Crystal Oscillators Explained

We’ve read a lot about oscillators, but crystal oscillators seem to be a bit of a mystery. Hobby-level books tend to say, build a circuit like this and then mess with it until it oscillates. Engineering texts tend to go on about loop gains but aren’t very clear about practice. A [circuit digest] post that continues a series on oscillators has a good, practical treatment of the subject.

Crystals are made to have a natural resonant frequency and will oscillate at that frequency or a multiple thereof with the proper excitation. The trick, of course, is finding the proper excitation.

The post starts with a basic model of a crystal having a series capacitance and inductance along with a resistance. There’s also a shunt or parallel capacitor. When you order a crystal, you specify if you want the resonant frequency in series or parallel mode — that is, which of the capacitors in the model you want to resonate with the inductor — so the model has actual practical application.

By applying the usual formula for resonance on the model you’ll see there is a null and a peak which corresponds to the two resonance points. The dip is the series frequency and the peak is the parallel. You can actually see a trace for a real crystal in a recent post we did on the Analog Discovery 2. It matches the math pretty well, as you can see on the right.

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Energy Sipping Neodymium Sphere Keeps on Spinning

At this point we’re sure you are aware, but around these parts we don’t deduct points for projects which we can’t immediately see a practical application for. We don’t make it our business to say what is and isn’t worth your time as an individual hacker. If you got a kick out of it, great. Learned something? Even better. If you did both of those things and took the time to document it, well that’s precisely the business we’re in.

So when [Science Toolbar] sent in this project which documents the construction of an exceptionally energy efficient spinning neodymium sphere, we knew it was our kind of thing. In the documentation it’s referred to as a motor, though it doesn’t appear to have the torque to do any useful work. But still, if it can spin continuously off of the power provided by a calculator-style photovoltaic cell, it’s still a neat trick.

But how does it work? It starts by cracking open one of those little solar powered toys; the ones that wave or dance around as soon as any light hits the panel in their base. As [Science Toolbar] explains, inside these seemingly magical little gadgets is a capacitor and the classic black epoxy blob that contains an oscillator circuit. A charge is built up in the capacitor and dumped into a coil at roughly 1 Hz, which provides just enough of a push to get the mechanism going.

In the video after the break, [Science Toolbar] demonstrates how you can take those internals and pair it with a much larger coil. Rather than prompting a little sunflower or hula girl to do its thing, the coil in this version provides the motive force for getting the neodymium sphere spinning. To help things along, they’re even using a junk box zero friction magnetic bearing made up of a wood screw and a magnetized screwdriver tip.

It’s an interesting example of how a tiny charge can be built up over time, and with a nice enough enclosure this will make for a pretty cool desk toy. We’ve previously seen teardowns of similar toys, which revealed a surprising amount of complexity inside that little epoxy blob. No word on whether or not the version [Science Toolbar] cannibalized was quite so clever, however.

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[Ben Krasnow] Gasses MEMS Chips, for Science

Why in the world does helium kill iPhones and other members of the Apple ecosystem? Enquiring minds want to know, and [Ben Krasnow] has obliged with an investigation of the culprit: the MEMS oscillator. (YouTube, embedded below.)

When we first heard about this, courtesy in part via a Hackaday post on MRI-killed iPhones, we couldn’t imagine how poisoning a micro-electromechanical system (MEMS) part could kill a phone. We’d always associated MEMS with accelerometers and gyros, important sensors in the smartphone suite, but hardly essential. It turns out there’s another MEMS component in many Apple products: an SiT 1532 oscillator, a tiny replacement for quartz crystal oscillators.

[Ben] got a few from DigiKey and put them through some tests in a DIY gas chamber. He found that a partial pressure of helium as low as 2 kPa, or just 2% of atmospheric pressure, can kill the oscillator. To understand why, and because [Ben] has a scanning electron microscope, he lapped down some spare MEMS oscillators to expose their intricate innards. His SEM images are stunning but perplexing, raising questions about how such things could be made which he also addresses.

The bottom line: helium poisons MEMS oscillators in low enough concentrations that the original MRI story is plausible. As a bonus, we now understand MEMS devices a bit better, and have one more reason never to own an iPhone.

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Synthesizing Mother Nature’s Sounds Like You’ve Never Seen Before

We’ve all heard the range of sounds to be made electronically from mostly discrete components, but what [Kelly Heaton] has achieved with her many experiments is a whole other world, the world of nature to be exact. Her seemingly chaotic circuits create a nighttime symphony of frogs, crickets, and katydids, and a pleasant stroll through her Hackaday.io logs makes how she does it crystal clear and is surely as delightful as taking a nocturnal stroll through her Virginia countryside.

