How Do You Fill The 1N34 Void?

The germanium point contact diode, and almost every semiconductor device using germanium, is now obsolete. There was a time when almost every television or radio would have contained one or two of them, but the world has moved on from both analogue broadcasting and discrete analogue electronics in its lower-frequency RF circuitry. [TSBrownie] is taking a look at alternatives to the venerable 1N34A point-contact diode in one of the few places a point-contact diode makes sense, the crystal radio.

In the video below the break, he settles on a slightly more plentiful Eastern European D9K as a substitute after trying a silicon rectifier (awful) and a Schottky diode (great in theory, not so good in practice). We’ve trodden this path in the past and settled on a DC bias to reduce the extra forward voltage needed for a 1N4148 silicon diode to conduct because, like him, we found a Schottky disappointing.

The 1N34 is an interesting component, and we profiled its inventor a few years ago. Meanwhile, it’s worth remembering that sometimes, we just have to let old parts go.

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Everything You Ever Wanted To Know About The ULN2003

The ULN2003 IC is an extremely versatile part, and with the help of [Hulk]’s deep dive, you might just get some new ideas about how to use this part in your own projects.

Each of the seven outputs works like this simplified diagram.

Inside the ULN2003 you’ll find seven high-voltage and high-current NPN Darlington pairs capable of switching inductive loads. But like most such devices there are a variety of roles it can fill. The part can be used to drive relays or motors (either brushed or stepper), it can drive LED lighting, or simply act as a signal buffer. [Hulk] provides some great examples, so be sure to check it out if you’re curious.

Each of the Darlington pairs (which act as single NPN transistors) is configured as open collector, and the usual way this is used is to switch some kind of load to ground. Since the inputs can be driven directly from 5 V digital logic, this part allows something like a microcontroller to drive a high current (or high voltage, or both) device it wouldn’t normally be able to interface with.

While the circuitry to implement each of the transistor arrays isn’t particularly complex and can be easily built by hand, a part like this is a real space saver due to how it packs everything needed in a handy package. Each output can handle 500 mA, but this can be increased by connecting in parallel.

There’s a video (embedded below) which steps through everything you’d like to know about the ULN2003. Should you find yourself wanting a much, much closer look at the inner secrets of this chip, how about a gander at the decapped die?

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Using A Spring As A Capacitive Touch Button

When [Daniel Eichhorn] designed the Pendrive S3 project, he wanted to use an off-the-shelf USB enclosure but also add a button for the user to start certain actions. Drilling a hole into the enclosure would be an option, but decided a touch sensor on the top of the enclosure would be much more elegant — not to mention better at keeping dirt and moisture out. To bridge the 6.3 mm spacing between the PCB and the top of the enclosure [Daniel] used a small, 7 mm PCB-mounted spring.

The spring used to create a capacitive touch sensor. (Source: JLCPCB parts)
The spring used to create a capacitive touch sensor. (Source: JLCPCB parts)

Although capacitive sensing works with just about  anything that’s electrically conductive, it’s important to get the conductive element as close to the user’s digits as possible. Using a spring here has the advantage that when the enclosure is closed up, the lid will push down onto the spring, which will not only compress slightly, but also provide the best capacitive sensing experience when e.g. the enclosure flexes or warps over time on account of always being pressed against the inside of the lid.

While hardly world-changing, this is another neat design tip when you’re looking to turn more surfaces into touch controls. Just keep in mind that capacitive sensing is notoriously fussy and any trace and spring are also excellent antennae for stray EMI. Nobody likes random capacitive button inputs, after all.

Introduction To MOSFET Switching Losses

Metal-oxide semiconductor field-effect transistors (MOSFETs) see common use in applications ranging from the very small (like CPU transistors) to very large (power) switching applications. Although its main advantage is its high power efficiency, MOSFETs are not ideal switches with a perfect on or off state. Understanding the three main sources of switching losses is crucial when designing with MOSFETs, with a recent All About Circuits article by [Robert Keim] providing a primer on the subject.

