Intro: Hermes' Caduceus
Every couple of years I get motivated to do something cool for Halloween. A couple of years ago I decided to be Hermes, which entailed a few items, one of which had to be the caduceus: the snakes wrapped around a winged staff. I couldn't JUST do a plain staff though - it had to have something special about it. The obvious answer, as everyone on here knows, was to add LEDs. I still wanted more. I wanted LEDs that only Hermes could activate, and they had to glow, not just flick on and off. It had to be magic after all.
Here's the list of things you'll need for this Instructable:
A 32"-36" length of 1" PVC pipe
A crystal about 1.5" diameter
A bag of feathers (from Michaels or similar store)
Two rubber snakes 32" - 48" long
Some fishing line
Silver spray paint (or whatever color you want)
Two single AA battery holders
Two 2N2222A transistors
One 130K 1/4W resistor
One 47K 1/4W resistor
One 100 uF capacitor
Two LEDs (I used amber)
One magnetic reed switch
One or two strong magnets
Two AA batteries, preferably alkaline because of the higher voltage
Now, on to what you do with all of this stuff ...
Step 1: Make the Staff
As you likely guessed, the PVC pipe is the shaft for the staff. Before you do anything, make sure that the battery holders slide easily, but snugly, into the pipe. If it looks like a go, cut it to the length that you want.
Step 2: Add Holes and Notches
Cut two notches at the top of the staff to stabilize the crystal when it's in place. Then drill about 6 holes down each side of the top of the staff as shown. The holes on the side are to hold the feathers, so you can vary the number and size according to how many feathers you want to use and how big they are at the base.
Once this is done, you can paint the staff in whatever color you like.
Step 3: Add Snakes
Wrap the snakes around the staff and secure them with a couple of lengths of the fishing line. Be sure to allow some room at the top for the feathers. You're going to hold the staff about 12 inches down from the top, so wrap the snakes with gaps where you can easily grab hold and your fingers can touch the PVC pipe. There's a reason for this.
(The picture shows feathers, but at this point you should not have the feathers in place.)
Step 4: Build the Electonics
My initial approach was to use a microcontroller. I made one of the various AVR programmers that plugs into a printer port and started work with an ATTiny85. In the end though, although I learned a lot about programming AVRs, this was just too dang complicated. So I simplified it and just used a couple of transistors, resistors, and a capacitor to get a glow-on and fade-off effect.
The schematic for what I came up with is attached. The fade effect is achieved with a simple R-C circuit. When the switch is closed, current flows through the 47K resistor to charge the capacitor. As the capacitor charges, the increasing voltage causes the transistors to ramp on, which pulls more and more current through the LEDs until they reach full brightness. When the switch is opened, the 130K resistor bleeds the capacitor down, gradually shutting off the current through the transistors and the LEDs appear to fade.
This circuit took quite a bit of tweaking to get the effect I wanted, so plan on playing around with the resistor and cap values until you get the effect you want. In order to get enough power through a transistor to really light up the LEDs, I chained two 2N2222As so that the first one can eventually saturate the base of the second one and really drive the LEDs to full brightness. Your results will depend upon the LEDs you use. I used amber ones. Remember that green and blue LEDs require a higher voltage to get full brightness, so you'll need to make adjustments if you use a different color.
As far as the physical shape, the idea here is that the circuit you will build must slide easily, but snugly, into the pipe from the top. The battery holders should have enough friction to hold the whole assembly in place. You can also coil some of the wire so that it's snug to the inside of the pipe. The battery holders should be at the bottom, then the magnetic reed switch, then at the top the resistors, cap, and transistors. The LEDs should be pointing upward from the top.
Here's the secret behind this whole thing: when you hold the staff, you will have the magnet(s) taped to one of your fingers. That finger will rest against the PVC over the magnetic switch, activating it. That's why you will be the only one who can activate the staff. When you build the electronics, make the wire long enough that it positions the magnetic switch right where you will hold it. When I built mine, I made it so that the switch was about 1/3 the way down from the top of the staff.
The picture shows about what it should look like. Make sure you test it before moving on to the next step!
Step 5: Put It Together
Once you're satisfied with the positioning, you can add the feathers. When you insert the feathers, they will help hold the electronics in place. Fortunately, feathers are generally no conductive.
Now add the crystal on top. The idea is for the LEDs to illuminate the crystal, but be hidden from view. You will need to adjust as necessary to get this effect. Use some fishing line threaded through the top feather holes to secure the crystal in place. The crystal I used had a hole drilled in one edge, which made it much easier to work with. Fishing line doesn't hold a knot very well, so you'll need to experiment there too. The batteries last a pretty long time, so you won't need to take the staff apart very often.
That's it! You should now have the finished product, ready to go. Here's a quick video, taken at our hackerspace (i3 Detroit), to show you the secret of how it works. ;)