A Plasma Speaker Using A TL494

We’re used to loudspeakers as circular components with a paper cone and a big magnet inside which is suspended a coil that is connected to our audio amplifier. But moving-coil speakers are not the only way to create sound from electricity, there are one or two other weapons in the audio designer’s arsenal.

One of the more spectacular and entertaining is the plasma speaker, and it’s one [Marcin Wachowiak] has been experimenting with. A continuous plasma in the form of a discharge between two electrodes is modulated with an audio signal, and the resulting rapid changes in the volume of plasma creates a sound. The value of a plasma speaker lies in the exceptionally low size and mass of the element producing the sound, meaning that while it can only effectively reproduce high frequencies it can do so from a much closer approximation to a point source than can other types of tweeter. For this reason it’s beloved of some audiophiles, and you will find a few commercially produced plasma tweeters at the high-end of the audio market.

[Marcin] isn’t in it for the audiophilia, instead he’s interested in the properties of the plasma. His plasma speaker does do the job well though, and in particular he’s put a lot of thought into the design of its drive circuit. At its heart is the ubiquitous TL494 PWM controller that you may be more familiar with in the context of switching power supplies, this one applies the audio drive as PWM to the gate of a MOSFET that switches the primary of a flyback transformer. He’s added refinements such as a gate discharge circuit and a second primary winding with a freewheel diode.

The result is an effective plasma speaker. It’s difficult to judge from his YouTube video below the break whether he’s achieved audiophile purity, but happily that’s not the point. We’ve shown you a few other plasma speakers in our time, if the subject interests you then take a look at this rotating plasma vortex, or a version using a 555 timer.

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Junk Bin Spin Coater uses Modded Case Fan

We’ve all been there: you need a specific tool or gadget to complete a project, but it’s not the kind of thing you necessarily want to fork over normal retail price for. It could be something you’re only going to use once or twice, or maybe you’re not even sure the idea is going to work and don’t want to invest too much money into it. You cast a skeptical towards the ever-growing pile of salvaged parts and wonder…

Inspiration and a dig through the junk bin is precisely how [Nixie] built this very impressive spin coater for use in his ongoing homemade semiconductor project. If you’ve never had first hand experience with a spin coater, don’t worry, not many people have. Put simply, it’s a machine that allows the user to deposit a thin layer of material on a disc by way of centrifugal force. Just place a few drops in the center of the disc, then spin it up fast enough and let physics do the rest.

[Nixie] only needs to spin up a fairly tiny disc, and realized the hub of a 40x40mm brushless case fan was just about the perfect size. A quick pass through the lathe stripped the hub of its blades and faced off the front. Once he found a tube that was the exact same diameter of the fan’s axle, he realized he could even use a small vacuum pump to hold his disc in place. A proper seal is provided by 10 and 16 mm OD o-rings, installed into concentric grooves he machined into the face of the hub.

With a way to draw a vacuum through the hub of the spinner he just needed the pump. As luck would have it, he didn’t have to wait for one to make the journey from China, as he had one of those kicking around his junk bin from a previous project. The only thing he ended up having to buy was the cheap PWM fan controller which he mounted along with the modified fan to a piece of black acrylic; producing a fairly professional looking little piece of lab equipment. Check out the video after the break for a brief demonstration of it in action.

This isn’t the first specialized piece of gear [Nixie] has produced in his quest for DIY chips. We’ve previously covered his DIY tube oven as well as his vacuum chamber complete with magnetically controlled manipulator arm.

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Circuit VR: An (Almost) Practical Buck Converter

In the last installment of Circuit VR, we walked around a simplified buck converter. The main simplification was using a constant PWM signal. The result is that the output voltage is a fixed fraction of the input voltage. For a regulator, the pulse width will need to depend on the output voltage so that any changes in the output are self-correcting. So this time, we’ll make a regulator, although we’ll still use a few Spice elements you’d have to replace in a practical design. In particular, we’ll assume you can generate a triangle wave, which is easy enough, and produce a stable 2.5 V reference.

The idea is to take a voltage reference and compare it to the output. We’ll think of the difference between the two as an error voltage, and use a comparator combined with a triangle wave generator to produce a PWM signal that is proportional to the error, and thus works to hold the output voltage constant.

