The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

  • Arduino Pro: Power Adaptation for FTDI Basic USB

    Arduino FTDI Basic on modified Arduino Pro
    Arduino FTDI Basic on modified Arduino Pro

    Some time ago, I bought a 5 V Arduino Pro board (about which you read earlier there) and a nominally compatible FTDI Basic USB-to-serial adapter. Turns out that they’re not quite a perfect match, although they do play nicely together in normal use.

    The FTDI Basic board produces a 3.3 V regulated output voltage that’s connected directly to the output of the Pro’s 5 V regulator. This doesn’t cause any particular problem, but one side effect is that you can’t shut the board’s power off: the USB power will keep the CPU alive, more or less.

    You should, of course, use a 3.3 V FTDI Basic board with a 3.3 V Pro, which would at least put two similar voltage sources head-to-head.

    The Pro is using a backup power supply that, for reasons that make perfectly good sense, backfeeds the Pro’s 5 V regulator: when the +12 V main supply Goes Away, the backup power supports VCC directly, rather than through the regulator. The regulator can take a joke like that, as witness the FTDI vs Pro situation; in my case, a diode isolates the two supplies in normal operation.

    For reasons that I don’t completely understand, some combination of voltage to the Pro regulator and the (diode isolated!) backup support voltage caused the FTDI chip to lock up with both TX and RX LEDs on solid.

    I suspect the FTDI chip’s internal 3.3 V regulator, in combination with the USB +5 V supply, in combination with the Pro board power, drove something outside its normal operation range. So I simply removed the 3.3 V pin from the connector, disconnecting that supply from the Pro’s overvoltage, and the thing now works fine.

    Side effects:

    • The FTDI board remains powered when the Pro board gets turned off, thus preventing Linux from changing the serial port device when the power comes back up again
    • I can actually turn the Pro power off, without having the FTDI supply keep it alive. Handy for soldering!

    The Pro pin labeled GND connects to the FTDI CTS line, an input that floats high when not connected. I yanked that pin and shorted CTS to GND on the FTDI board: one less pin to worry about, for reasons that you’ll see tomorrow.

    There are many different versions of the boards and USB adapters, so current production probably doesn’t match what I have. Pay attention to what you have, though…

  • Debugging Tube Circuitry: Open Resistor

    Open 2.2 meg resistor
    Open 2.2 meg resistor

    I dropped in to mooch some female header strips from my buddy Eks (which is not nearly as obscene as it sounds) and got the story behind this innocent-seeming 2.2 megohm carbon-composition resistor.

    It seems he was debugging a defunct tube-based audio amplifier. He’d probed everything and discovered that the grid bias on one of the tubes was totally wrong, which caused protracted headscratching over the associated circuitry.

    Now, in semiconductor work, a 2.2 meg resistor is an open circuit compared to the other circuit impedances. In fact, you can use pretty nearly any resistor with green or blue in the third band as a standoff in Manhattan-style construction in place of those small insulated pads.

    Megohm-value resistors are actually useful in tube circuitry; you’ll see plenty of green and blue bands sprinkled around those sockets. Although we didn’t get into details, I suspect this one was part of a grid-leak bias circuit that holds the grid voltage just a bit below the cathode; the bias comes from the few electrons that whack into the grid wires rather than passing through, so the total DC current is in the microamp range.

    After more headscratching, Eks yanked this resistor, measured it, and found it was a completely open circuit. A 2.2 meg resistor isn’t all that much different from an open circuit (it’s hard to tell the difference with an in-circuit measurement) when used in a transistor circuit, but the difference separates correct function from failure for a tube amp.

    Eks swapped in a new resisistor and the amp worked fine. Case closed!

    The digital multimeter in my desk drawer tops out at 2000 kΩ, which shows you just how much demand there is for high-value resistors these days…

  • Nichicon SMD Electrolytic Capacitor Polarity

    Nichicon electrolytic capacitors
    Nichicon electrolytic capacitors

    This should be obvious, but isn’t. The black bar marks the negative terminal and the corner-cut side of the base marks the positive terminal.

    How much would it cost to put little hyphens down the middle of the black bar?

    The data sheet for a related, not identical, series of caps is there.

  • Slitting Brass Tubing

    Casting Wood's Metal in brass tube
    Casting Wood's Metal in brass tube

    I needed a brass tube with a lengthwise slit to serve as an electrostatic shield around a ferrite bar antenna. There are many wrong ways to do this, all of which produce terrible results, pose a serious risk of personal injury, or both. I say that with some confidence, having tried some of them over the years.

