How Simple Can A Superhet Be

October 30, 2025 by Jenny List 20 Comments

If you cultivate an interest in building radios it’s likely that you’ll at some point make a simple receiver. Perhaps a regenerative receiver, or maybe a direct conversion design, it’ll take a couple of transistors or maybe some simple building-block analogue ICs. More complex designs for analogue radios require far more devices; if you’re embarking on a superhetrodyne receiver in which an oscillator and mixer are used to generate an intermediate frequency then you know it’ll be a hefty project. [VK3YE] is here to explode that assumption, with a working AM broadcast band superhet that uses only two transistors.

The circuit diagram of the radio — It doesn’t get much simpler than this.

A modern portable radio will almost certainly use an all-in-one SDR-based chip, but in the golden age of the transistor radio the first stage of the receiver would be a single transistor that was simultaneously RF amplifier, oscillator, and mixer. The circuit in the video below does this , with a ferrite rod, the familiar red-cored oscillator coil, and a yellow-cored IF transformer filtering out the 455 kHz mixer product between oscillator and signal.

There would normally follow at least one more transistor amplifying the 455 kHz signal, but instead the next device is both a detector and an audio amplifier. Back in the day that would have been a germanium point contact diode, but now the transistor has a pair of 1N4148s in its biasing. We’re guessing this applies a DC bias to counteract the relatively high forward voltage of a silicon diode, but we could be wrong.

We like this radio for its unexpected simplicity and clever design, but also because he’s built it spiderweb-style. We never expected to see a superhet this simple, and even if you have no desire to build a radio we hope you’ll appreciate the ingenuity of using simple transistors to the max.

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2025 Component Abuse Challenge: The Opto Flasher

October 29, 2025 by Jenny List 6 Comments

There’s a part you’ll find in almost every mains powered switch mode power supply that might at first appear to have only one application. An optocoupler sits between the low voltage and the high voltage sides, providing a safely isolated feedback. Can it be used for anything else? [b.kainka] thinks so, and has proved it by making an optocoupler powered LED flasher.

If a part can be made to act as an amplifier with a gain greater than one, then it should also be possible to make it oscillate. We’re reminded of the old joke about it being very easy to make an oscillator except when you want to make one, but in this case when an optocoupler is wired up as an inverting amplifier with appropriate feedback, it will oscillate. In this case the rather large capacitor leading to a longish period, enough to flash an LED.

We like this circuit, combining as it does an unexpected use for a part, and a circuit in which the unusual choice might just be practical. It’s part of our 2025 Component Abuse Challenge, for which you just about still have time to make an entry yourself if you have one.

All Hail The OC71

October 29, 2025 by Jenny List 50 Comments

Such are the breadth of functions delivered by integrated circuits, it’s now rare to see a simple small-signal transistor project on these pages. But if you delve back into the roots of solid state electronics you’ll find a host of clever ways to get the most from the most basic of active parts.\

Everyone was familiar with their part numbers and characteristics, and if you were an electronics enthusiast in Europe it’s likely there was one part above all others that made its way onto your bench. [ElectronicsNotes] takes a look at the OC71, probably the most common PNP germanium transistor on the side of the Atlantic this is being written on.

When this device was launched in 1953 the transistor itself had only been invented a few years earlier, so while its relatively modest specs look pedestrian by today’s standards they represented a leap ahead in performance at the time. He touches on the thermal runaway which could affect germanium devices, and talks about the use of black silicone filling to reduce light sensitivity.

The OC71 was old hat by the 1970s, but electronics books of the era hadn’t caught up. Thus many engineers born long after the device’s heyday retain a soft spot for it. We recently even featured a teardown of a dead one.

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2025 Component Abuse Challenge: A Bistable Flip-Flop With A Fuse

October 28, 2025 by Jenny List 8 Comments

The flip-flop, in whichever of its several forms you encounter it, is a staple of logic design. Any time that you need to hold onto something, count, or shift bits, out it comes. We expect a flip-flop to be an integrated circuit if we use one, but most of us could knock one together with a couple of transistors.

You aren’t restricted to transistors of course, a relay will do just as well, but how about a fuse? [b.kainka] has made a functioning set/reset flip-flop using a pair of PTC self-resetting fuses.

