Half Crystal Radio, Half Regenerative Radio

A rite of passage in decades past for the electronics experimenter was the crystal radio. Using very few components and a long wire antenna, such a radio could pick up AM stations with no batteries needed, something important in the days when a zinc-carbon cell cost a lot of pocket money. The days of AM broadcasting may be on the wane, but it’s still possible to make a crystal set that will resolve stations on the FM band. [Andrea Console] has done just that, with a VHF crystal set that whose circuit also doubles as a regenerative receiver when power is applied.

The key to a VHF crystal set lies in the highest quality tuned circuit components to achieve that elusive “Q” factor. In this radio that is coupled to a small-signal zero voltage threshold FET that acts as a detector when no power is applied, and the active component in a regenerative radio when it has power. The regenerative radio increases sensitivity and selectivity by operating at almost the point of oscillation, resulting in a surprisingly good receiver for so few parts. Everyone should make a regenerative radio receiver once in their life!

The Simplest Way To Spot 2.4GHz RF

When the cool kids are showing off their SDRs it’s easy to forget that a radio receiver can be very simple indeed. The crystal set is one of the earliest forms of radio receiver, a tuned circuit and a diode that would pick up those AM broadcast stations no problem. But lest you imagine that these receivers can only pick up those low frequencies, here’s Hackaday alum [Ted Yapo] with a handy 2.4GHz receiver that picks up strong WiFi and microwave oven leakage.

It’s about as simple as it gets, an LED with a UHF diode in reverse across it. The clever part lies in the wire leads, which are cut to resonate as a dipole at 2.4 GHz. The resulting RF voltage is rectified by the UHF diode, leaving enough DC for the LED to flash. If you are wondering why the LED alone couldn’t do the job as a rectifier you would of course be on to something, however its much worse high frequency performance would make it not up to the job at this frequency.

The glory days of analogue broadcasting may now be in the past, but it’s still possible to have fun with a more conventional crystal radio. If you are adventurous, you can even make one that works for the FM, band too.

Cheap DIY High Impedance Earphones

Crystal radios can feel magical, given their ability to tune in audio from distant stations with nothing but the energy from the radio signal itself. However, to achieve this feat, they typically rely on a high-impedance earphone to produce an audible sound with very little current. These earphones are hard to find, and thus can be expensive. However, [Billy] figured out a way to build them on the cheap. 

The build starts with a common piezoelectric buzzer. It’s torn down and the extraneous circuitry inside is removed. The piezo element itself is then directly hooked up to a mono audio jack for use with one of [Billy’s] crystal radios. To make it into a usable earpiece, the tip of a pen is cut off and glued to the buzzer’s plastic housing. Then, a rubber in-ear cup from regular modern earbuds is used to ensure a tight, comfortable fit in the ear.

It’s a great way to build something that’s now hard to source, and we bet that [Billy’s] design is more comfortable than the hard plastic models that shipped in Radio Shack kits in the 90s. Of course, there’s other ways to build high-impedance drivers, as we’ve featured before. And, if you’re looking to build a crystal radio, it’s hard to go past [Billy’s] credit card chip build. Video after the break.

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How Early Radio Receivers Worked

If you’ve ever built a crystal radio, there’s something magical about being able to pull voices and music from far away out of thin air. If you haven’t built one, maybe you should while there’s still something on the AM band. Of course, nowadays the equivalent might be an SDR. But barring a computer solution, there are not many ways to convert radio waves into intelligence. From a pocket radio to advanced RADAR to a satellite in orbit, receiving a radio wave is accomplished in pretty much the same way.

There are, however, many ways to modulate and demodulate that radio wave. Of course, an AM radio works differently than an FM radio. A satellite data downlink works differently, too. But the process of capturing the radio wave from the air and getting them into a form ready for further processing hasn’t changed much over the years.

In this article, I’ll talk about the most common radio receiver architectures you may have seen in years past, and next week I’ll talk about modern architectures. Either way, understanding receiver architectures will help you design new radios or troubleshoot them.

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Hackaday Podcast 070: Memory Bump, Strontium Rain, Sentient Solder Smoke, And Botting Browsers

Hackaday editors Elliot Williams and Mike Szczys bubble sort a sample set of amazing hacks from the past week. Who has even used the smart chip from an old credit card as a functional component in their own circuit? This guy. There’s something scientifically devious about the way solder smoke heat-seeks to your nostrils. There’s more than one way to strip 16-bit audio down to five. And those nuclear tests from the 40s, 50s, and 60s? Those are still affecting how science takes measurements of all sorts of things in the world.

Take a look at the links below if you want to follow along, and as always, tell us what you think about this episode in the comments!

Direct download (60 MB or so.)

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Credit Card Chip Used To Make Crystal Radio

Perhaps the simplest radio one can build is the crystal radio. Using a diode as a detector, the design generally uses less than 10 components and no battery, getting its power to run from the radio signal itself. [Billy Cheung] decided to build a crystal radio using a rather unconventional detector – the smart chip in a common credit card.

This is possible because the smart chip on many credit cards contains a diode. It’s then a simple matter of hooking up the right pads on the credit card to the rest of a crystal radio circuit, and you’re all set. Of course, [Billy] goes the whole hog, building the entire radio on a single credit card. Other cards are cut up to create bobbins for winding coils to form a variable inductor, used to tune the radio. Doing this allows for a much cleaner, thinner design, rather than using a variable capacitor which is comparatively hard to find. Turning the dial allows stations to be tuned in, and with a high impedance earbud hooked up, you’re listening to AM radio. Oh, and don’t forget an antenna!

[Billy] breaks down the details for anyone wishing to replicate the feat, going so far as to wind the coils in real time in his Youtube video. Cutting templates and other details are available on Github. While it’s not going to be the most replicated hack, as it requires the destruction of a credit card to achieve, we love the ingenuity. And, if society does collapse, we’ll all have a great source of diodes when the ATMs have all become useless. Video after the break.

[Thanks to Zane Atkins for the tip!]

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[Eric] Talks Crystal Radios

The AM broadcast band doesn’t have a lot of mainstream programming on it across much of the United States today. That’s a shame because a lot of kids got their first taste of radio and electronics by building simple crystal radios. [Eric Wrobbel] has a well-done page discussing some of the crystal radio kits and toys that have been around.

[Eric] should know, as he’s written two books on toy crystal radios. The pictures range from a 1945-era “Easy Built Radio Kit” which looks like a piece of masonite with a coil, some Fahnestock clips, and a cat whisker, to a very slick looking Tinymite from 1949. Honestly, though, the one we really want is the X-50 Space Helmet Radio that comes in a box marked “For Young Moon Travelers.”

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