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

  • Homebrew V-750 Dosimeter Charger Pedestal: Overview

    Although my V-750 dosimeter charger cleaned up reasonably well, I wanted to see if I could build a high-voltage supply from more-or-less contemporary parts to charge the dosimeters. The circuit is easy enough, but the charging pedestal that connects to the dosimeter turned into an interesting shop project.

    V-742 dosimeter charging contact
    V-742 dosimeter charging contact

    Pencil-style electrometer radiation dosimeters, like the V-742 shown here, have a charging contact pin embedded in a transparent plastic (?) end cap recessed in the bottom. Inside the dosimeter a mighty spring (or, perhaps, the plastic cap itself) holds the pin outward so that it does not make electrical contact with the gold-coated quartz fiber in normal use.

    This baffled me at first, because I did not understand why the charge didn’t just leak off the fiber through the charging pin. In order to dump charge onto the fiber, you must first press the pin inward by about 1 mm against the internal spring: no pressure, no contact, no charge.

    Duh…

    The dosimeter’s innards must be kept scrupulously clean and full of dry air. After you pull the pin out to admire it, the dosimeter won’t hold a charge ever again. I yanked the pin out of a dosimeter that simply didn’t work and, after a bit of fiddling, the dosimeter can now be set to zero, but the charge leaks off in a matter of hours rather than days.

    Charging contact pedestal
    Charging contact pedestal

    The V-750 charging pedestal has an outer sleeve (the negative contact) and a central pin (the positive contact) that fit neatly into the end of the dosimeter. The pin stands about 2 mm proud of the plastic insulator that pipes light into the dosimeter to illuminate the scale. The sleeve, insulator, and pin move as a single unit: the dosimeter presses them down into the V-750 against two stacked springs.

    A 1-lb spring holds the insulator in place by pressing the whole cylinder outward against its shoulder. The charger turns on when the dosimeter reaches that spring’s limit of travel at about 1 mm, but it’s not firm enough to press the dosimeter pin into contact with the quartz fiber. That’s the position you use to read the dosimeter: the light is on, but the fiber won’t move yet.

    In order to charge the fiber, the dosimeter must move down an additional 3 mm against an 8-lb leaf spring until it seats against the pedestal’s threaded shell. Holding the dosimeter steady against that pressure while twiddling the voltage knob to adjust the dosimeter fiber to the zero point of the scale is more challenging than you might expect: grab it in your fist and hold on tight. It’s a good idea to wear glasses, as the dosimeter optics provides maybe 5 mm worth of eye relief: you can easily poke yourself in the eye with the fool thing if your grip loosens.

    So, basically, a new charging pedestal must include a shell that meets the dosimeter’s body and a central shaft consisting of a sliding outer sleeve, a transparent insulator, and a central pin. The shaft must be pushed against the dosimeter by a really stiff spring to close the charging contact.

    Not-quite-as-built cross section sketch
    Not-quite-as-built cross section sketch
    Finished charging pedestal
    Finished charging pedestal

    The overall plan looked something like this, at least before I started cutting metal…

    What changed:

    • a larger spring surrounds the LED
    • no need for the weak spring
    • no switch: the voltage-adjust knob has one
    • a single slot in the side to prevent rotation
    • screws, not solder, holding bolt to EMT shell
    • no sleeve inside the bolt: it’s a copper bolt

    But, all in all, it worked out OK.

    Charging pedestal components
    Charging pedestal components

    Here’s what the final result looked like, all spread out so you can see the innards…

    The next few posts will show various bits & pieces, with notes & asides.

  • Laser Pointer Annoyances

    Laser pointer battery contact
    Laser pointer battery contact

    Maybe it’s just me, but all of the laser pointers I’ve bought, even the relatively spendy ones, have crappy switches and unstable battery contacts.

    For example, this is the business end of a $12 (!) pen-style pointer. The battery contact was off-center and poorly secured; I pried the white plastic retainer out, bashed the spring into submission, and replaced the retainer with a length of heat-shrink tubing. It wasn’t pretty.

    This pointer has an actual mechanical switch module inside, with a clicky mechanism actuated by the external button. Cheaper pointers seem to rely on bare PCB contacts bridged by the button’s base. Ugh.

    Laser pointer battery orientation: positive DOWN
    Laser pointer battery orientation: positive DOWN

    Memo to Self: The AAA cells fit into the housing with the positive terminal away from the laser head. The white plastic plug has a molded cross that could be mistaken for a + symbol, but it’s not.

  • Capacitor Plague Up Close

    A friend dropped off a dead eMachines Celeron for my next recycling trip. Peering inside, what do my wondering eyes behold but a nasty case of Capacitor Plague!

    Herewith, some pix of the victims within the box. Note the bulging tops ready to blow along the pressure-relief grooves, the distinct tilt caused by the bulging bottom plug, and the right-hand cap near the power supply on countdown for launch!

    More background on the plague is there.

