Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The original V-750 pedestal is a threaded bushing around the cylinder that contacts the dosimeter. Hard to make from scratch, but it’s basically a bolt with hole in the middle. I can do that…
A foray into the parts heap produced a copper bolt threaded 1/2″-20 and a matching steel nut. I bandsawed the nut in half, doing a surprisingly good job of cutting it parallel to the surfaces, and filed off the obvious blems. The thin washer fit a 7/16″ bolt until I filed the hole out; the OD is a bit undersized for a 1/2″ head and looks much better in this application.
I grabbed the bolt threads in the lathe and turned down the head for a slip fit in the EMT. Turns out the head wasn’t exactly concentric with the threads, but now the rounded-off hexagon tips are. Drill out the middle for a slip fit around the 11/32″ brass tubing, break the edges, and it’s all good.
Drilling EMT mounting holes in bolt head
The bolt threads need to be barely long enough to go through the aluminum box I’ll eventually mount this thing in and pass through the nut, so I sawed the bolt off to 3/8″, more or less, and cleaned up the end in the lathe.
I thought about soldering the bolt to the EMT shell, but, fortunately, came to my senses before doing any damage. Instead, I drilled & tapped three 4-40 holes in the head that will match with similar clearance holes in the EMT. This is the sort of thing that works really well with “manual” CNC: get the first side lined up, then just type G0 A120 and you’re at the next face. A manual G83 peck drill cycle pokes the hole exactly where it’s needed.
Manual tapping, a bit more edge breaking, some cleanup, and the thing looks pretty good.
Pedestal mount – oblique viewPedestal mount – top view
The cylindrical center of the pedestal must conduct light into the dosimeter, conduct a positive charge into the contact pin, and push that pin hard enough to make contact inside the dosimeter.
The general notion is to turn an acrylic rod to a slip fit inside an 11/32″ telescoping brass tube, glue a wider acrylic disk to the bottom to take up the spring pressure, and run a 4-40 machine screw right down the axis to carry the current. There’s also a screw in the side to prevent the shaft from rotating.
Although the bulk of screw and solderless lug looks like it should block much of the central shaft’s view of the LED in the base, enough light gets around to illuminate the dosimeter’s scale. The acrylic doesn’t need an optically perfect finish, either, as diffuse light works fine.
Turning contact base ringTurning central contact post
I used a hole saw to extract a disk from a piece of acrylic that used to contain one of those crappy desk clocks they give out as awards when money’s too tight to mention. The diameter should be a bit larger than the EMT’s ID so you can turn it to a slip fit.
Chuck the disk up reasonably square and drill out the center to a bit over 11/32″, so the tubing will bear against the rod rather than the base.
The acrylic rod has two slip fits: into the brass tubing and into the disk. Neither will be particularly fussy, so don’t lose much sleep over perfection here. Apply some good solvent adhesive to the rod’s large end and slide it into the disk. Pause for a day while it cures: it’s a big joint.
After it’s cured, chuck up the rod and turn the disk so it’s nice & square & neatly finished.
You’ll need two more disks: one for the pedestal base and another to act as a collet. I made them from 3/8″ polycarbonate sheet, of which I have what may turn out to be a lifetime supply. The base disk will be another slip fit in the EMT, the collet must match the actual OD you just turned on the contact disk. Saw a slot in the collet disk to convert it into a (crude) collet.
Drilling 4-40 clearance hole
The original V-750 pedestal has a nice stamped rod running the length of the central post, but I figured a long 4-40 screw would work as well. However, there’s no reason to thread the entire length of the post, so drill out a 4-40 clearance hole from the disk to within about 1 cm of the other end. This is where the collet disk comes in handy; you can see the saw slit at the bottom, between two jaws.
Take the rod out out and thread the end.
Drilling rotation stop hole
The last step is drilling & tapping a 4-40 hole in the side for the rotation stop screw. This will fit into a corresponding slot in the EMT shell to prevent you from twisting the contact wire off.
Put everything together, with a dab of cyanoacrylate adhesive to keep the brass tubing in place, and you’re done with this part.
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
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
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.
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
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.
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!
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
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
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.
The Smell of Molten Projects in the Morning 