Consider that humble device, the digital alarm clock. It does its job, telling the time with four simple digits, but it doesn’t have a whole lot of soul. This is why handmade Nixie-tube alarm clocks still sell to collectors: The glow from a gas-discharge tube is supernaturally special compared with that weary workhorse of digital electronics, the seven-segment numeric display. People enjoy funky-looking retro gear that has character. If it glows in the dark, so much the better.
Light-emitting devices have been on my mind, lately, as I’ve been writing about them in the Encyclopedia of Electronics, Volume Two (to be published in time for Christmas, I hope). Digging around in parts catalogues and in my own basement, I came across oddball displays and indicators that could give any new project some geek appeal. I’m talking about the kind of thing that makes someone stop and say, “Wait a minute. What is that?”
Long ago, miniature light bulbs fulfilled the function that is now served by LEDs. Surprisingly, those baby bulbs are still available, for a wide variety of voltages, for less than $1 in some cases. The sample in Figure A is super-tiny, only 3mm in diameter, terminating in two little pins that you can insert into a breadboard. The bulb in Figure B is of similar size, with thin wire leads. Or you can choose a screw-in base, as shown in Figure C, allowing easy replacement if the bulb burns out. All three of these incandescent indicators run on 5VDC (or 5VAC — they’re not fussy).
They draw slightly more current than an LED, and may be rated for 25,000 hours instead of 50,000. But they don’t need series resistors, there’s no polarity to worry about, and the pale yellow radiance is special.
Search an online catalogue for “incandescent” and you’ll find all kinds of goodies.
Nicer in Neon
The word neon may make you think of Las Vegas, but I’m still thinking about indicators. A neon bulb is a gas-discharge tube in which a relatively high voltage (80V and up) ionizes the gas, emitting photons. Neon bulbs aren’t super-bright, and their discharge comes in only one color (orange), but that’s part of the retro appeal. You can buy tiny ones, or medium-size ones, or full-size light bulbs containing electrodes in fanciful shapes — like the one in Figure D, which I picked up at a yard sale. The little lamp in Figure E is more practical, about a quarter-inch in diameter, sold with a series resistor preattached. It needs 110VAC but draws hardly any current, and it can be switched with the smallest available 5VDC solid-state relay.
Exercise caution with 110 volts: One wiring error will zap your digital components instantly, and can also zap you.
Nine Segments Too Many
While seven-segment LEDs manage to do a minimal job of displaying numerals, they were never intended for letters of the alphabet. Manufacturers responded to this need by adding extra segments — but 16-segment alphanumeric LEDs were so ugly, few people wanted to look at them. Consequently they are now almost obsolete, which makes them a great nostalgia item. The display in Figure F, pictured while attempting to represent the letter N, is 0.8″ tall. If you put eight of them together you can multiplex them through a controller chip such as the Maxim MAX6954, which has a 16-segment alphabet stored in ROM.
Imagine a fortune-telling toy that offers its advice from a random selection of single words, using creepy green 16-segment displays that are soooo 1990s. Maybe you can arrange for some of the segments to malfunction, creating a flickering effect that is tantalizingly ambiguous. What did it just say? Was it HOPE, or NOPE? Or maybe COPE?
Consider adding a vibration sensor so that if you bang on the box, the display will stabilize itself momentarily. Building a retro device that pretends to malfunction can open up a world of possibilities.
Pimp My Chip
The TIL311 is a numeric LED display designed by Texas Instruments back in 1972. Send it a binary number from 0000 through 1001 and its internal logic generates a decimal digit from 0 through 9. Current-limiting for the LEDs is built in, no decoder is needed, and there’s no tricky serial communication protocol. Just drive its four inputs directly from a counter chip or a microcontroller, and you have an instant no-hassle numeric output. If you really want to geek out, the chip can represent hexadecimal values by using letters A through F.
It generates decent-looking dot-matrix characters (as shown in Figure G), and its body is molded from translucent red plastic, revealing the conductors inside. With gold-plated pins, it has a pimped-out look, like cheap jewelry. Just the thing for wearable electronics.
Because TI doesn’t make it anymore, it has become a scarce and coveted item, with U.S. suppliers charging a minimum of $20 apiece for old stock. Fortunately you can find them on eBay from Asian sources at one-quarter the price. I have to wonder how those funky old chips ended up in China. Maybe they were pulled from trashed 1970s minicomputers. I just bought a bunch of them, and I think you’ll find there are still a few left, at least for now.
Around 1973 my father acquired one of the first Commodore calculators. I ran across it recently, opened it up, and was blown away by its trippy little vacuum fluorescent display (VFD), shown in Figure H. Nine tiny digits reside in a sealed capsule, proudly made in Japan. (In those days, China was still ruled by Chairman Mao, and was barely capable of building houses, let alone manufacturing electronics.)
The display probably takes about 20V, but the keypad interfaces with the calculating chip using 5VDC. So, you could repurpose it as an output display for a microcontroller. Just follow the conductors from the keys to the chip, cut the traces, and make new connections. And if you didn’t share my good fortune of inheriting a vintage digital artifact from your dad, you can find them online for maybe $5 each. Most are not considered collectible (yet), despite the totally cool ‘70s styling shown in Figure I.
I’ll bet you can think of some other idiosyncratic indicators to add pedigree to your next project. No matter what you build, it will surely benefit from the mystique of light-emitting technology that is almost, but not quite, obsolete.