The circuit is simplicity itself, a pair of incandescent bulbs in series, each in turn in parallel with a momentary action switch and a PTC fuse. On start-up both fuses are conducting, so one or other of them will do its job as a fuse and go high impedance. At that point its bulb will light and the other fuse will remain low impedance so its bulb will stay dark. Press the switch across the lit bulb for a few seconds however, and the circuit resets itself. The other fuse goes high impedance while the first fuse returns to low impedance, and the other bulb lights.

We’re not sure we can see much in the way of practical application for this circuit, but sometimes merely because you can is reason enough. It’s part of our 2025 Component Abuse Challenge, for which you just about still have time to make an entry yourself if you have one.

Know Audio: Lossy Compression Algorithms And Distortion

October 28, 2025 by Jenny List 37 Comments

In previous episodes of this long-running series looking at the world of high-quality audio, at every point we’ve stayed in the real world of physical audio hardware. From the human ear to the loudspeaker, from the DAC to measuring distortion, this is all stuff that can happen on your bench or in your Hi-Fi rack.

We’re now going for the first time to diverge from the practical world of hardware into the theoretical world of mathematics, as we consider a very contentious topic in the world of audio. We live in a world in which it is now normal for audio to have some form of digital compression applied to it, some of which has an effect on what is played back through our speakers and headphones. When a compression algorithm changes what we hear, it’s distortion in audio terms, but how much is it distorted and how do we even measure that? It’s time to dive in and play with some audio files. Continue reading “Know Audio: Lossy Compression Algorithms And Distortion” →

A 3D Printed 16mm Movie Camera

October 27, 2025 by Jenny List 4 Comments

The basic principles of a motion picture film camera should be well understood by most readers — after all, it’s been well over a hundred years since the Lumière brothers wowed 19th century Paris with their first films. But making one yourself is another matter entirely, as they are surprisingly complex and high-precision devices. This hasn’t stopped [Henry Kidman] from giving it a go though, and what makes his camera more remarkable is that it’s 3D printed.

The problem facing a 16mm movie camera designer lies in precisely advancing the film by one frame at the correct rate while filming, something done in the past with a small metal claw that grabs each successive sprocket. His design eschews that for a sprocket driven by a stepper motor from an Arduino. His rotary shutter is driven by another stepper motor, and he has the basis of a good camera.

The tests show promise, but he encounters a stability problem, because as it turns out, it’s difficult to print a 16mm sprocket in plastic without it warping. He solves this by aligning frames in post-processing. After fixing a range of small problems though, he has a camera that delivers a very good picture quality, and that makes us envious.

Sadly, those of us who ply our film-hacking craft in 8mm don’t have the luxury of enough space for a sprocket to replace the claw.

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Exploding The Mystical Craftsman Myth

October 27, 2025 by Jenny List 85 Comments

As a Hackaday writer, I see a lot of web pages, social media posts, videos, and other tips as part of my feed. The best ones I try to bring you here, assuming of course that one of my ever-vigilant colleagues hasn’t beaten me to it. Along the way I see the tropes of changing content creator fashion; those ridiculous pea-sized hand held microphones, or how all of a sudden everything has to be found in the woods. Some of them make me laugh, but there’s one I see a lot which has made me increasingly annoyed over the years. I’m talking of course about the craftsman myth.

No. The Last True Nuts And Bolts Are Not Being Made In Japan

If you don’t recognise the craftsman myth immediately, I’m sure you’ll be familiar with it even if you don’t realise it yet. It goes something like this: somewhere in Japan (or somewhere else perceived as old-timey in online audience terms like Appalachia, but it’s usually Japan), there’s a bloke in a tin shed who makes nuts and bolts.

But he’s not just any bloke in a tin shed who makes nuts and bolts, he’s a special master craftsman who makes nuts and bolts like no other. He’s about 120 years old and the last of a long line of nut and bolt makers entrusted with the secrets of nut and bolt making, father to son, since the 8th century. His tools are also mystical, passed down through the generations since they were forged by other mystical craftsmen centuries ago, and his forge is like no other, its hand-cranked bellows bring to life a fire using only the finest cedar driftwood charcoal. The charcoal is also made by a 120 year old master charcoal maker Japanese bloke whose line stretches back to the n’th century, yadda yadda. And when Takahashi-san finally shuffles off this mortal coil, that’s it for nuts and bolts, because the other nuts and bolts simply can’t compare to these special ones. Continue reading “Exploding The Mystical Craftsman Myth” →