Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
I favor a small cylindrical earbud with a good seal inside my ear for use with the amateur radio on my bike. These things come with back vents that allegedly improve their bass response; that’s not a concern for communications-grade audio and, worse, the vent produces a tremendous amount of wind noise.
Earbud with back vent
The solution is straightforward: put some tape over the vent!
Kapton tape over vent
I used Kapton tape, because I have it, but in point of fact the snippet of duct tape I applied on the first ride (having forgotten to do it on the bench) worked just fine. A drop of epoxy would be fine, too, if you were a bit careful about not letting it ooze down inside the case while it cured.
Despite the fancy appearance, this is a random pick from the assortment of earbuds I’ve bought at $10 or less over the last few years. According to my golden-eared assistant, the audio quality varies dramatically among the assortment, but they all work reasonably well between 300-3000 Hz. I suspect the insanely cheap ones on eBay are essentially the same things, although IMO they’re intended to collect large quantities of high positive ratings: caveat emptor.
Speaking of caveats, insert the usual safety caveats here.
Note that we’re using one earbud for tactical comm, not boppin’ to the music, and the audio level is low enough we (well, I) can’t hear diddly at speeds over 15 mph. Your jurisdiction may prohibit “headphones” or “earphones” or some such, so behave accordingly.
All the officers I’ve met think the radios are a great idea, if that means anything.
This is my latest attempt to come up with a robust electret mic capsule mount for our bike helmets.
The general idea is to put the capsule in a small brass tube (from my box o’ random cutoffs) soldered to the end of a copper-wire boom lashed to the helmet. The tube provides alignment and physical protection, the boom doesn’t pose a poking hazard, and some decent electrical tape secures the mic cable to the boom.
The mic capsule has back vents that allegedly provide ambient noise reduction, so the brass tube must be open on both ends. This does not implement the “waterproof” part of the spec; I still haven’t figured that out yet.
I annealed a length of 12 AWG copper wire to make it easy to bend around the helmet’s contours; two passes with a propane torch to red heat does the deed. It will work-harden quickly and maintain its shape after that.
AWG 12 wire is 0.080 inches in diameter, close enough to 2 mm that I poked a hole in the brass tubing with a 2 mm end mill. Filed the end of the wire flat, stuffed it in the hole, fluxed the joint, applied the big soldering gun to the wire, flowed some silver solder, and it’s all good. Fairly obviously, this meets my “the bigger the blob, the better the job” soldering criterion…
Mic rear
The capsule has two layers of Kapton tape wrapped around it to snug up the fit, although I doubt that insulating it from the brass tube makes any difference.
Mic front
The windscreen is a ball snipped from an open-cell acoustic foam sound deadening panel that has contributed myriad mic windscreens over the years. The mic fits into a slit cut with an X-acto knife; no finesse required. The nylon cable tie will disintegrate from sun exposure at about the same time the foam rots away, which takes about two years.
Mic foam windscreen ball
Despite what you might think, the helmet attachment is dramatically less butt-ugly than in years gone by…
Boom-to-helmet detail
The trick is lashing the bent portion of the boom to the helmet, which prevents the entire boom from rotating around its long axis. That keeps the mic aimed directly at your mouth, regardless of how you bend the boom.
The earbud wire loops around the mic boom a few times, with the first loop over the boom to take advantage of its rounded surface. With any luck, that will delay the inevitable fatigue failure. Mary favors old-style cylindrical earbuds, rather than newer flat or round ones.
The USB cable (this is not, repeat not a USB headset) gets lashed to various parts of the helmet foam and routed out to the middle of the back, with the male connector a few inches below the helmet. That puts the cable over the back of the Tour Easy’s seat frame, leaving the bulk of the cable hanging behind the seat. The cable length from the female connector to the radio interface is a delicate trade off between being
Long enough to let you stand up and
Short enough to stay out of the rear wheel.
