Engineering

602 Articles

Diodes: The Switch You Never Knew You Had

December 9, 2016 by 62 Comments
Vishay's take on the 1N4148 data sheet (PDF), describing it as a switching diode.
Vishay’s take on the 1N4148 data sheet (PDF), describing it as a switching diode.

When looking across the discrete components in your electronic armory, it is easy to overlook the humble diode. After all, one can be forgiven for the conclusion that the everyday version of this component doesn’t do much. They have none of the special skills you’d find in tunnel, Gunn, varicap, Zener, and avalanche diodes, or even LEDs, instead they are simply a one-way valve for electrical current. Connect them one way round and current flows, the other and it doesn’t. They rectify AC to DC, power supplies are full of them. Perhaps you’ve also used them to generate a stable voltage drop because they have a pretty constant voltage across them when current is flowing, but that’s it. Diodes: the shortest Hackaday article ever.

Not so fast with dismissing the diode though. There is another trick they have hiding up their sleeves, they can also act as a switch. It shouldn’t come as too much of a shock, after all a quick look at many datasheets for general purpose diodes should reveal their description as switching diodes.

So how does a diode switch work? The key lies in that one-way valve we mentioned earlier. When the diode is forward biased and conducting electricity it will pass through any variations in the voltage being put into them, but when it is reverse biased and not conducting any electricity it will not. Thus a signal can be switched on by passing it through a diode in forward bias, and then turned off by putting the diode into reverse bias.

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Probing high voltage

Measuring High Voltage In Millimeters (and Other HV Probe Tricks)

December 8, 2016 by 43 Comments

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.

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Taking It To Another Level: Making 3.3V Speak With 5V

December 5, 2016 by 51 Comments

If your introduction to digital electronics came more years ago than you’d care to mention, the chances are you did so with 5V TTL logic. Above 2V but usually pretty close to 5V is a logic 1, below 0.8V is a logic 0. If you were a keen reader of electronic text books you might have read about different voltage levels tolerated by 4000 series CMOS gates, but the chances are even with them you’d have still used the familiar 5 volts.

This happy state of never encountering anything but 5V logic as a hobbyist has not persisted. In recent decades the demands of higher speed and lower power have given us successive families of lower voltage devices, and we will now commonly also encounter 3.3V or even sometimes lower voltage devices. When these different families need to coexist as for example when interfacing to the current crop of microcontroller boards, care has to be taken to avoid damage to your silicon. Some means of managing the transition between voltages is required, so we’re going to take a look at the world of level shifters, the circuits we use when interfacing these different voltage logic families.

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Glues You Can Use: Adhesives For The Home Shop

November 28, 2016 by 72 Comments

A while back I looked at lubricants for the home shop, with an eye to the physics and chemistry behind lubrication. Talking about how to keep parts moving got me thinking about the other side of the equation – what’s the science behind sticking stuff together? Home shops have a lot of applications for adhesives, so it probably pays to know how they work so you can choose the right glue for the job. We’ll also take a look at a couple of broad classes of adhesives that are handy to have around the home shop. Continue reading “Glues You Can Use: Adhesives For The Home Shop” →

Squoze Your Data

November 22, 2016 by 44 Comments

I have a confession to make. I enjoy the challenge of squeezing software into a tiny space or trying to cut a few more cycles out of a loop. It is like an intricate puzzle. Today, of course, there isn’t nearly as much call for that as there used to be. Today even a “small” microcontroller has a ton of memory and resources.

Even so, there’s still a few cases where you need to squeeze those last few bytes out of memory. Maybe you are trying to maximize memory available for some purpose. Maybe you are anticipating mass production and you are using the smallest microcontroller you can find. Or maybe you’re doing the 1 kB Challenge and just want some advice.

One way to find techniques to maximize resources is to look at what people did “in the old days.” Digital Equipment computers once had a special character set called Squoze (or sometimes DEC Radix-50). This technique can be useful when you need to get a lot of strings into memory. The good news is that you can reliably get 3 characters into 2 bytes (or, as DEC did, 6 characters into 4 bytes). The bad news is that you have to pick a limited character set that you can use. However, that’s not always a big problem.

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The Little Mechanism That Made Precise Time-keeping Possible

November 15, 2016 by 61 Comments

There are few things to which we pay as much attention as the passage of time. We don’t want to be late for work, or a date. Even more importantly, we don’t want to age and die. Good time keeping is an all important human activity, and we started to worry about it as soon as we abandoned our hunter-gatherer lifestyle and agriculture and commerce emerged.

By de:Benutzer:Flyout - own work, http://de.wikipedia.org/wiki/Bild:Kerzenuhr.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1783765
A candle clock

Measuring time needs two things: a repetitive process to mark equal increments of time, and a way of tracking and displaying the result. The first timekeeping devices relied of course on the movement of the sun. Ancient Egyptians, around 3500 BC, built obelisks that, by casting a shadow on the ground at different positions, gave an approximate idea of the time. Next came the use of some medium that was consumed at a regular pace: candle, incense, water and sand clocks are examples. A great advancement came with the advent of the mechanical clock, and here is where the escapement mechanism appears.

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