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.
Electrical & Electronic gadgets
If this is true, I can scrap out my roll of mu metal shielding:
I think they mean the sleeve protects the magnetic stripe from mechanical damage, but wedging those two sentences together certainly suggests the envelope has serious anti-magnetic mojo…
In that version of the GPS+voice interface, I sprinkled 100 nF and 100 pF SMD caps across the input lines in the hope that they’d reduce EMI on the audio board. The board worked fine for years, but now that it’s time to build another board & box, I figured it’d be good to know a bit more about their actual response.
So I cobbled up a test fixture with a 3 dB pad from the tracking generator output and a 20 dB pad to the spectrum analyzer input (both of those are bogus, because the cap impedance varies wildly, but work with me on this):
Pulled an assortment of 100 nF ceramic caps from the stockpile:
And rubbed them against the HP8591 spectrum analyzer & tracking generator:
Their self-resonant frequencies are much lower than I expected:
The attenuators produce about 17 dB of loss with no cap in the circuit, so the disk caps are pretty much asleep at the switch from VHF on up. The small bypass cap in the top photo is OK and the SMD cap is pretty good, but they’re all well past their self-resonant frequency and acting like inductors.
The relevant equations:
The drill goes a little something like this:
In round numbers, the 100 nF SMD cap has L=2 nH and ESR=60 mΩ.
Now, it turns out a 100 pF SMD cap resonates up at 300 MHz, between the VHF and UHF amateur bands:
So I think the way to do this is to pick the capacitance to put the self-resonant frequency in the VHF band, parallel another cap to put a second dip in the UHF band, and run with it. A back of the envelope calculation suggests 470 pF and 47 pF, but that obviously depends on a bunch of other imponderables and I’ll just interrogate the heap until the right ones step forward.
Just to show the test fixture isn’t a complete piece of crap, here’s a 12 pF cap resonating up around 850 MHz:
For the combination of components, sweep speeds, bandwidths, and suchlike in effect, the spectrum analyzer’s noise floor is down around -75 dBm. I think the 12 pF cap is actually better than it looks, but I didn’t fiddle around with a narrower resolution bandwidth.
A cheap auto escape hammer (IIRC, free in the bottom of a tag-sale box filled with stuff I could actually use) has been kicking around the back of the bench for far too long; it had a feeble single-cell incandescent bulb flashlight with the cheapest possible non-switch. I ripped all that out, carved out enough plastic to fit a CR123 lithium cell, hot-melt-glued a real pushbutton switch and 10 mm white LED in place, and soldered it up:
The CR123 puts out enough juice to light up the LED, but it’d be happier with a bit more current. There’s no limiting resistor, so the LED gets what it gets.
Augment the screws with a few snippets of Kapton tape, use some real 3M Velcro tape, and it’s all good (albeit ugly on a stick):
Now, there’s no way to test the hammer part of it (perhaps I could visit a junkyard and whack out a few windows for practice?), but at least now we have a disposable flashlight in the van…
For some odd reason, Linear Tech’s LTSpice IV simulator has a hotkey to stop a simulation (Ctrl-H), but not one to start the run.
Add one:
Done!
Well, it turns out that the DVD drive I stuffed into that case really does require a whole bunch of current. I tried playing a DVD and got erratic results, including weird keyboard (!) failures. Finally, I hitched a bench supply to the coaxial power jack on the case and caught it in the act:
That jack normally connects to the power-only USB cable, which implies an upper limit of 100 mA. A bit of poking around inside shows that the coaxial power jack simply parallels the USB jack’s VCC line, so there’s no fancy negotiation or current sharing going on.
When the keyboard went nuts it was sharing an unpowered USB hub with this thing, which means that the overcurrent dragged down the hub’s supply. I was permuting all the choices to see if the failures suggested anything; eventually it did.
A bit of rummaging in the Basement Laboratory Warehouse Wing uncovered a 5.0 V 3.7 A wall wart switching power supply that is grossly in excess of the drive’s 1.5 A rating. Amazingly, it even had the correct coaxial power plug on the end of the cable, which never happens.
Alas, because the external supply back-powers the USB data cable, it lights up the Q150’s power button when the PC is turned off. I think I can insert an isolation diode into the USB power trace to isolate it from the jack, somewhat along the lines of that hack. However, that seems to require removing the USB connector to uncover a very well protected top trace. For now, I’ll just unplug the drive.
I cable-tied the mic/earphone cable on Mary’s bike helmet to a rib on the fancy air vents near the back end, hoping that would reduce the inevitable flexing. Alas, it didn’t work out that way and the cable lasted only two seasons. This cut-away view shows the pulverized shield braid inside the jacket:
The symptoms were totally baffling: the mic worked perfectly, but the earphones cut out for at most a few syllables. Of course, I can’t wear her helmet and it only failed occasionally while riding. I barked up several wrong trees, until it got so bad that I could make it fail in the garage while listening to the local NWS weather radio station.
I spliced in a new USB male-A connector and (re-)discovered that the braid seems to be aluminum, rather than tinned copper. In any event, the wire is completely unsolderable; I crimped the braid from the new connector to a clean section of the old braid. The braid serves only as an electrostatic shield, as it’s not connected to anything on the helmet end. That should suffice until I rebuild the headsets this winter.
The keyboard on my trusty HP 48GX calculator finally deteriorated to the point of unusability, so I tore the thing apart following the useful instructions there. The warning about applying force to the rivets that hold the case halves together gives you not the faintest concept of how much force is actually required to pry the mumble thing apart at the battery compartment; I finally invoked force majeure with a chisel scraper…
I expected the calculator would not survive this operation and I wasn’t disappointed.
An HP 50g is now in hand. Here in late 2011 I’d expect HP’s top-of-the-line RPN calculator to sport a crisp high-resolution display, but noooo the low-contrast 131×80 LCD seems teleported directly from the latter part of the last millennium. The manuals are PDFs, which is OK, but their content is far inferior to the HP 48GX manuals. In particular, the editing / proofreading is terrible. I infer that the HP calculator division can barely fog a mirror and is on advanced life support; HP’s diverting all their money to, uh, executive buyouts or some other non-productive purpose.
The fact that HP sells new-manufacture HP 15C calculators doesn’t crank my tractor, even though I lived and died by one for many years. A one-line 7-segment display doesn’t cut it any more, even if the new machinery inside allegedly runs like a bat out of hell.
My HP 16C, now, that one you’ll pry out of my cold, dead hands. At one point in the dim past, I’d programmed the Mandelbrot iteration into it to provide bit-for-bit verification of the 8051 firmware for the Mandelbrot Engine array processor I did for Circuit Cellar: slow, but perfect. That calculator has a low duty cycle these days, but when I need it, I need it bad.