This vertiginous shot looks down at the helmet hanging on the seat of Mary’s bike. Yup, that’s her bright new homebrew seat cover to the upper left…
I’m in the process of reworking the interface box between the amateur radio HTs on our bikes and our helmet-mounted earbud & mic lashup. Mary needed a new helmet before I got the new interface ready, soooo there’s an adapter cable in the middle.
This time around, the helmet cable uses a male USB-A connector, rather than a female 6-pin Mini-DIN PS/2 keyboard connector. Either one is cheap & readily available as assembled cables, which gets me out of soldering teeny little connector pins. These days, though, USB cables are more common.
The motivation for a non-latching, low-extraction-force connector at the helmet is that when (not if) you drop the bike, the helmet doesn’t tie your head to the bike and snap your spine. Falls on a recumbent are much less exciting than on an upright bike, but you still want the bike to go that-a-way while you go this-a-way. Been there, done that.
The old helmet cable connector: female 6-pin mini-DIN. The wire color code is not standardized. Viewed from rear of female connector or the front of the male connector, with the key slot up:
ear com - Gn 5 |_| 6 K - ear hot
mic com - Or 3 key 4 Y - mic hot
gnd - Bn 1 2 R - gnd
The new helmet cable connector: male USB-A. Mercifully, they standardized the wire colors. Looking at the front of the male USB-A connector with the tab down and the contacts up, the pins are 4 3 2 1:
1 – R – ear hot
2 – W – mic hot
3 – G – mic com
4 – K – ear com
The female USB-A connector is exactly the same.
That arrangement should produce the proper twisted pairs in a USB 2.0 cable, but all the USB cables I’ve seen so far lay all four wires in a common twist inside the shield. Maybe it’s the cheap junk I buy, huh?
It’s worthwhile to scribble some color in the background of the trident USB symbol so it’s easier to mate the connectors.
Easy-align USB connectors
Memo to Self: verify the connections & proper operation before shrinking the tubing!
This Tenergy automatic NiMH charger is typical of the breed: pick a charging current to match the cell / pack capacity, then stand back and let it determine full charge.
The instruction sheet reads thusly:
For battery pack between 1100 mah and 2100 mah, please use the low level switch — charging rate: 0.9 A
For battery pack over 2100 mah, please use the high level switch — charging rate: 1.8 A
Pop quiz: what charging current should you use for a battery pack made from nominal 2300 mAh cells?
I thought so, too, but consider this graph (the full post is there):
Tenergy RTU Pack A Tests – Aug 2009
The actual capacity is more like 1600 mAh, not 2300 mAh. Do you set the charge current based on the wildly overoptimistic cell rating or the actual measured capacity?
As you might expect: charge based on the actual measured capacity, because that’s what the battery can handle.
The higher rate actually worked with new cells, but as the packs aged the charger would sometimes grossly overheat them. Bad for the packs, not to mention a bit scary.
The lower rate worked perfectly, although it took me a while to figure that out.
This story begins years ago, as mentioned there. I’d retrieved the offending outlet expander / extension cord from my mother’s apartment and tossed it in my big box of Extension Cords.
I recently plugged it in and was rewarded with a flash-bang inside the box. Taking it apart reveals two more blackened outlet compartments (in the lower right), but no more missing contact blades.
It turns out that the black (hot) wire got caught between a stiffening rib on the back plate and the edge of the box supporting the brass plate connecting the white (neutral) wire to the contacts. Here’s reconstructed view after I cut off the extension cord.
Crushed wire
Flipping the wire over shows the spot where the copper conductor eventually poked through the insulation.
Exposed conductor
It touched the sharp corner of the brass strip just to the left of the divider in this view. The notch in the divider channeled the jet of burning debris across the far wall of the right-hand compartment. The left-hand compartment is completely smudged.
Short-circuit point and debris jets
Looks like I get credit for this one… but even seeing how I did it, I’m not sure there’s any way to know none of the wires got crushed while reassembling the box.
