I made this desktop pen caddy for a friend. When all the pens are in the box, the Nixie tube indicates nothing in the display. Remove a pen and it lights up the corresponding letter to the pen that is missing. It also has a WWII NOS Blinky oxygen flow meter from a B17. This one just lights up and doesn't blink like the other one I did in my military box (reference my other instructable). It has a pilot light to inidcate the USB charger is on as well (neon bulb).
Wood - Hickory and Oak - fatigued and tung oiled. - $8
Blinky flowmeter $14 eBay
Nixie tube $3
Nixie driver - disposable camera driver as powered by a rechargeable battery charger.
Reed switches - $3
Assorted scrap brass and copper, electrical ground connectors, misc. electrical connectors...
Build the box AFTER you build the internal working triggers. It was very hard to make everything fit well and not interfere with the mechanism. See next step. To patina the copper I used heat from a torch and muriatic acid. I fatigued the box with a torch and letter punches, and the fittings with acid and copper sulfate. Afterwards I finished it with tung oil and a good coat of wax.
Step 1: The Trip Mechanism
A simple teeter-totter design I figured by trial and error. I used the magnets from old ultrasonic tooth brush heads as they are very powerful and small in size. Epoxy works well to hold them and the reed switches are hot melt glued to the bottom platform. When the pens are in, the magnets are up and so no reed switch is closed. Each lead to a different letter on the Nixie is wired to the switch for that pen. Make sure the spacers have washers between them and the mechanism is free from binding.
Step 2: The Internal Assembly
The insides are basically the following:
Nixie Tube - Disposable camera voltage driver - and rechargeable battery charger.
Pen trip mechanism wired for each reed switch to the corresponding lead on the nixie.
The USB charger
The power cord terminals
I power the LED to light the blinker from the 5vdc from the USB... I just tap off of it. The toggle switch powers the USB so it isnt always on. But the battery cahrger is always hot so the Nixie can work 24/7 to keep your pens safe!