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Archive for May 11th, 2009

Homebrew V-750 Dosimeter Charger Pedestal: LED Holder, Spring, and Assembly

Drilling mounting holes in base ring

Drilling mounting holes in base ring

The charger pedestal includes an LED to light up the dosimeter’s graticule. I seated a 10 mm white LED into a polycarbonate ring that also serves as the base for the stiff spring that presses the contact assembly against the dosimeter’s internal spring.

I made the base while I was doing the lathe work for the contact assembly, then grabbed it in the Sherline mill’s 3-jaw chuck to drill the 4-40 holes with a touch of manual CNC.

As before, I manually tapped the holes, but it’s a lot easier with each hole at the right location and pointed in the right direction!

Step bit making an annular ring

Step bit making an annular ring

I described the step-drilling that produced the correct hole and shoulder sizes there. That won’t work every time, but in this situation it was just about perfect.

The LED power wires pass through the central hole in the ring. I used a blob of hot-melt glue to hold the LED in place; epoxy would be more in keeping with the nuclear weapons theme, but HMG is just fine with me.

There’s another hole just to one side of the LED, more or less centered between the mounting screws, that passes the wire from the dosimeter charging contact out of the pedestal. This wire starts at the center of the top, passes inside the spring, and must not be pinched along the way.

LED in base with spring positioning ring

LED in base with spring positioning ring

I added an aluminum cylinder as a positive stop to prevent the dosimeter contact assembly from getting pushed too far into the pedestal. The length matches up with the anti-rotation slot in the EMT: the screw doesn’t quite hit the top or the bottom of the slot.

A wrap of green electrical tape around the outside made the cylinder a slip fit inside the EMT shell. It shouldn’t move at all.

The cylinder also holds the spring in place so it can’t rub against the charging wire, but I’m pretty sure that isn’t necessary.

The spring comes from my parts heap. It must provide a bit under 8 pounds of force to activate the dosimeter charging spring with about 3 mm of travel. I picked the length of the EMT shell to preload the spring to make the answer come out right, which also affects the length of the aluminum cylinder.

The spring OD must fit into the EMT and the ID must clear the 10 mm LED and charging wire in the base. Your mileage will most certainly vary.

Charging pedestal components

Charging pedestal components

Assembly is straightforward, but goes much more easily with three hands.

  • Screw the panel mount bolt into place
  • Attach the charging wire to the central contact & remove the anti-rotation screw
  • Slide the central contact in place, reinstall the screw through the slot
  • Slide the spring & aluminum cylinder in place, wire in the middle
  • Pass the wire through the LED base ring
  • Press the base assembly into position and hold while installing the screws
Finished charging pedestal

Finished charging pedestal

The charger I built turns the LED and charger power on with a push-to-activate digital encoder knob, so there’s no need for the 1 lb spring & switch found in the V-750 charger.

To read the dosimeter, just hold it loosely atop the pedestal, push the twiddle knob down, and the LED comes on.

To zero the dosimeter, press it firmly and twiddle the knob for zero!

I’ll describe the charger circuity at some point; it’s detailed in my Circuit Cellar column in the August 2009 issue.

V-742 Dosimeter set to Zero

V-742 Dosimeter set to Zero

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