It’s safely in the trash and the cord is in my big box of Random Power Cords.
Memo to Self: Make sure the box fits together smoothly?
Here’s the status of the AA NiMH packs I’ve been using with the radios on our bikes, plus three packs I made up last year and have been keeping on the desk to measure their long-term storage characteristics. Click for more detail.
Bike Radio Pack Status – 2010-03
The “Tenergy 09 x” packs are new & unused with, frankly, disappointing capacity of about half their 2.6 Ah rating. That’s not much better than the used Tenergy packs (T9x and RTU x), which is either a Good Thing (they have good long-term stability) or a Bad Thing (they’re grossly over-rated to begin with).
The two Duracell packs are far better than any of the Tenergy packs.
The three 6-cell packs along the bottom are fading fast.
The previous test runs are there, albeit with a 1 A discharge.
This season I’ll use some Li-Ion packs that weigh twice as much with three times the capacity… plus a built-in charge gauge, pessimistic though it may be.
A comment on yesterday’s post about quartz crystal measurements prompted me to destroy a crystal in the name of science…
The question is, what effect does exposing a crystal to the air have on its performance? I would have sworn it would never work right again, because it’s normally running in an inert atmosphere and maybe a partial vacuum. One measurement being worth a kilo-opinion, here’s what happened.
I picked random crystal from the bottom of the crystal box, based on it having a solder seal that I could dismantle without deploying an abrasive cutoff wheel or writing some G-Code to slice the can off with a slitting saw. The crystal was labeled HCI-1800 18.000 MHz and probably older than most of the folks who will eventually read this… younger than some of us, though.
The center frequency is 18.0050 MHz (at this rather broad span) and it has some ugly spurs out there to the right.
A closeup of the series-resonant peak:
HCI-1800 18 MHz – Baseline BW
The bandwidth is 1.50 kHz at 17.99950 MHz at this span.
Naked HCI-1800 18.0 MHz Crystal
Then I applied a soldering iron around the seal and yanked the case off. I think that didn’t involve whacking the crystal with the case en passant, but I can’t be sure. In any event, it looks undamaged and seems to operate properly.
A pair of spring clips attach to the electrodes and hold the quartz disk in position. They’re just the cutest little things and quite unlike the other holders I’ve seen. I think the solder blobs fasten the spring ends together and don’t bond to the electrodes, but what do I know?
HCI-1800 Crystal Overview
The quartz disk has a few small chips near the edge:
HCI-1800 Crystal Edge Chips
I think those are Inherent Vice… simply because:
They’re not in a position where I could have whacked the disk and
I doubt I could whack it that delicately
Anyhow, with the can off, here’s what the series resonant peak looks like:
HCI-1800 18 MHz – Opened BW
The resonant frequency is now 17.99968, 180 Hz higher, which may be due to instability in the HP8591 spectrum analyzer’s not-stabilized-for-ten-hours ovenized oscillator. The bandwidth is 1.55 kHz, 50 Hz wider, although I think that’s one resolution quantum of difference.
Here are the two bandwidth traces overlaid.
HCI-1800 18 MHz – Overlaid BW
The peak has been centered in both, so you can’t tell they’re slightly different. The interesting point is the difference in the slope to the low-frequency side of the peak, which is slightly higher for the open-case condition. Seeing as how the missing case completely changes the usual stray capacitance situation, I’m not surprised.
Anyhow, I admit to being surprised: there’s not that much difference after opening the case. I’ll put the naked crystal in a small container in a nominally safe place for a while, then retest it to see what’s happening.
Memo to Self: A “safe place” is nowhere near the Electronics Workbench!
Here are some other naked crystals:
Naked Crystals
Notice the tarnished (presumably) silver electrodes on the crystal in the lower left. That one’s been sitting on my monitor and in other hazardous locations for a few years. I can’t find these anywhere right now, but if they turn up I’ll test them, too.
The Smell of Molten Projects in the Morning 