ChatControl Gets Coup-De-Grace

November 1, 2025 by Jenny List 37 Comments

Possibly the biggest privacy story of the year for Europeans and, by extension the rest of the world, has been ChatControl. Chatcontrol is a European Union proposal backed by Denmark for a mandatory backdoor in all online communications. As always with these things, it was touted as a think-of-the-children solution to online child abuse material, but as many opposed to it have warned, that concealed far more sinister possibilities. For now, it seems we can breathe easily as the Danes are reported to have formally backed away from the proposal after it was roundly condemned by the German government, sending it firmly into the political wilderness.

Hackaday readers are likely vastly more informed on this matter than many of the general public, so you’ll have no need for a primer on the obvious privacy and security concerns of such a move. From our point of view, it also suffered from the obvious flaw of being very unlikely to succeed in its stated aim. Even the most blinkered politician should understand that criminals would simply move their traffic to newly-illegal encrypted forms of communication without government backdoors. Perhaps it speaks volumes that it was the Germans who sounded its death-knell, given that state surveillance on that level is very much within living memory for many of them.

The mood in European hackerspaces has been gloomy of late on the subject, so it’s something of a cause for celebration on the continent. If only other governments on the same side of the Atlantic could understand that intrusive measures in the name of thinking of the children don’t work.

European flags: Šarūnas Burdulis, CC BY-SA 2.0 .

2025 Component Abuse Challenge: An Input Is Now An Output

October 31, 2025 by Jenny List 20 Comments

Part of setting up a microcontroller when writing a piece of firmware usually involves configuring its connections to the outside world. You define a mapping of physical pins to intenral peripherals to decide which is an input, output, analogue, or whatever other are available. In some cases though that choice isn’t available, and when you’ve used all the available output pins you’re done. But wait – can you use an input as an output? With [SCART VADER]’s lateral thinking, you can.

The whole thing takes advantage of the internal pull-up resistor that a microcontroller has among its internal kit of parts. Driving a transistor from an output pin usually requires a base resistor, so would it be possible to use the pullup as a base resistor? If the microcontroller can enable or disable the resistor on an input pin then yes it can, a transistor can be turned off and on with nary an output to be seen. In this case the chip is from ATmega parts bin so we’re not sure if the trick is possible on other manufacturers’ devices.

As part of our 2025 Component Abuse Challenge, this one embodies the finest principles of using a part in a way it was never intended to be used, and we love it. You’ve still got a few days to make an entry yourself at the time of writing this, so bring out your own hacks!

The Time Of Year For Things That Go Bump In The Night

October 30, 2025 by Jenny List 23 Comments

Each year around the end of October we feature plenty of Halloween-related projects, usually involving plastic skeletons and LED lights, or other fun tech for decorations to amuse kids. It’s a highly commercialised festival of pretend horrors which our society is content to wallow in, but beyond the plastic ghosts and skeletons there’s both a history and a subculture of the supernatural and the paranormal which has its own technological quirks. We’re strictly in the realm of the science here at Hackaday so we’re not going to take you ghost hunting, but there’s still an interesting journey to be made through it all.

Today: Fun For Kids. Back Then: Serious Business

A marble carved skull on a 17th century monument in the church of st. Mary & st. Edburga, Stratton Audley, Oxfordshire. — English churches abound with marble-carved symbols of death.

Halloween as we know it has its roots in All Hallows Eve, or the day before the remembrance festivals of All Saint’s Day and All Soul’s Day in European Christianity. Though it has adopted a Christian dressing, its many trappings are thought to have their origin in pagan traditions such as for those of us where this is being written, the Gaelic Samhain (pronounced something like “sow-ain”). The boundary between living and dead was thought to be particularly porous at this time of year, hence all the ghosts and other trappings of the season you’ll see today.

Growing up in a small English village as I did, is to be surrounded by the remnants of ancient belief. They survive from an earlier time hundreds of years ago when they were seen as very real indeed, as playground rhymes at the village school or hushed superstitions such as that it would be bad luck to walk around the churchyard in an anticlockwise manner.

As a small child they formed part of the thrills and mild terrors of discovering the world around me, but of course decades later when it was my job to mow the grass and trim the overhanging branches in the same churchyard it mattered little which direction I piloted the Billy Goat. I was definitely surrounded by the mortal remains of a millennium’s worth of my neighbours, but I never had any feeling that they were anything but at peace. Continue reading “The Time Of Year For Things That Go Bump In The Night” →

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.

Continue reading “How Simple Can A Superhet Be” →

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.

Continue reading “All Hail The OC71” →

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.