A Working Intercom From Antique Telephones

February 9, 2026 by Bryan Cockfield 13 Comments

Although it can be hard to imagine in today’s semiconductor-powered, digital world, there was electrical technology around before the widespread adoption of the transistor in the latter half of the 1900s that could do more than provide lighting. People figured out clever ways to send information around analog systems, whether that was a telegraph or a telephone. These systems are almost completely obsolete these days thanks to digital technology, leaving a large number of rotary phones and other communications systems relegated to the dustbin of history. [Attoparsec] brought a few of these old machines back to life anyway, setting up a local intercom system with technology faithful to this pre-digital era.

These phones date well before the rotary phone that some of us may be familiar with, to a time where landline phones had batteries installed in them to provide current to the analog voice circuit. A transformer isolated the DC out of the line and amplified the voice signal. A generator was included in parallel which, when operated by hand, could ring the other phones on the line. The challenge to this build was keeping everything period-appropriate, with a few compromises made for the batteries which are D-cell batteries with a recreation case. [Attoparsec] even found cloth wiring meant for guitars to keep the insides looking like they’re still 100 years old. Beyond that, a few plastic parts needed to be fabricated to make sure the circuit was working properly, but for a relatively simple machine the repairs were relatively straightforward.

The other key to getting an intercom set up in a house is exterior to the phones themselves. There needs to be some sort of wiring connecting the phones, and [Attoparsec] had a number of existing phone wiring options already available in his house. He only needed to run a few extra wires to get the phones located in his preferred spots. After everything is hooked up, the phones work just as they would have when they were new, although their actual utility is limited by the availability of things like smartphones. But, if you have enough of these antiques, you can always build your own analog phone network from the ground up to support them all.

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An Introduction To Analog Filtering

December 4, 2025 by Bryan Cockfield 8 Comments

One of the major difficulties in studying electricity, especially when compared to many other physical phenomena, is that it cannot be observed directly by human senses. We can manipulate it to perform various tasks and see its effects indirectly, like the ionized channels formed during lightning strikes or the resistive heating of objects, but its underlying behavior is largely hidden from view. Even mathematical descriptions can quickly become complex and counter-intuitive, obscured behind layers of math and theory. Still, [lcamtuf] has made some strides in demystifying aspects of electricity in this introduction to analog filters.

The discussion on analog filters looks at a few straightforward examples first. Starting with an resistor-capacitor (RC) filter, [lcamtuf] explains it by breaking its behavior down into steps of how the circuit behaves over time. Starting with a DC source and no load, and then removing the resistor to show just the behavior of a capacitor, shows the basics of this circuit from various perspectives. From there it moves into how it behaves when exposed to a sine wave instead of a DC source, which is key to understanding its behavior in arbitrary analog environments such as those involved in audio applications.

There’s some math underlying all of these explanations, of course, but it’s not overwhelming like a third-year electrical engineering course might be. For anyone looking to get into signal processing or even just building a really nice set of speakers for their home theater, this is an excellent primer. We’ve seen some other demonstrations of filtering data as well, like this one which demonstrates basic filtering using a microcontroller.

2025 Component Abuse Challenge: Digital Logic With Analog Components

October 4, 2025 by Bryan Cockfield 10 Comments

[Tim] noticed recently that a large number of projects recreating discrete logic tend to do so with technology around 70 years old like resistor-transistor logic (RTL) or diode-transistor logic (DTL). To build something with these logic families nowadays requires an intense treasure hunt of antique components bordering on impossible and/or expensive. Rather than going down this rabbit hole he decided to invent a somewhat new logic system using analog components in this entry in our Component Abuse Challenge.

The component in question here is an analog multiplexer, which is normally used to select one of two (or several) signal lines and pass them through to an output. Unlike digital multiplexers which only pass 1s and 0s, analog multiplexers can pass analog signals since the transistors aren’t driven to saturation. He has come up with an entire system of logic gates using these components, with trickier devices like latches eventually implemented with help from a capacitor.

The first attempt at using this logic system had a small mistake in it which caused these latches to behave as oscillators instead, due to a polarity mistake. But a second attempt with simplified design and reduced component count ended up working, proving out [Tim]’s concept. Not only that but his second prototype is functioning at an impressive 15 MHz, with a possibility of an even higher clock speed in future designs. Not bad!

A photo of the front-panel with a bunch of lamps and knobs.

The Making Of A Minimalist Analog Drum Machine

October 1, 2025 by John Elliot V 15 Comments

Our hacker [Moritz Klein] shows us how to make a minimalist analog drum machine. If you want the gory details check out the video embedded below and there is a first class write-up available as a 78 page PDF manual too. Indeed it has been a while since we have seen a project which was this well documented.

A typical drum machine will have many buttons and LEDs and is usually implemented with a microcontroller. In this project [Moritz] eschews that complexity and comes up with an analog solution using a few integrated circuits, LEDs, and buttons.

