Entry tags:

circuits, halloween, howto, steampunk

Steampunk Cane, Part 3
Or, how to craft a keyed flux channel for your aetheric jumpship.
(This is part three of a how-to series. Start at Part 1 or continue to Part 4!)

The Vertical Circuit

Now, you need to start thinking three dimensionally. I found it very helpful to start sketching out the schematic in the vertical format. You can't do it for everything (the IC with the two op-amps, for example, can't really be drawn flat), but it's still useful. It gets you familiar with which pieces are connected to what, how to orient the circuit, and where your tight spots will be. In this case, I knew that the bits having to do with the potentiometer should be on the bottom, since that's where those rails are. I knew that the LEDs needed to start at the top, but run to about the middle, so that's where those two nodes needed to go. My inductor would not fit inside the five rails, but would fit inside three, so it had to go near the top, as well. The constant current source, then, was on the bottom, followed by the current compare and PWM, and the buck converter was spread over the upper 1/3 of the rails. The LEDs then led back down to the current compare, which is back at the op-amp. Basically, start at the bottom of the rails with the top of the circuit schematic, and proceed in a clockwise spiral through the schematic while going up on the rails. When you hit the LEDs, go back down.

Current Source

At this point I snugged up nuts on the rails and got them oriented the way I wanted them. It would have been a really great time to remember that I was putting terminal lugs on the bottom of the acrylic base, too. I had to go back and disconnect the rails and tighten them again after doing the whole circuit. Bad move - it worked out all right, but was much more trouble than it should have been.

Work with three rails for as long as you can. The front two aren't used for much, anyways. I waited until I was putting the IC in before putting those rails up. That made soldering more difficult, but it meant that I knew that everything would fit and not short out.

We have lots of vertical space. Use it, but don't waste it. Vertical space is much more plentiful than horizontal, though, and resistors are longer than you give them credit for - longer than the gap between rails, for instance. Rely on those leads to let components move the circuit upwards. It's tough to make a physical connection before soldering in this little space, but give those leads a twist when you can. Make sure to use some sort of a clamp (pliers, etc.) to protect your ICs and transistors from the heat of your soldering iron. Get a fine tip—you'll need it to solder between the rails. Also, tinning your leads before soldering and keeping a clean tip are absolutely essential. You're working in tight spaces with little physical support and some sensitive components - everything you do should be about minimizing the solder time.

When connecting to the rails, a great way to secure components is to grab a space piece of rod and wrap the leads around it to make a little spring. Remove it from the scrap rod, and slightly tighten this spring to cause a good 'grip' when you slip it on the rail. Slide it down where you need, and solder it. This is another reason why it's a good idea to work from the bottom up. Be careful, because it takes much more heat to solder to the rail than it does to solder two components together. Make sure your solder actually bonds with the brass, and doesn't just sit on it in a little ball.

Comparators

To reduce its horizontal footprint (that is, oriented as we are, the dimension from point of leads to top of package), I bent the leads on the op-amp outwards at the point where they get thinner. This had the added advantage of giving me two soldering surfaces if I needed. In this space, it's just about impossible to solder three things directly to each other. Rather, use the long lead of a resistor (or a separate piece of wire, if absolutely necessary) to create a separate solder joint for each component that joins a node. I made exceptions to this where convenient, but it was a good general rule.

After you get the IC mounted, screw the front rails in place, and connect them up to the op-amp where necessary. You won't likely be able to get a heat sink on the IC. I'd like to recommend a socket so that this is not a problem, but I don't believe one would fit.

LEDs

After the IC, it's all downhill. Not only is the rest of the circuit relatively straightforward, but you'll have gained familiarity with how things have to be laid out. I had all sorts of trouble trying to fit the area around the inductor in my head while planning on paper, but it just flew together when I actually sat down to do the work.

As of when I bought these components, it was very difficult to find efficient, bright, white LEDs with a diffuse lens. The white LED market appears to be all about brightness, and a frosted LED scatters its light all over. If you're making a desk lamp, a focused light might be appropriate, but a 30° spread on the LEDs in this cane would give people spots if they looked down the top of the cane, and would put a spotlight on the ceiling and/or floor when just carrying it around.

I took matters into my own hands by chucking the LEDs into my drill press, running it on its slowest setting, and giving them a good scrubbing with 220 grit sandpaper. You can't really sand it too much, but you can definitely sand too little, so don't be shy. This is especially true on the dome of the LED, which is where the lens is. After you unchuck the bulb, examine it under a bright light, looking for reflections. Any shiny spot is a spot where the light is going to shine straight through instead of diffusing.

After you get your LEDs unpolished, wire them up. I chose to point two up and two down - mostly for aesthetics, but also because even when scuffed up, the LEDs still put out more light along their axis than otherwise. Double- and triple- check your soldering and component placement. Make sure no wires are touching, and no components are brushing up against rails. Give it a little shake to make sure nothing will jostle against anything else (though it should be pretty solid at this point). Then...fire it up! This circuit doesn't have any backwards voltage protection, so be extra careful when attaching the 9Vs.

Give a twist to your pot, and you should get a gentle glow. If not, you still have your prototype breadboarded, I hope. Personally, I went at 1/3 speed while soldering and planning, checking every node configuration against the breadboard at least three times. Just before applying power, I noticed I had one resistor wrong, going across the legs of the pot instead of connecting one of those legs to ground. Fortunately, that ended up being the only mistake. I wouldn't relish having to rework the circuit around the IC, for example. Soldering up this circuit took me three long evenings, just to give a sense of the deliberateness I felt it was due.

Add the Bulb

Now that you know the circuit works, slide the bulb over the circuit and check the fit. You might need to rock it or turn it from time to time to get the irregularities of the rails to fit in the irregularities of the tube, but try not to twist or shake it too much. The ends of the rods, solder spikes, and ends of leads will put small scratches in the bulb, and they add up over time.

To help the fit in the union, I found a 1 1/4" ID compressible rubber O-ring in the plumbing section of Ace Hardware that fits perfectly. It helps snug things up, since the flare + base are not quite thick enough to meet the black pipe, due to the taper of the pipe threads. If you're super confident, you can weld the tube to the base at this time, so that the bulb will not have the opportunity to twist and scratch any more. So far, I have not been that confident.

The top assembly is complete! Set it aside for now.