Simplest Speaker Oscillator, Now Even Simpler

It never fails. Lay down some kind of superlative — fastest, cheapest, smallest — around this place and someone out there says, “Hold my beer” and gets to work. In this case, it’s another, even simpler audio oscillator, this time with just a loudspeaker and a battery.

Attentive readers will recall the previous title holder was indeed pretty simple, consisting only of the mic and speaker from an old landline telephone handset wired in series with a battery. Seeing this reminded [Hydrogen Time] of a lucky childhood accident while experimenting with a loudspeaker, which he recreates in the video below. The BOM for this one is even smaller than the previous one — just a small speaker and a battery, plus a small scrap of solid hookup wire. The wire is the key; rather than connecting directly to the speaker terminal, it connects to the speaker frame on one end while the other is carefully adjusted to just barely touch the flexible wire penetrating the speaker cone on its way to the voice coil.

When power is applied with the correct polarity, current flows through the wire into the voice coil, which moves the cone and breaks the circuit. The speaker’s diaphragm resets the cone, completing the circuit and repeating the whole process. The loudspeaker makes a little click with each cycle, leading to a very rough-sounding oscillator. [Hydrogen Time] doesn’t put a scope on it, but we suspect the waveform would be a ragged square wave whose frequency depends on the voltage, the spring constant of the diaphragm, and the spacing between the fixed wire and the voice coil lead.

Yes, we realize this is stretching the definition of an audio oscillator somewhat, but you’ve got to admit it’s simple. Can you get it even simpler?

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Op-Amp Drag Race Turns Out Poorly For 741

When it was first introduced in 1968, Fairchild’s 741 op-amp made quite a splash. And with good reason; it packed a bunch of components into a compact package, and the applications for it were nearly limitless. The chip became hugely popular, to the point where “741” is almost synonymous with “op-amp” in the minds of many.

But should it be? Perhaps not, as [More Than Electronics] reveals with this head-to-head speed test that compares the 741 with its FET-input cousin, the TL081. The test setup is pretty simple, just a quick breadboard oscillator with component values selected to create a square wave at approximately 1-kHz, with oscilloscope probes on the output and across the 47-nF timing capacitor. The 741 was first up, and it was quickly apparent that the op-amp’s slew rate, or the rate of change of the output, wasn’t too great. Additionally, the peaks on the trace across the capacitor were noticeably blunted, indicating slow switching on the 741’s output stage. The TL081 fared quite a bit better in the same circuit, with slew rates of about 13 V/μS, or about 17 times better than the 741, and nice sharp transitions on the discharge trace.

As [How To Electronics] points out, comparing the 741 to the TL081 is almost apples to oranges. The 741 is a bipolar device, and comparing it to a device with JFET inputs is a little unfair. Still, it’s a good reminder that not all op-amps are created equal, and that just becuase two jelly bean parts are pin compatible doesn’t make them interchangeable. And extra caution is in order in a world where fake op-amps are thing, too.

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No Inductors Needed For This Simple, Clean Twin-Tee Oscillator

If there’s one thing that amateur radio operators are passionate about, it’s the search for the perfect sine wave. Oscillators without any harmonics are an important part of spectrum hygiene, and while building a perfect oscillator with no distortion is a practical impossibility, this twin-tee audio frequency oscillator gets pretty close.

As [Alan Wolke (W2AEW)] explains, a twin-tee oscillator is quite simple in concept, and pretty simple to build too. It uses a twin-tee filter, which is just a low-pass RC filter in parallel with a high-pass RC filter. No inductors are required, which helps with low-frequency designs like this, which would call for bulky coils. His component value selections form an impressively sharp 1.6-kHz notch filter about 40 dB deep. He then plugs the notch filter into the feedback loop of an MCP6002 op-amp, which creates a high-impedance path at anything other than the notch filter frequency. The resulting sine wave is a thing of beauty, showing very little distortion on an FFT plot. Even on the total harmonic distortion meter, the oscillator performs, with a THD of only 0.125%.

This video is part of [Alan]’s “Circuit Fun” series, which we’ve really been enjoying. The way he breaks complex topics into simple steps that are easy to understand and then strings them all together has been quite valuable. We’ve covered tons of his stuff, everything from the basics of diodes to time-domain reflectometry.

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Back To Basics With A 555 Deep Dive

Many of us could sit down at the bench and whip up a 555 circuit from memory. It’s really not that hard, which is a bit strange considering how flexible the ubiquitous chip is, and how many ways it can be wired up. But when was the last time you sat down and really thought about what goes on inside that little fleck of silicon?

If it’s been a while, then [DiodeGoneWild]’s back-to-basics exploration of the 555 is worth a look. At first glance, this is just a quick blinkenlights build, which is completely the point of the exercise. By focusing on the simplest 555 circuits, [Diode] can show just what each pin on the chip does, using an outsized schematic that reflects exactly what’s going on with the breadboarded circuit. Most of the demos use the timer chip in free-running mode, but circuits using bistable and monostable modes sneak in at the end too.

