunki] has been hard at work building a Teaddy Top – an Apple IIc Plus modified for a road warrior. It has a 3.5 inch disk drive, runs at a blistering four megahertz, and has a beautiful integrated color LCD. It would be a shame to have such a great machine and no way to play games as they were intended, so [Quinn] set about building a game pad for her lovable Apple II.

The Apple II joystick port isn’t as simple as an Atari or Commodore joystick port. Where the bog-standard Atari joystick is basically just a bunch of switches connected to pins, the Apple II joystick is analog. Weird, and even weirder is the value of the pots in these joysticks: 150kΩ. Somehow or another, nobody makes pots in this value any more. Luckily the hardware in these joysticks is well documented, and shoehorning in modern components isn’t that bad.

The Apple joystick has a bit of circuitry – a 556 timer chip that reads the values of each pot and converts that into a stream of 0s and 1s for the Apple. The joystick [Quinn] found for her game pad is an analog thumb stick on a neat breakout board manufactured by Parallax. This analog joystick has 10kΩ pots in it, and that just won’t work with the 556 timer chip. However, since this is just resistors and a 556 chip, adjusting some of the values on the original schematics does the trick. [Quinn] added a few capacitors to her circuit, and everything worked beautifully.

With the electronics down, she turned her attention to the case for her Apple II road warrior enclosure. She recently picked up a 3D printer, which means she’s new to 3D printing. After spending a few hours designing a controller in 123D Design, she sent the files over to the printer. Warping happened. She tried an ABS slurry. The part was stuck to the bed. It took a few tries (purple glue sticks are awesome, [Quinn]), but she eventually got her plastic enclosure printed out, and the circuitry installed. The result is a portable computer, with a custom controller, playing Lode Runner. Can’t beat that.

Monitoring Power With A 555

[Diederich] is running a Raspberry Pi loaded up with Pimatic, a great home automation server that does just about anything you can throw at it. One thing it doesn’t do is monitor electricity and gas directly from the meter – you’re going to need hardware for that. [Diederich] stepped up to the plate and built that hardware using just a 555 timer. The total cost of adding this to his Pimatic setup was less than a dollar.

The 555 can be used as a timer, a trigger, and a bunch of them can be cobbled together into a CPU. [Diederich] isn’t using some fancy logic here; he’s just using the 555 as a Schmitt trigger with a phototransistor and his electricity meter. The output of the 555 is connected to the GPIO of the Raspberry Pi, and a Python script ties into Pimatic.

It’s a neat solution that only costs a dollar, and using the 555 has a few advantages: the 555 makes it possible to use long and thin wires back to the Pi, which means [Diederich]’s Pi doesn’t have to be located right next to his meter.