Spam (madbodger) wrote,

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Random hackery

I have been a little hack monkey lately.

Vacuum Fluorescent Displays

I've been playing with some ex Russian military vacuum fluorescent displays, (ИВЛМ-1/7), which are basically little vacuum tubes (triodes and diodes) with fluorescent anodes. They need 20-30 volts DC on the plates (and grids, if any), and a small AC voltage on the filament. These are oddball voltages for digital logic, which is why I was reverse enginnering random cheap power supply modules from surplus companies.

Here's the result:

Work Related

In order to finish my digitizer code, which digitizes several signals together, I needed a signal source. The real signal source isn't available, but the company does have a simulator. The signals (both real and simulated) are voltages that the digitizer can't accept directly. There's a hardware engineer on staff, who is designing an adaptor board to convert the signals, avoid ground loops, and that sort of thing. But he isn't finished. So I bashed together a quick circuit to adapt and isolate things sufficiently, so I could continue.

I went to the local Radio Shack (i.e. my back room):

There, I grabbed a couple of likely parts, an LM339 quad comparator, and an LM324 quad Norton op-amp. For all the signals, I needed new fewer than six BNC jacks, and there weren't that many in stock. So I found an old video distribution amp (apparently unloved, the AC cord had been cut off it before I acquired it), and removed its innards. Then I breadboarded a quick lashup with the 339, variable attenuators for the inputs and a resistive voltage divider for a reference. No dice. No output. How annoying. I checked the supply voltage, all the signals, but still no output. I finally decided my NOS 339 was DOA, and, as the Radio Shack didn't have any more 339s, I rebuilt it using the 324. This didn't seem to work either. My trigger signal didn't reappear. I checked one of the other signals, and it looked fine, but with a low slew rate and appreciable delay. True, the 324 is not a fast chip. And it looked like my fast trigger signal was too brief for it to see.

So I went to look at more complete docs than the terse ones on the back of the cards the parts came on. Aha! I had forgotten a "feature" of the 339: "The output of the LM139 series is the uncommitted collector of a grounded-emitter NPN output transistor." To get any output from this chip, I needed pull-up resistors! So I re-rebuilt the circuit, putting back the 339 and including pull-up resistors this time. And it worked! For the curious, here's the schematic:

And a picture of the breadboard sitting inside the gutted video amp (fizzygeek pronounced it officially "cute"):

iGo + MacBook

I love my iGo for travelling, because I can just bring along one power adaptor and power/charge most my gadgets with it: cellphones, cameras, laptops, iPod, whatever. It also lets me run from wall power, car power, or plane power. But it only lets me run two things at once. This is inconvenient when I want to charge a camera and a cellphone while using my laptop. So I bought a splitter, that lets me run two "A" (8 watt) devices. But the phone adaptor (A01) doesn't work with the splitter, and the iPod adaptor (B01) is a "B" (15 watt) device. They said the phone adaptor revision A didn't work with the splitter, implying that there was another revision that worked. It turns out that the version that does work (A95) is a completely different part, one that doesn't show up on their web site. However, I easily scored a pair of 'em on the secondary market. I also found that there is an A adaptor (A61) for iPods, so I picked that one up, too. But I still wanted more (like charging an A and a B device at once). I figured that, although their connectors are quite proprietary, the "dualpower" tap uses an ordinary barrel connector. I'd just have to get a plug, two jacks, and make a Y splitter, then I could run two dualpower adaptors at once. If I put splitters on both of them, I could run 5 devices at once! However, it turns out to be an odd size barrel (5.0mm x 2.1mm, Radio Shack size "K"). Plugs are hard to get (RS doesn't stock 'em, but I had 'em ship one to my local store [a real one, not my back room] for free). Jacks are even harder to find. I could use a 5.5mm jack and deal with a loose fit, but even those aren't common, and cable mount ones I haven't found anywhere.

Then I got my employer to buy me a MacBook Pro, and went looking for the iGo adaptor for it. There isn't one (yet, the S32 is currently under development). So I changed my tack somewhat. I connected the barrel plug to a cigarette lighter type socket. Now I can plug the Macbook auto adaptor into that, and juice it up with the right voltage by using one of the otherwise-useless PC tips that came with my iGo, since the dualpower tap receives the same voltage that the main tip provides. I chose the S5 tip, which delivers the right voltage and current for a Macbook.

This lashup also makes it easy to dope out the voltages for all the tips. Additionally, I can use an ordinary cigarette lighter splitter cord to let me plug in two dualpower units, along with their car adaptors, achieving my original goal. Note that you shouldn't plug most car accessories into this lashup, as it will deliver whatever voltage the tip programs, not the 12-14V these devices expect. The dualpower rigs are designed to accept this range of voltages, so they're okay, and since I chose a tip to match the Macbook's AC supply's output, that works too.

Side note: I found that I could get the oddball tips I wanted for the best price by buying bulk lots of tips. So I have a box full of random iTips I don't need. If folks out there have iGo stuff and need tips, let me know, if I have yours, you're welcome to it.

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