    I must build an ESR tester one of these days…

  • Quick-and-easy IR-passing / Visible-blocking Optical Filter

    Gel filters - normal visible light
    Gel filters – normal visible light

    When you don’t need high optical quality for an IR filter, you can superimpose red and blue stage-lighting filters: pure black to the eye, transparent to IR.

    You know they’re IR-transparent because they’re generally snuggled right up against huge incandescent bulbs: if the filter material absorbed any IR, it’d burn right up.

    I’ve used Lee Filters Congo Blue (181) and Primary Red (106) to good effect. They may be available from a stagecraft outlet near you, but around here that stuff is a mailorder deal. You’ll get a lifetime supply, so maybe you can pass some out to your cronies; techies always enjoy odd presents like that.

    A more optically flat (and durable and expensive) option would be a photographic-image-quality glass IR filter suitable for camera mounting. I got one after I dropped a homebrew plastic filter down a sewer grate.

    For examples, go to Adorama, click on Filters in the left column, then select Infra-Red Filters, then maybe refine the search to the cheaper Hoya brand before your budget runs away in fear.

    Gel filters - IR view with visible light
    Gel filters – IR view with visible light

    Make sure your camera doesn’t have an IR blocking filter behind the lens. I think most consumer-grade digital cameras do have an IR-blocking filter and most video cameras don’t, but I’m sure those general rules don’t hold in all cases. Indeed, I bought a Sony DSC-F717 specifically for its IR mode; fortunately, the CCD sensor failed shortly before the factory recall ended.

    The pictures show the same scene under normal lighting, with the camera set to its IR mode, and IR mode with an IR filter in front of the lens.

    The gel filters appear dark-gray in the middle image because the camera sets the exposure (1/60 f2.4 ISO100) based on the visible light entering the lens. They’re transparent in the bottom image because the exposure (1/30 f2.4 ISO1000) is based on only the IR illumination, which is pretty dim. The gratuitous greenish cast is how Sony reminds you that the image was in IR mode

    Gel filters - Pure IR view
    Gel filters – Pure IR view

  • Electronic Fluorescent Ballast: Zowie!

    Back in 2000, I replaced the ballast in our bathroom light; the old one failed after a mere 45 years. The casing didn’t sport any PCB-free labels (no surprise there), so I disposed of the carcass at a town hazmat day.

    Under normal circumstances you’d replace the whole fixture, but this is a slender 4-foot chromed steel base with a matching chromed shield over a 4-foot fluorescent tube: charming, in a retro-mid-50s sort of way. We couldn’t find anything suitable at the local big-box home supply stores, so I just cleaned it up and stuck a new ballast inside.

    I indulged in the luxury of a warm-white tube so I didn’t look quite so dead in the morning.

    That ballast just failed, after a mere 9 years, which I confirmed by swapping in a new tube. It seems nothing lasts any more.

    We went through the same “should we get a new fixture?” exercise and, unwilling to drop more than $150 on a really cheesy two-tube fixture that would be way too bright, I bought Yet Another Ballast from, oddly enough, the same manufacturer and possibly even the same Mexican town.

    This time I got an electronic ballast, with an A sound rating which comes mostly for free without that big magnetostrictive iron core. Costs twice what the magnetic ballast does, but I figure you only go around once, right?

    It comes with a scary label telling you to insulate the unused lead (it can drive two tubes) “for 600 V”. That turns out to be the standard wire-nut rating, so I clipped off the exposed copper end and screwed the nut in place over the insulation. Wired the leads up per the diagram and that’s the end of that story.

    Now, I’m here to tell you that going from a nearly dead magnetic ballast to a shiny new electronic ballast is a wonder to behold: the tube pops on at full brilliance, far brighter than it ever was before, and is (no surprise) flicker-free.

    It’s almost enough to make me preemptively re-ballast the kitchen fixtures …

    Update: Which I did, a few months later. The 4-tube kitchen light pops on and is much brighter. However, that may be due to new tubes as much as anything; the ballasts wanted T8 tubes. Alas, I couldn’t find 3000 K warm-whites and had to settle for 3500 K soft-whites. All in all, a good improvement.

    More on electronic ballast adventures there.

  • Clock-radio Backup Battery vs Current Drain

    Clock-radio battery hack
    Clock-radio battery hack

    The little red Battery Sentinel LED on our old Realistic (a.k.a. Tandy a.k.a. Radio Shack) clock radio was on this morning, which means that, once again, the backup battery needs attention.

    It’s supposed to use an ordinary 9V battery, but it ate two or three of those a year. Given the absurd cost of 9V batteries relative to AA cells, that stopped making sense pretty quickly.

    Most devices with backup batteries draw essentially zero power from them during normal operation. This gadget draws 6 µA.

    An alkaline 9V battery has a capacity of about 500 mAh, maybe more with a low-drain load like this. That should last for a few years:

    500e-3 / 6e-6 = 83k hours = 500 weeks = 10 years

    Alas, the clock battery monitor is really fussy and triggers the LED when the voltage drops under about 8.5 V.