Yes, this is basic stuff, but there’s a lot of value in looking at things like this with a fresh set of eyes. We’re impressed by [DiodeGoneWild]’s presentation; while most 555 tutorials focus on component selection and which pins to connect to what, this one takes the time to tell you why each component makes sense, and how the values affect the final result.

Curious about how the 555 came about? We’ve got the inside scoop on that.

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Altoids Tin Spy Radio Goes Solid State

[Helge Fykse (LA6NCA)] has a type, as they say. At least as far as radios are concerned, he seems to prefer elegant designs that keep the BOM to the minimum needed to get the job done. And Altoids tins — he really seems to like putting radios in Altoids tins.

This QRP transceiver for the 60-meter amateur radio band is a perfect example of that ethos. For the unfamiliar, QRP is Morse code shorthand for decreased power, and is generally used when hams are purposely building and operating radios that radiate very little power, typically below a watt. For this transceiver, [Helge] chose to use modern components, a marked but interesting departure from his recent tube-powered spy radios. The design is centered on a custom oscillator board he designed using an Arduino Pro Mini and an Si5351 oscillator chip. Other components include an ADE-1ASK frequency mixer, an antenna tuner module that can be swapped out for operating on different bands, a receiver that’s little more than a couple of op-amps, and a Darlington pair for an RF power amplifier. Everything fits neatly on a piece of copper-clad board inside the tin box.

As is his tradition, [Helge] was on the air in the field with this radio almost before the solder had time to cool. His first contact was a 240-km shot to a friend, who reported a fine signal from this little gem. And that’s with just powering it off a 9-volt battery when it’s designed to the typical 12-volt supplies hams favor; he estimates this resulted in a signal of about 200 mW. Not too shabby.

Honestly, we’d love to learn more about that oscillator board [Helge] used, and maybe get a schematic for it. We found a little bit about it on his web page, but not the juicy details. If you’re out there, [Helge], please share the wealth.

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Clock Hack Gives DEC Rainbow A New Lease On Life

In retrocomputing circles, it’s often the case that the weirder and rarer the machine, the more likely it is to attract attention. And machines don’t get much weirder than the DEC Rainbow 100-B, sporting as it does both Z80 and 8088 microprocessors and usable as either a VT100 terminal or as a PC with either CP/M or MS-DOS. But hey — at least it got the plain beige box look right.

Weird or not, all computers have at least a few things in common, a fact which helped [Dr. Joshua Reichard] home in on the problem with a Rainbow that was dead on arrival. After a full recapping — a prudent move given the four decades since the machine was manufactured — the machine failed to show any signs of life. The usual low-hanging diagnostic fruit didn’t provide much help, as both the Z80 and 8088 CPUs seemed to be fine. It was then that [Joshua] decided to look at the heartbeat of the machine — the 24-ish MHz clock shared between the two processors — and found that it was flatlined.

Unwilling to wait for a replacement, [Joshua] cobbled together a temporary clock from an Arduino Uno and an Si5351 clock generator. He connected the output of the card to the main board, whipped up a little code to generate the right frequency, and the nearly departed machine sprang back to life. [Dr. Reichard] characterizes this as a “defibrillation” of the Rainbow, and while one hates to argue with a doctor — OK, that’s a lie; we push back on doctors all the time — we’d say the closer medical analogy is that of fitting a temporary pacemaker while waiting for a suitable donor for a transplant.

This is the second recent appearance of the Rainbow on these pages — [David] over at Usagi Electric has been working on the graphics on his Rainbow lately.

An LM386 Oscillator Thanks To Tungsten Under Glass

Once ubiquitous, the incandescent light bulb has become something of a lucerna non grata lately. Banned from home lighting, long gone from flashlights, and laughed out of existence by automotive engineers, you have to go a long way these days to find something that still uses a tungsten filament.

Strangely enough, this lamp-stabilized LM386 Wien bridge oscillator is one place where an incandescent bulb makes an appearance. The Wien bridge itself goes back to the 1890s when it was developed for impedance measurements, and its use in the feedback circuits of vacuum tube oscillators dates back to the 1930s. The incandescent bulb is used in the negative feedback path as an automatic gain control; the tungsten filament’s initial low resistance makes for high gain to kick off oscillation, after which it heats up and lowers the resistance to stabilize the oscillation.

For [Grug Huler], this was one of those “just for funsies” projects stemming from a data sheet example circuit showing a bulb-stabilized LM386 audio oscillator. He actually found it difficult to source the specified lamp — there’s that anti-tungsten bias again — but still managed to cobble together a working audio oscillator. The first pass actually came in pretty close to spec — 1.18 kHz compared to the predicted 1.07 kHz — and the scope showed a very nice-looking sine wave. We were honestly a bit surprised that the FFT analysis showed as many harmonics as it did, but all things considered, the oscillator performed pretty well, especially after a little more tweaking. And no, the light bulb never actually lights up.

Thanks to [Grug] for going down this particular rabbit hole and sharing what he learned. We love builds like this that unearth seemingly obsolete circuits and bring them back to life with modern components. OK, calling the LM386 a modern component might be stretching things a bit, but it is [Elliot]’s favorite chip for a reason.

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