Just in case you haven’t spent two painful minutes talking to me, I should probably come out and admit that I’m a huge dork. As a huge dork, I have hugely dorky interests, like woefully obsolete display technologies and woefully primitive cars. It seems like the right thing to do would be to combine these interests. Luckily, I have a plan.
My plan is this: I want to make a digital speedometer for my 1973 Volkswagen Beetle. But I don’t want just any kind of digital speedometer — I want something that could have been, hypothetically, period-correct for a car from the early 1970s. I also have a number of aesthetic desires as well — I don’t want it to look like the usual, expected 7-segment numerical displays we’ve been seeing everywhere since the 1980s.
So, if I want to avoid the usual 7-segment number look and be something period-accurate, what are my options? There were a number of different ways of displaying changing numbers in the late ‘60s and early ‘70s. Seven-segment displays did exist, but they were still generally expensive. The Apollo Command Module computer display used them, and other sorts of equipment, but most consumer-grade stuff used other options, like the popular flip-card displays you may have seen on clock radios forgotten in your grandmother’s closet.
I don’t really want to use flip-card displays — they’re too slow and noisy for a speedometer. Nixie tubes are an interesting option. They look great, and fit the period well, but they also demand pretty high voltage power and are kind of expensive now.
Happily, chance has provided me with the perfect solution. A few years ago, my friend Tom Jennings gave me these fascinating and very-forgotten bits of old display hardware called one-plane displays. Here’s an old Soviet kiloohmmeter that uses them:
They’re display devices that use tiny scrims and lenses to project an image onto a screen. Each display unit can contain many individual images, any of which can be projected onto the screen by giving power to one of the light bulbs positioned to illuminate that particular image. The one I have even uses Futura for the font for the numbers, which also happens to be VW’s corporate font for that era, and is the font used in the original 1973 speedometer and fuel gauge.
The result is a fast-changing display that’s relatively compact, and has a distinct look all its own. Even better, one of the units he gave me was a two-digit numerical display — perfect for a speedometer.
Here, it’s easy to just show these to you. Here’s a video I made when I first got these:
As you can see from the video, I’ve actually been thinking about doing this for years, and thanks to a rigorous life-plan of procrastination, kid-having, and general large-task inertia, I’ve managed to put off starting it. Until now. Our launch of the Garage has inspired me to get off my ass and actually make this inane dream happen. So let’s start by figuring out what I need to do:
• Get the one-plane display to physically fit in the housing of an old Beetle speedo
• Replace all the old incandescent bulbs on the display with LEDs
• Find some way to get data from the wheel to compute speed (hall-effect sensor?)
• Get the data from the wheel sensor to an Arduino
• Have the Arduino convert the wheel RPMs to MPH, then take the (pretty likely) two-digit MPH number and read each digit separately
• The Arduino then needs to convert each digit into the appropriate LED to illuminate
• Rinse and repeat
Here’s the same basic idea in sketch-form:
You’ll notice a couple extra things in there — I added a multiplexer between the Arduino and the one-plane display because the Arduino only has, um, 14 digital output pins — the things I need to turn a given LED (which for my purposes means a numerical digit on my speedometer) on or off. And, if I hope to go more than 49 MPH, I’ll need to have at least, say 18 pins available.
With 18 pins, I can have 0-9 in the ones side and 1-8 on the tens side. I only need 0 in the ones slot, and as much as I’d love to delude myself, I’m probably not going to be getting the Beetle into the 90s any time soon. Though, actually, it won’t be much harder to add that, so fuck it. Sure, I’ve only had it up to 86 or so MPH ever, with a tailwind, downhill, but maybe I’ll get lucky. So, if I have 19 outputs available, I can have all the digits up to 99.
Now, I could cheat by going by 2s or 5s or whatever for the ones, but I’d rather have a more accurate speedo, I think. But who knows, I may change my mind. Right now, there’s a number of ways to multiplex the Arduino pins to get more output, so I’ll try that first. If I fail, I’ll think of something else.
Now, my programming skills are pretty much limited to Applesoft BASIC on the Apple II — though I still do some goofy projects with that, I really do need to bone up on what the Arduino uses, which is a Java-based language. It’ll be a learning curve, but there’s tons of resources out there, and much of what I’m doing programming-wise, has been done before.
The physical side should be a little more straightforward. The one-plane display will just about fit in the stock speedometer housing, which I’ll gut. After fitting in the display unit, I’ll make a new inside-speedo face, with cutouts for the one-plane display screen and a stock VW fuel gauge. I’ll also add windows for the turn signal arrows, and the high-beam indicator. For the remaining two lights, the alternator light and the oil pressure light, I’m lucky because the one-plane display includes two circles in each digit window — red or green.
I did a quick test of the digits by just shining an LED into the light holes, and you can sort of see the red/green circles here, too:
I’ll have the left one be red, and stand for the alternator light and the right one be green for oil. This fits the old color and placement convention of these lights on 68 and earlier Beetle speedometers, anyway. Here’s a mockup of what I think it’ll look like:
You may notice the lack of an odometer, as I did just now. I also just now realized I don’t really care; my odometer hasn’t worked for years, and my life quality doesn’t seem to have suffered much. I’m also now amazed at how few instruments I have: speedo, gas, four lights. That’s it. Oh, there’s a brake warning light off the speedo cluster, and a long-disabled seat belt warning light, but for the main cluster, this is pretty much all I need.
I think my next step here will to be to get the basic setup working on the bench. I’ll get the 19 LEDs connected in a multiplexed array and mounted in the display unit, and see if I can at least get these digits to light up. After that, I’ll tackle the splitting a two-digit number into discreet digits and displaying those, and after that I’ll work on the actual RPM/speed sensor.
Once it all works on the bench, I’ll see about getting it physically crammed into the speedo housing, get a new faceplate printed, and put all the crap together.
Okay! Here’s hoping I don’t procrastinate more, and hopefully soon I’ll have the only Beetle in the world with a one-plane speedometer display! All suggestions welcome.
Contact the author at firstname.lastname@example.org.