Detect Lightning Strikes With An Arduino

September 1, 2021 by Bryan Cockfield 20 Comments

Lightning is a powerful and seemingly mysterious force of nature, capable of releasing huge amounts of energy over relatively short times and striking almost at random. Lightning obeys the laws of physics just like anything else, though, and with a little bit of technology some of its mysteries can be unraveled. For one, it only takes a small radio receiver to detect lightning strikes, and [mircemk] shows us exactly how to do that.

When lightning flashes, it also lights up an incredibly wide spectrum of radio spectrum as well. This build uses an AM radio built into a small integrated circuit to detect some of those radio waves. An Arduino Nano receives the signal from the TA7642 IC and lights up a series of LEDs as it detects strikes in closer and closer proximity to the detector. A white LED flashes when a strike is detected, and some analog circuitry supports an analog galvanometer which moves during lightning strikes as well.

While this project isn’t the first lightning detector we’ve ever seen, it does have significantly more sensitivity than most other homemade offerings. Something like this would be a helpful tool to have for lifeguards at a pool or for a work crew that is often outside, but we also think it’s pretty cool just to have around for its own sake, and three of them networked together would make triangulation of strikes possible too.

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A Superheterodyne Receiver With A 74xx Twist

April 19, 2021 by Dan Maloney 31 Comments

In a world with software-defined radios and single-chip receivers, a superheterodyne shortwave radio might not exactly score high on the pizzazz scale. After all, people have been mixing, filtering, and demodulating RF signals for more than a century now, and the circuits that do the job best are pretty well characterized. But building the same receiver using none of the traditional superhet trappings? Now that’s something new.

In what [Micha] half-jokingly calls a “74xx-Defined Radio”, easily obtained discrete logic chips, along with some op-amps and a handful of simple components, take the place of the tuned LC circuits and ganged variable capacitors that grace a typical superhet receiver. [Micha] started by building an RF mixer out of a 74HC4051 analog multiplexer, which with the help of a 2N3904 phase splitter forms a switching mixer. The local oscillator relies on the voltage-controlled oscillator (VCO) in a 74HC4046 PLL, a chip that we’ve seen before in [Elliot Williams]’ excellent “Logic Noise” series. The IF filter is a simple op-amp bandpass filter; the demodulator features an op-amp too, set up as an active half-wave rectifier. No coils to wind, no capacitors to tune, no diodes with mysterious properties — and judging by the video below, it works pretty well.

It may not be the most conventional way to tune in the shortwave bands, but we always love the results of projects that are artificially constrained like this one. Hats off to [Micha] for the interesting trip down the design road less travelled.

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A One-Transistor Ham Transmitter Anyone Can Build

January 20, 2021 by Dan Maloney 65 Comments

What attracts a lot of people to amateur radio is that it gives you the ability to make your own gear. Scratch-building hams usually start by making their own antennas, but eventually, the itch to build one’s own radio must be scratched. And building this one-transistor transmitter is just about the simplest way to dive into the world of DIY radio.

Of course, limiting yourself to eight components in total entails making some sacrifices, and [Kostas (SV3ORA)]’s transmitter is clearly a study in compromise. For starters, it’s only a transmitter, so you’ll need to make other arrangements to have a meaningful conversation. You’ll also have to learn Morse code because the minimalist build only supports continuous-wave (CW) mode, although it can be modified for amplitude modulation (AM) voice work.

The circuit is flexible enough that almost any part can be substituted and the transmitter will still work. Most of the parts are junk-bin items, although the main transformer is something you’ll have to wind by hand. As described, the transformer not only provides feedback to the transistor oscillator, but also has a winding that powers an incandescent pilot lamp, and provides taps for attaching antennas of different impedances — no external tuner needed. [SV3ORA] provides detailed transformer-winding instructions and shows the final build, which looks very professional and tidy. The video below shows the rig in action with a separate receiver providing sidetone; there’s also the option of using one of the WebSDR receivers sprinkled around the globe to verify you’re getting out.

This little transmitter looks like a ton of fun to build, and we may just try it for our $50 Ham series if we can find all the parts. Honestly, the hardest to come by might be the variable capacitor, but there are ways around that too.

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Antenna Pulls In AM Stations

August 19, 2020 by Al Williams 26 Comments

While we can’t argue that FM has superior audio quality and digital streaming allows even higher quality in addition to worldwide access, there’s still something magic about hearing a weak and fading AM signal from thousands of miles away with nothing between the broadcaster’s antenna and yours. If you can’t have a big antenna — or even if you can — a loop antenna can help your big antenna fit in less space. In the video after the break, [TheOffsetVolt] covers an AM loop and shows how it can pull in distant AM stations.