As it’s a primer, the subthreshold mode of MOSFET modes of operation is omitted, leaving the focus on the linear (ohmic) mode where the MOSFET’s drain-source is conducting, but with a resistance that’s determined by the gate voltage. In the saturated mode the drain-source resistance is relatively minor (though still relevant), but the turn-on time (RDS(on)) before this mode is reached is where major switching losses occur. Simply switching faster is not a solution, as driving the gate incurs its own losses, leaving the circuit designer to carefully balance the properties of the MOSFET.

For those interested in a more in-depth study of MOSFETs in e.g. power supplies, there are many articles on the subject, such as this article (PDF) from Texas Instruments.

Custom Polyurethane Belts Made Easy

If you need to make polyurethane belts in custom lengths, it’s not too hard. You just need to take lengths of flexible polyurethane filament, heat the ends, and join them together. In practice, it’s difficult to get it right by hand. That’s why [JBVCreative] built a 3D printed jig to make it easy. 

The jig consists of two printed sliders that mount on a pair of steel rods. Each slider has a screw-down clamp on top. The clamps are used to hold down each end of the polyurethane filament to be joined. Once installed in the jig, the ends of the filament can be heated with a soldering iron or other element. and then gently pushed together. The steel rods simply enable the filament to be constrained linearly so the ends don’t shift during the joining process.

The jig doesn’t produce perfect belts. There’s still a small seam at the join that is larger than the filament’s base diameter. A second jig for trimming the belt to size could be helpful in this regard. Still, it’s a super useful technique for making custom belts. This could be super useful to anyone needing to restore old cassette decks or similar mechanical hardware.

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Power Resistance Isn’t Futile

As [Electronoobs] points out, everything has resistance. So, how hard can it be to make a high-power resistor? In the video below, he examines a commercial power resistor and how to make your own using nichrome wire.

Sure, in theory, you can use a long piece of wire, but normally, you want to minimize the amount of space occupied. This leads to winding the wire around some substrate. If you just wind the wire, though, you get an inductor. This can cause nasty voltage spikes when there is a change in current through the resistor. You can get “noninductive” wire wound resistors that use either two opposing windings or alternate the turn direction on each turn. This causes the magnetic fields to tend to cancel out, reducing the overall inductance.

Nichrome wire has more resistance per millimeter and can dissipate more power. Modern digital meters can measure the resistance of a wire if you account for the test leads. To make a substrate, [Electronoobs] got creative since he anticipated generating a lot of heat. The final product even uses water cooling.

Why do you want a big resistor? Maybe you need a dummy load, or you want to drain some batteries. If you want to recycle nichrome wire, it is much more common than you might expect.

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Protoboard Z80 Computer Teaches The Basics

As curious people, we’re all incredibly fortunate to live in an age where information can so easily be obtained. If you want to learn how something works, from a cotton gin to an RBMK reactor, you’re just a few keystrokes away from articles, diagrams, and videos on the subject. But as helpful as all of that information can be, we also know that there’s no substitute for hands-on experience.

While we can’t recommend you try building a miniature graphite-moderated nuclear reactor, there’s plenty of other devices that you can study by constructing your own functioning model. For example, when [Jorisclayton] wanted to really know what was going on inside a computer, they decided to go back to basics and build their own Z80 machine. To maximize the experience, they skipped any of the existing kit designs and instead wired the whole thing up by hand across a few perfboards.

The main board contains a 4 MHz Z80 processor, paired with 32K ROM and 64K RAM. Here you’ll also find the clock generator, I/O decoder, serial port, voltage regulator, and a trio of expansion slots that use a long strip of 2.54 mm pin headers as the interface. In the first expansion slot you’ve got a primordial “graphics card” based around the TMS9918 video display controller (VDC) and 16K of additional RAM. The second expansion card has a CompactFlash reader and an LED array mapped to I/O address 0x00h so it can be used for various notifications.

[Jorisclayton] says the final expansion board is still being worked on, but the idea is for it to handle user input through a PS/2 keyboard connector, as well as provide ports for a pair of Super Nintendo (or compatible) controllers. Everything is held together with a minimalist 3D printed frame to show off all that careful soldering.

Obviously there’s no PCB design files to share for this one, but [Jorisclayton] has posted a schematic for wiring everything up if you’re looking for resources to build your own Z80 computer. Sure the chips themselves might no longer be in production, but that doesn’t mean this venerable CPU is going anywhere just yet.