    Here’s one right way: fill the tube with Wood’s Metal, thus turning it into a solid rod, then cut the slit with a slitting saw.

    Wood’s Metal is a moderately toxic alloy that melts in hot water, which turns casting into a simple workbench operation. You might not want to cast it in the kitchen, but that’s your call. Clean up the scraps, wash the counter even though you used newspaper, wash your hands, and don’t suck your thumb.

    As shown, I just poured the molten metal into the brass tube atop a steel block, broke off whatever seeped out, and remelted the scraps. Turns out I had just barely enough for the job.

    Slitting brass tubing - overview
    Slitting brass tubing – overview

    My buddy Eks gave me a stack of slitting saws a while ago and I modified a standard Sherline holder to fit them. Turns out there’s just barely enough room for everything within the mill’s working envelope; the saws are a bit over 3 inches in diameter.

    So I cut the back of the tubing, making the pictures somewhat disorienting.

    The tubing fit neatly into an old V-block (evidently homebrewed by a better machinist than I), held down by ordinary Sherline clamps on perilously long studs screwed into the tooling plate. The saw had just enough reach to clear the rather broad V-block’s shoulder.

    The tubing is 0.630 OD with a 15-mil wall and the saw blade is pretty nearly 32 mils thick. I touched off Z=0.331 (630/2 + 32/2) with the blade atop the tubing, then jogged away to Y=+1 and drove down to Z=0 to cut exactly through the middle of the tube.

    Slit 0.015 inch deep
    Slit 0.015 inch deep

    The V-block is aligned with the front of the table, but I did a bit of nudging to persuade it into final alignment. Of course, the saw wasn’t quite centered on the holder, so a blade or three tinged on the tubing when I did a Y=0 trial pass at low RPM.

    For lack of anything smarter, I cut at 500 RPM and fed at 5 inch/min. That’s painfully slow, but correspondingly boring… remember, in machine shop work, boring is good.

    I did five passes: one trial at Y=0, three cuts at 5-mil steps, and a cleanup cut. The picture shows the 15-mil pass left a very thin web at the far end. A final 2-mil cut removed that web, leaving only a few burrs. You could do it in one pass, but I wanted to minimize the depth-of-cut into the Wood’s Metal.

    Unclamp, discover that the cast metal rod slides right out, touch up the edges with a file, and it’s all good. A lovely slit, perfectly aligned, without bent metal or bloodshed.

    As a bonus, I get a nice Wood’s Metal ingot out of the operation. The line along the rod is just barely perceptible with a fingernail; it’s more of a polished line than an actual cut.

    Slit tube with Wood's Metal ingot
    Slit tube with Wood's Metal ingot

    Turns out the shield works a bit too well: it cuts out the WWVB signal, too. I think the tubing is too close a fit to the ferrite rod and detunes the winding. More experimentation is in order…

  • Hammered Solder Ribbons

    I needed more solder ribbon for resistance soldering, so I figured I should make a batch of the stuff. Put a length of silver solder between folded paper, hammered it on the vise anvil with a polished brass hammer, and it worked fine.

    Flushed with success, I did the same with some ordinary rosin-core lead solder for the next time I must solder a shield or some such.

    Solder Ribbons
    Solder Ribbons

    Just snip off the appropriate length and fire up the iron…

  • Alpha-Geek Clock: Ready for the Mass Market

    Packaged Alpha-Geek Clock
    Packaged Alpha-Geek Clock

    OK, I had to do it. The Alpha-Geek Clock WWVB receiver circuitry, such as it is, now resides in a nice Pactec enclosure, with a bright red LED.

    All time, all the time, and nothing but the time.

    I should put it up on etsy.com for fifty bucks…

  • Digital Concepts AAA NiMH Cells: Craptastic!

    The AAA cells I mentioned there bubbled to the top of the heap on my desk again, so I charged them, let them sit around for a few days to stabilize, then ran a discharge test.

    The top (black) trace is all four AAA cells in series; the two steps correspond to the two weakest cells failing. The red trace is the surviving two cells. The green trace is the strongest cell, which supplied current during all three traces.

    They’re nominally 900 mAh, but the results are pretty much what you’d expect.

    No-name AAA NiMH - Sequential Discharge
    No-name AAA NiMH – Sequential Discharge

    The most durable cell, the last one to fail with the green trace, had a capacity of a bit over 500 mAh: slightly over half the rating. The weakest cell (the first step on the black trace) failed after a mere 250 mAh.

    Junk. Pure junk. I’ll give ’em another charge just to see what happens, but don’t hold your breath anticipating a resurrection.