Continuing with the educational radio he’s talked about before, he adds a loop antenna that is two feet on each side of a square, making it four square feet in area. Although he calls it an amplifier, it’s really just a passive tuned circuit that couples to the radio’s built-in antenna. There’s no actual connection between the antenna and the radio.

We aren’t sure if the reradiation explanation is really what’s happening, or if it is just transformer coupled to the main antenna. But either way, it seems to work well. You can think of this as adding a preselector to the existing radio. Loop antennas are directional, so this design could work as a direction finder.

We have seen many loop antennas, some with novel construction methods. Some even tune themselves.

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Pulling A Crystal By Grinding It

May 10, 2020 by Jenny List 29 Comments

If you own a radio transmitter, from a $10 Baofeng handheld to a $1000 fancy all-band transceiver, setting the frequency is simply a case of dialing in where you want to go. A phase-locked-loop frequency synthesizer or a software-defined radio will generate your frequency, and away you go. There was a time though when synthesizers were impossibly complex and radio amateurs were faced with a simple choice. Use an LC oscillator and put up with drifting in frequency, or use a crystal oscillator, and be restricted to only the frequencies of the crystals you had. [Mark Erdle, AE2EA] modified a 1950s broadcast AM broadcast transmitter for the 1.8MHz amateur band, and his friend [Andy Flowers, K0SM] thought it needed its crystal back for originality rather than the external frequency source [Mark] had provided. He documents the process of modifying a crystal oven and moving a crystal frequency in the video below the break.

A crystal oven is a unit containing the crystal itself alongside a thermostatic heater, and in this one, the crystal was a 1970s-vintage hermetically sealed HC6 device. He modified the oven to take a socket for older FT243 crystals because the quartz element can easily be accessed. [Andy] picked a crystal as close as he could find below the required frequency. He then ground it down with very fine grit on a glass plate, reducing its mass and thus its resonant frequency. We’re taken through the process of getting it close to frequency, but sadly don’t see the etching that he uses for the very last stage. At the end of the video, we see a QSO on the transmitter itself, which is something of an oddity in an age when AM on amateur bands has been supplanted by other modes for decades.

If you’re curious about the transmitter there’s a video thread following its restoration, and if the guts of older radio gear interests you then take a look at this aircraft receiver lovingly brought back to life.

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An FPGA And A Few Components Can Make A Radio

April 18, 2020 by Jenny List 18 Comments

There was a time when making a radio receiver involved significant work, much winding of coils, and tricky alignment of circuitry. The advent of Software Defined Radio (SDR) has moved a lot of this into the domain of software, but there is of course another field in which a radio can be created via code. [Alberto Garlassi] has created a radio receiver for the AM and HF bands with a Lattice MachXO2 FPGA and minimal external components.

He describes it as an SDR, which given that it’s created from Verilog, is a term that could be applied to it. But instead of using an SDR topology of ADC and digital signal processing, it implements a surprisingly traditional direct conversion receiver.

It has a quadrature AM demodulator which has a passing similarity to an SDR with I and Q phased signals, but that’s where the similarity ends. Frequency selection is via an oscillator controlled from a serial port, and there is even a PWM amplifier on board that can drive a speaker. The result can be seen in the video below, and as you can hear the direct conversion with quadrature demodulator approach makes for a very effective AM receiver.

If this is a little much but you still fancy a radio with minimal components, you should have a look at the Silicon Labs range of receiver chips.

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A Dangerous Demonstration Of The Power Of Radio

April 16, 2020 by Dan Maloney 56 Comments

Terrestrial radio may be a dying medium, but there are still plenty of listeners out there. What would a commute to or from work be without a check of “Traffic on the Eights” to see if you need to alter your route, or an update of the scores from yesterday’s games? Getting that signal out to as many listeners as possible takes a lot of power, and this dangerous yet fascinating demo shows just how much power there is on some radio towers.

Coming to us by way of a reddit post, the short video clips show a crew working on a 15,000-Watt AM radio tower. They appear to be preparing to do tower maintenance, which means de-energizing the antenna. As the engineer explains, antennas for AM radio stations in the medium-wave band are generally the entire tower structure, as opposed to the towers for FM and TV stations, which generally just loft the antenna as high as possible above the landscape. The fun starts when the crew disconnects a jumper and an arc forms across the clamp and the antenna feed. The resulting ball of plasma acts like a speaker, letting us clearly hear the programming on the station. It’s like one of the plasma speakers we’ve seen before, albeit exceptionally more dangerous.

It’s an impressive display of the power coursing through broadcast towers, and a vivid reminder to not mess with them. Such warnings often go unheeded, sadly, with the young and foolish paying the price. There’s a reason they put fences up around radio towers, after all.

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