Estimated reading time: 8 minutes

Maxed Out: Max's Display-O-Meter Takes Shape

Archived column by: Clive "Max" Maxfield

OK, to ensure that we're all tap dancing to the same drum beat (I know whereof I speak, because my dad used to be a tap dancer before WWII) let's briefly recap the situation.

Way back in the mists of time, someone created a pseudo electronic machine purported to demonstrate the difference between men and women (Figure 1).

Figure 1. The difference between men and women.

I decided that it would be fun to create something similar (but WAY cooler). Thus, in Part 1 of this mini-series we discussed the overall "look and feel" that I'm aiming at. As part of this I showed a mega-cool brass panel with antique analog meters and switches, which forms part of a "Dr. Who TARDIS Control Console" that my friend Douglas is building in California (Figure 2).

Figure 2. One potential look and feel for my project.

I should emphasize that Douglas' panel is only a starting point. Achieving a consistent and coherent look and feel for this sort of thing is a non-trivial task. Thus, I've turned this part of the project over to my friend Denis (it's a French spelling pronounced "Den-ee").

Denis is an incredibly skilled artist who understands all sorts of things about styles from different eras, which means he tends to waffle on about "Late-1800s UK Industrial" vs. "1930s German Industrial." So Denis is currently working on a look and feel that utilizes the parts I have acquired while also satisfying my craving for brass. We'll talk more about these parts shortly.

Meanwhile, we also need some way to control the beast (to monitor the state of the switches and knobs and any other input devices and to drive the lights, meters, and any other output devices). Thus, in Part 2 we discussed my use of a PICAXE microcontroller and the building of a prototype device.

When last I left you, I'd wired up a single 8-bit output shift register that was being used to drive 8 bits on a 10-bit bar graph LED display, and I was using my PICAXE microcontroller to flash the LEDs (Oooh, shiny!). So, what's been happening since then? Well, read on!

The Prototype Evolves

First of all I've continued to develop the prototype. I now have a chain of five 8-bit output shift registers wired up and I'm using these to drive 40 LEDs presented in four 10-bit bar-graph dual-in-line packages as illustrated in Figure 3.

Figure 3. The prototype evolves.

Also, not shown here, I have four 10-bit switches (presented in dual inline packages) driving five 8-bit input shift registers. This means that I can now play around using my PICAXE microcontroller to read the state of the input switches and use whatever values it finds as the basis to decide what to display on the output LEDs.

It's important to remember that this is only a prototype. The real Display-O-Meter (we really have to pick another name) is going to employ antique - or antique-looking - switches and knobs and suchlike for its inputs, and antique meters for its outputs.

We're also going to use LEDs as output devices, but they won't look like LEDs by the time we've finished with them. With regard to the LEDs, there are two projects ongoing as we speak. Initially I was considering using only uni-color LEDs; for example, a toggle switch might have both green and red LEDs associated with it. The switch being active would light the green LED; inactive would light the red LED. But then we'd end up with some LEDs being off, which is sort of boring.

So, eventually I decided to go with tricolor LEDs (at first I thought they'd be expensive, but I managed to find a bag of 50 for only $10 on eBay). Each four-terminal device contains a red, green, and blue LED, and mixing the intensities of these allows you to achieve a whole gamut of colors. The only downside is that maintaining subtle control over the intensities of the three colors is non-trivial, because it requires the use of pulse width modulation (PWM) techniques.

I wasn't able to find any off-the-shelf control chips that could do what I wanted, so I turned to a friend named Joe in the UK. Joe is an expert with PIC microcontrollers, and he's come up with a scheme by which each tricolor LED will have an associated 8-bit PIC to control it. I know, you think this is an expensive hobby, but the cheap-and-cheerful microcontrollers we're using cost only around $0.50 each, which really isn't so bad.

The end result is that I'll be able to use my main PICAXE controller to issue a command along the lines of "LED number 23, I want you to display turquoise," and that LED's personal microcontroller will handle the nitty-gritty details.

