Archive for May 8th, 2009

Homebrew V-750 Dosimeter Charger Pedestal: Insulator & Light Pipe

Contact Detail - Bottom View

Contact Detail - Bottom 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 ring

Turning contact base ring

Turning central contact post

Turning 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

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

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.

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