    [Update: the clock does a “battery test” every day, which probably accounts for the short battery life. I haven’t measured that current… or the duration of the test.]

    Fortunately, the clock case has a recessed bottom that fits a standard AA cell holder like a glove. I wired up 1-1/2 4-cell holders (yes, I should have used 7 cells, but I wasn’t sure what the upper voltage limit might be) to a standard 9V battery snap connector and screwed the assembly to the case.

    Now all I must put up with are the weak AA cells I got from batteries.com; the most recent order was a disappointment.

    Memo to Self: That snap connector has red = negative / black = positive!

  • Old Kenmore Sewing Machine Foot Control Repair

    Foot control - inside view
    Foot control – inside view

    One of Mary’s first investments when she got out of college was a sewing machine and she’s been using it ever since. Of late, it’s gotten a bit sporadic and the foot control seemed to be at fault.

    The symptoms were that the foot control required too much travel (equivalently: foot pressure) to get up to speed, it started abruptly (poor speed regulation), and sometimes cut out without warning.

    So I took it apart to see what I could do.

    Two pins in the side hold the top cover in place and serve as pivots. Loosen the two visible screws in the center of two of the bottom feet, hold the top half of the case down, and slide the pins out.

    A wedge on the top half presses down on the middle of the steel bar, pressing it into the rheostat. A dab of silicone lube on the wedge greatly improved that action.

    Rheostat graphite wafers and contacts
    Rheostat graphite wafers and contacts

    The speed control itself is brutally simple: a carbon-pile rheostat in series with the 120 VAC 1 A sewing machine motor. The ceramic case and heatsink tab tell you that things get pretty toasty inside that Bakelite case.

    Disassembly is obvious, which is one of the nice things about old electrical gadgets: you can puzzle out how they work and how the parts fit together just by looking. A slew of graphite disks slides out from two cylindrical tunnels in the ceramic case, followed by two graphite contact buttons. The brass fittings on the front have carbon dust on their raised surfaces, but are basically just stamped & machined metal parts.

    No fancy electronics, no firmware, just a high-power (and utterly non-inductive!) carbon variable resistor.

    The rheostat has three modes, in increasing order of pressure:

    • Off — no pressure on the foot control
    • Resistive speed control — resistance varies with foot pressure
    • Full throttle — rheostat resistance shorted by front switch
    Rheostat speed control contacts
    Rheostat speed control contacts

    With no pressure on the foot control, there’s a generous gap between the contact bar on the back surface and the two graphite buttons sticking out of the ceramic case. There’s no way for the contacts to close by shaking or accident.

    A bit more foot pressure connects those two buttons through the shorting bar across the back. Light pressure on the graphite disks means a relatively high resistance, on the order of several hundred ohms, and relatively low current to the motor. Of course, that also means the motor has poor starting torque, but … a sewing machine doesn’t need a lot of torque.

    Increasing foot pressure squeezes the disks together and decreases the resistance. It drops to a few tens of ohms, perhaps lower, but it’s hard to get a stable measurement. The motor averages all that out and trundles along at a reasonably steady pace.

    Rheostat full-speed contacts
    Rheostat full-speed contacts

    Finally, the brass disk in the central case tunnel shorts the tabs on the two brass end contacts and lets the motor run at full speed. Increasing the foot pressure beyond that point doesn’t change anything; the spring-loaded shaft can’t deform the tabs.

    The steel shaft and contact disk can short one or the other of the two piles, but that just decreases the already small resistance by about half. That might give the motor a speed boost instantly before jumping to full speed.

    As nearly as I can tell, the carbon disks evaporated over the decades, as the piles seems quite loose and required a lot of foot motion to reach the first contact point. I lathe-turned a pair of brass disks about three wafers thick, so that they’d take up the empty space in the piles.

    I also filed the brass end fittings flat so that they contact the disks over more of their surface. The first two disks looked like they had hot spots: loose carbon collected in the areas where the contacts didn’t quite touch them. I doubt that actually improved anything, but it’s the thought that counts.

    The spacers worked reasonably well, although I wound up removing one graphite disk from each pile to ensure the full-speed contacts would close properly. They’re in a small plastic bag tucked under the aluminum heatsink tab, where they can’t get lost. With any luck, the bag won’t melt around them.

    Rheostat with brass spacer button
    Rheostat with brass spacer button

    A few days later, the sewing machine stopped working entirely. The foot control itself seemed to be working correctly, but a bit of poking around showed that the cord had a broken conductor just outside the strain relief. I cut the cord off at the strain relief, hacksawed the strain relief apart, then rewired it. The cord is now four inches shorter and everything works fine again.

    I think this would be a nice candidate for a PWM controller, but then I’d have to shoehorn all that circuitry into the base of the sewing machine or add another cord to the foot control. Ptui, this works well enough.

    Memo to Self: Replace the entire cord next time.