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Arduino Powered Arcade Button Lighting Effects

As if you already weren’t agonizing over whether or not you should build your own arcade cabinet, add this one to the list of compelling reasons why you should dedicate an unreasonable amount of physical space to playing games you’ve probably already got emulated on your phone. [Rodrigo] writes in to show off his project to add some flair to the lighted buttons on his arcade controller. (Google Translate)

The wiring for this project is about as easy as you’d expect: the buttons connect to the digital inputs on the Arduino, and the LEDs on the digital outputs. When the Arduino code sees the button getting pressed, it brings the corresponding LED pin high and starts a fade out timer using the SoftPWM library by [Brett Hagman].

It’s worth noting that the actual USB interface is being done with a stand-alone controller, so the Arduino here is being used purely to drive the lighting effects. The more critical reader might argue that you could do both with a single microcontroller, but [Rodrigo] was in a classic “Use what you’ve got” situation, and already had a USB controller on hand.

Of course, fancy lit arcade buttons won’t do you much good without something to put them in. Luckily we’ve covered some fantastic looking arcade cabinets to get you inspired.

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Behind The Pin: How The Raspberry Pi Gets Its Audio

Single board computers have provided us with a revolution in the way we approach computing as hardware creators. We have grown accustomed to a world in which an entire microcomputer has become a component in its own right rather than a complex system, and we interface to them as amorphous entities through their exposed interfaces. But every pin or socket on a single board computer has something behind it, so following up on a recent news-inspired item in which we took a look at what lies behind the Ethernet jack on a Raspberry Pi, we’d like to continue that theme by looking behind more pins and interfaces. So today we’ll stay with the Raspberry Pi, and start with an easy target by taking a look down its audio jack.

All the main Raspberry Pi board releases since 2012 with the exception of the Pi Zero series, have featured a 3.5mm jack carrying line-level audio. The circuits are readily accessible via the Raspberry Pi website, and are easy enough to understand because of course all the really hard work is done within the silicon of the Broadcom system-on-chip. Looking at the audio circuitry, we’ll start by going back to the original Pi Model B from 2012 (PDF) because though more recent models have seen a few changes, this holds the essence of the circuitry.

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555 Ways to Speed Control A DC Motor

The 555 timer IC is a handful of active components all baked into one beautifully useful 8 pin package. Originally designed for timing purposes, they became ubiquitous parts that can achieve almost anything. In this case, they’re being used to create a  basic PWM motor controller.

The trick is to set the 555 up in astable mode, and use diodes and a potentiometer in the charge/discharge loop. By hanging a diode off either side of a potentiometer, leading to the charge and discharge pins, and connecting the center lug to the main capacitor, you can vary the resistance seen by the capacitor during charge and discharge. By making charging take longer, you increase the pulse width, and by making discharge take longer, you reduce the pulse width. The actual frequency itself is determined largely by the capacitor and total resistance of the potentiometer itself.

This is a very old-school way to generate a PWM signal, which could be used to vary intensity of a light or make noise on a buzzer. However, in this case, the output of the 555 is connected to a MOSFET which is used to vary the speed of a computer fan motor.

It’s an excellent way to learn about both PWM motor control and the use of 555 timers, all with a very low parts cost and readily available components. We’ve seen such setups before, used as easy-to-build dimmer switches, too.

Need a Thousand Extra PWM Pins?

If your Arduino runs out of I/O lines, you can always add one of the several I/O expander chips that takes a serial interface to set its several pins. Or perhaps you could buy something like an Arduino Mega, with its extra sockets to fulfil your needs. But what would you do if you really needed more pins, say a thousand of them? Perhaps [Brian Lough] has the answer. OK, full disclosure: If you really need a thousand, the video isn’t exactly for you, as he shows you how to add up to 992 PWM outputs. The chip he uses works with any microcontroller (the video shows an ESP8266), and we suppose you could use two daisy chains of them and break the 1,000 barrier handily.

We like how short the video is (just two minutes; see below) as it gets right to the point. The PCA9685 chip gives you 16 12-bit PWM channels via an I2C interface. You can daisy chain up to 62 of the boards to get the 992 outputs promised.

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