The heart of the build are the integrated circuits which include two TL074 quad op amps, a TL072 dual op amp, a CD4520 binary counter, and eight CD4015 shift registers. Fifteen switches and buttons are used along with seven LEDs. And speaking of LEDs, our hacker [Moritz] seems to have an LED schematic symbol tattooed to his hand, and we don’t know about you, but this screams credibility to us! :)

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Checking Out A TV Pattern Generator From 1981

September 2, 2025 by Donald Papp 11 Comments

The picture on a TV set used to be the combined product of multiple analog systems, and since TVs had no internal diagnostics, the only way to know things were adjusted properly was to see for yourself. While many people were more or less satisfied if their TV picture was reasonably recognizable and clear, meaningful diagnostic work or calibration required specialized tools. [Thomas Scherrer] provides a close look at one such tool, the Philips PM 5519 GX Color TV Pattern Generator from 1981.

This Casio handheld TV even picked up the test pattern once the cable was disconnected, the pattern generator acting like a miniature TV station.

The Philips PM 5519 was a serious piece of professional equipment for its time, and [Thomas] walks through how the unit works and even opens it up for a peek inside, before hooking it up to both an oscilloscope and a TV in order to demonstrate the different functions.

Tools like this were important because they could provide known-good test patterns that were useful not just for troubleshooting and repair, but also for tasks like fine-tuning TV settings, or verifying the quality of broadcast signals. Because TVs were complex analog systems, these different test patterns would help troubleshoot and isolate problems by revealing what a TV did (and didn’t) have trouble reproducing.

As mentioned, televisions at the time had no self-diagnostics nor any means of producing test patterns of their own, so a way to produce known-good reference patterns was deeply important.

TV stations used to broadcast test patterns after the day’s programming was at an end, and some dedicated folks have even reproduced the hardware that generated these patterns from scratch.

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The Tape Speed Keyboard

August 4, 2025 by Bryan Cockfield 10 Comments

For those who experienced any part of the 1960s, even if it’s just experiencing the music from that era here in the future, the sound of the Mellotron is immediately recognizable. The Moody Blues were famous for using the tape-based instrument, and the Beatles and David Bowie produced hits with it as well. It’s haunting sounds are still highly prized today, but the complexity, cost, and maintenance requirement for the tape loops and other moving parts can put many musicians off from owning one. But [Japhy Riddle] has built an instrument without these downsides called the Tape Speed Keyboard.

Unlike the Mellotron which used a tape loop for each of its keys, the Tape Speed Keyboard uses only a single cassette tape. As the name implies, it changes the pitch of the sound by modulating the speed of the single tape housed in its own tape deck. The keyboard itself started off life as a Casio MT-35 but since this is a completely analog instrument, it was rewired so each key is connected to a potentiometer whose output voltage is tuned to a specific tape speed. [Japhy] reports that this is similar to tuning an analog piano and the process can be equally temperamental.

With everything electronic working, [Japhy] turned to making this a more acceptable musical instrument. Predictably, turning the motor on and off for each key press came with a bit of delay, causing the sound to come out goofy and muddy. To solve this problem he changed the design to make the tape play continuously rather than start and stop for a key press, and then modified other keys to be on-off switches for sound output. Since cassette tapes have two sides, he can also play either of two sounds in this way.

With the final polish on, the Tape Speed Keyboard is able to produce completely unique compositions that separate it from even the venerable Mellotron. Be sure to check out the video linked below to hear its sound. There have been plenty of other musical projects based around tape decks as well, including this one inspired by the original Mellotron and this tape deck-based guitar effects pedal.

Thanks to [splashbun] for the tip!

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A sine wave and triangle wave on a black background

2025 One Hertz Challenge: Op-Amp Madness

July 31, 2025 by Adam Zeloof 7 Comments

Sometimes, there are too many choices in this world. My benchtop function generator can output a sine, square, or saw wave anywhere from 0.01 Hz up to 60 MHz? Way too many choices. At least, that’s what we suspect [Phil Weasel] was thinking when he built this Analog 1 Hz Sinewave Generator.

Rendering of a PCB — A KiCad rendering of [Phil]’s design

[Phil]’s AWG (which in this case stands for Anything as long as it’s a 1 Hz sine Wave Generator) has another unique feature — it’s built (almost) entirely with op-amps. A lot of op-amps (37, by our count of the initial schematic he posted). His design is similar to a Phased Locked Loop (PLL) and boils down to a triangle wave oscillator. While a 1 Hz triangle wave would absolutely satisfy judges of the One Hertz Challenge, [Phil] had set out to make a sine wave. Using a feedback loop and some shaping/smoothing tricks (and more op-amps), he rounded off the sharp peaks into a nice smooth sine wave.

Sometimes we make things much more complicated than we need to, just to see if we can. This is one of those times. Are there much simpler ways to generate a sine wave? Yes — but not exclusively using op-amps! This entry brings stiff competition to the “Ridiculous” category of the 2025 One Hertz Challenge.