The other aspect with regard to the LEDs is to make them look antique. For this we're considering a variety of different options, such as fronting the LED with a small disk of frosted glass mounted behind a brass bezel. But the ultimate solution depends on the look-and-feel determined by Denis.

Switches and Meters and Knobs, Oh My!

To be honest, when I started out on this project, I didn't realize just how difficult it was going to be to track down the antique parts ... trust me, it can be a real pain. Fortunately, I know a few people who have odd things stashed away in their electronics treasure chests. Also, I've made a friend at my local technology recycling store, which is located no more than a mile from my office. The end result is that I've managed to pull together a small collection of rather interesting meters as illustrated in Figure 4.

Figure 4. Collecting analog meters.

To provide a sense of scale, the large round meter in the upper left-hand corner is 4.5 inches in diameter, while the rectangular meter in the upper-center is 4.75 inches wide. Also, not shown here, I have four small antique meters (two round, two square, presented as matched pairs) winging their way to me as we speak.

In the case of knobs, I was disgruntled to discover that these little scamps can be ferociously expensive. I'm told that you can easily pay $10 or $20 for an antique knob on eBay. This was a bit of a show-stopper, because like most folks, my funds are somewhat limited at the moment.

And then, as luck would have it, I saw an entire box of the little scamps being auctioned off on eBay. It seems that these were stock from some old electronics repair shop that had gone out of business decades ago. So I hung around in stealth mode watching the auction proceed, and then pounced right at the end with seconds to spare (Figure 5).

Figure 5. This is what 14 pounds of antique knobs looks like.

But what about the switches? Well, truth to tell, this is where I've run into a bit of a stumbling block. There's no problem using modern rotary switches, because these will be hidden behind the antique knobs. The issue is finding antique toggle or lever switches. I did manage to pick up 10 rather interesting toggle switches with little balls on the ends. but that's about as far as I've gotten.

What I really want is a bunch of antique telephone lever switches of different shapes and sizes. I'd also like some small industrial control type switches, but this is a work in progress and I'm sure that something will come along.

On the bright side, I did manage to acquire a miniature copper-on-bakelite Frankenstein-type knife switch - you know, the type Igor throws to reanimate the monster. This will look wonderful as my main on/off switch (Figure 6).

Figure 6. This will make a GREAT master On/Off switch!

Once everything comes together, yet another friend, Tim, a master machinist, has a full machine shop located just a few minutes from my office. Tim tells me, "If Denis can draw it, I can fabricate it!"

So, that's where we are at the moment. I have an ongoing quest to locate antique switches. (I'm also interested in acquiring more meters and suchlike in case I decide to build more than one unit. It could be fun to do the same thing but in multiple styles and then display them side by side. In the meantime, I'm happily playing with my prototype trying to hone in on the algorithm(s) I'm going to use to control the various output displays.

Actually, that's what I'd originally planned to talk about in this column, but you know me - just wind me up and watch me go! At some time in the future when things have progressed a little farther I'll do a followup column to bring you up to date with this project. But for now, I have a mega-cool column planned for next time. Until then, have a good one!

P.S. If you happen to have any old switches or suchlike, or if you've thought of a better name for the Display-O-Meter, please don't hesitate to send me an e-mail at max@techbites.com.

About the Author

Clive "Max" Maxfield is president of TechBites Interactive, a marketing consultancy firm specializing in high technology. Max is the author and co-author of a number of books, including Bebop to the Boolean Boogie (An Unconventional Guide to Electronics), The Design Warrior's Guide to FPGAs (Devices, Tools, and Flows), and How Computers Do Math featuring the pedagogical and phantasmagorical virtual DIY Calculator.

In addition to being a hero, trendsetter, and leader of fashion, Max is widely regarded as being an expert in all aspects of computing and electronics (at least by his mother). Max was once referred to as "an industry notable" and a "semiconductor design expert" by someone famous who wasn't prompted, coerced or remunerated in any way. To contact the author, click here.