If you’ve ever used an old tube radio, you might be familiar with that mysterious little green display that helps you to tune exactly to a station. That display is called a tuning indicator, or magic eye tube; in essence it’s a minimalistic cathode ray tube that can sweep its electron beam along only one axis. It thereby outputs a kind of bar graph that varies with the input voltage.
With few modern uses other than being pretty, it only makes sense that these tubes find their way into works of art: [Patrice] used one to make an insect-like piece of circuit sculpture. The tube he used is an EM34, which is one of the most common indicator tubes around and has a circular, iris-like display area. This becomes a large eye, peering forward from the bug’s body. The legs are made from 1.5 mm thick brass wire, while a DC/DC converter generates the 210 Volts DC needed to operate the tube.
An interesting “touch” is the addition of two antennae that are hooked up in such a way that the tube’s image changes when you push them; this interactivity makes the bug come alive a little bit. Speaking of touch, we think it would be prudent to put some insulation around the 210 V wires; even though the bug is battery-powered, touching the high voltage and ground wires simultaneously would deliver a nasty shock.
Nevertheless, the bare-wire retro design looks beautiful and would make a great ornament for any electronics-lover’s office. We’ve seen magic eye tubes being used for various purposes: you can turn them into a spectrum analyzer, measure capacitors with them, or simply use them as a bar-graph display.
New Old Stock (NOS) magic-eye tubes are still easy to get, around $5-$15 bucks per tube. There are even kits with the driving circuitry included.[1] See [2] for an entire Web site dedicated to magic-eye tubes. I never knew there were so many different display types.[3]
* References:
1. Magic-eye tube stuff on Ebay:
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Magic+Eye+Tube
2. All About Magic-Eye Tubes:
http://magiceyetubes.com/
3. 2. Magic-Eye Tubes – Display Patterns:
http://magiceyetubes.com/patterns.htm
If you have an old gadget with one that has weak emission or tired phosphor after all these years, these tubes can be hard to get and expensive when you find them. Some enterprising young whippersnapper ought to design a replacement using a smart watch display and a tiny computing thingie, using the 6 volt filament supply as power.
We are lucky to live in an age where small color displays are getting cheap, and Arduino Nanos are cheaper. One of the dead parts in my Central Electronics 20A when I got it was the 6E5 green eye. Well, it wasn’t totally dead, but you had to have the lights off to see the faint glow from the phosphor. I did have an NOS tube in the junk box. When this one dies, I think I’ll put in a display.
I recently picked up three volumes of “Modern Practical Radio and Television”, 1952 edition from my local charity shop.
It has a nice few paragraphs dedicated to how these indicators are hooked up to a superhet, which closes the circle on my boyhood fascination with my late grandad’s “hifi”, a massive walnut cabinet with AM _and_ FM radio, with a magic eye tuner. Oh, and at the top – a cocktail cabinet! Xmas round at my grannies had it’s compensations!