Homemade piezo buzzer with amplifierThe visual and aural sensations of the video below will surely tempt you further, but in case it doesn’t, here’s a taste. When Radio Shack went out of business, she lost her source of very specific piezo buzzers and so had to reverse engineers theirs to build her own, right down to making her own amplifiers on circular circuit boards and vacuum forming and laser cutting the housings. For the sounds, she starts out with a simple astable multivibrator circuit, demonstrating how to create asymmetry by changing capacitors, and then combining two of the circuits to get something which sounds just like a cricket. She then shows how to add katydids which enhance the nighttime symphony with percussive sounds much like a snare drum or hi-hat. It’s all tied together with her Mother Nature Board built up from a white noise generator, Schmitt trigger, and shift registers to turn on and off the different sound circuits, providing a more unpredictable and realistic nighttime soundscape. The video below shows the combined result, though she admits she’ll never really be finished. And be sure to check out even more photos and videos of her amazing work in the gallery on her Hackaday.io page.

For the more familiar range of sounds, though no less varied, check out our own [Elliot William’s] series, Logic Noise, where he takes us through an extensive exploration of a less Mother Naturely soundscape.

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An Arduino Watch Without A Clock

When you show up at a party wearing this bare PCB watch, there are effectively two possible reactions you might receive from the other people there. Either they are going to snicker at the nerd who’s wearing a blinking circuit board on their wrist in public, or they are going to marvel at the ridiculously low part count. We’ll give you one guess as to which reaction you’d likely get at any event Hackaday is involved in.

Designed and built by [Electronoobs], this extremely simple watch consists of a ATmega328P microcontroller, a dozen LEDs with their associated 200 Ω resistors, and a battery. There’s also a single push button on the front which is used to not only set the watch, but turn the LEDs on when you want to check the time. Short of dropping down to one LED and blinking out the time, it’s hard to imagine a timepiece with fewer components than this.

You’re probably wondering how [Electronoobs] pulled this off without an external clock source for the ATmega328P chip. The chip actually has an internal 8 MHz oscillator that can be used, but you need to flash the appropriate bootloader to it first. Accordingly, the backside of the PCB has both SPI and a UART solder pads for external bootloader and firmware programming.

As you might expect, there’s a downside to using the internal oscillator: it’s not very good. The ATmega328P spec sheet claims a factory calibrated accuracy of ±10%, and [Electronoobs] has found that equates to a clock drift of around 15 seconds per day. Not exactly great, but considering the battery only lasts for two days anyway, it doesn’t have much of an impact in this case.

Compared to other “analog” LED watches we’ve seen, the simplicity of this build is really quite remarkable. The closest competitor we’ve seen so far is this slick binary watch.

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A Crystal Oscillator For A Stable Bench Reference

[Paul] likes a precise oscillator. His recent video shows a crystal oscillator with a “watch crystal” and a CMOS counter, the CD4060. Using such a circuit can produce very stable frequencies and since the 32.768 kHz crystal is a power of 2, you get nice divisions out of the counter.

We’ve seen the same trick done with decade counters (like the 4518B) to divide by 10 instead of powers of two to make frequency standards. A 1 MHz crystal can easily generate 100 kHz, 10 kHz, etc.

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A Graph Plotting Metal Detector

Metal detectors can be a great source of fun, and occasionally even found wealth. They allow the detection of metal objects at a distance, enabling hidden treasures to be discovered. They’re also highly critical to the work of minesweepers and unexploded ordnance disposal teams. [Andrius] wanted to add such a device to his kit when motorcycling through the woods of Lithuania, and thus decided to undertake a build of his own.

The detector is a thoroughly modern one – fans of the 555 may want to look away now. A Collpits oscillator, built from two transistors, is used to generate a frequency that is passed through the detection coil. This frequency is measured by an Arduino that plots a graph of the received frequency on an OLED display. As the coil is passed near metal objects, the oscillator frequency changes, and this is visible on the frequency plot on-screen.

Not only is it a quick and easy build that is achievable from what are now junkdraw components, it’s also one that would be readily usable by the hearing-impaired, too. It’s a great project to tackle if you’re looking to get to grips with basic oscillators, frequency measurement, or just microcontroller programming in general.

Still need more inspiration? We’ve seen a similar concept executed before.