Simple Headphone Mixer

I was having a hard time finding a way to connect several audio sources into one pair of headphones. Using splitters can cause damage, passive mixers would have poor audio quality, and real mixers are overkill and expensive. I feel an electronics project coming on...



I'm planning to fit it in an Altoids tin.  It will be my first project using SMD instead of through-hole.  It's using single-supply op-amps, and I'm planning to filter the power down to 4V, so it won't be able to drive especially high-impedance headphones, but I'm hoping it will have good noise immunity and quality. I will configure it with unity gain, so ideally the volume will be the same as whatever the inputs are. It has jumpers so that you can use either two regular op-amps, or one op-amp and one special headphone amp, and also so you can bypass the voltage regulator for a bit more power (at the expense of noise).

New and Improved 3D Printed Clock

Third time's the charm! The third version of my 3d-printed clock is working great:



The biggest change was the winding system. It's a Huygens endless-chain maintaining power, a nifty idea from 300 years ago. The main weight and a tensioning weight both hang on pulleys, and the chain is a loop that goes over the great wheel (in my case, the hour wheel) and also a wheel with a ratchet. You can wind it up without affecting the driving force at all. It also simplifies things by putting the ratchet-and-click on its own wheel.

I've also made some nice-looking weights. The main weight, about 4 kg, is a 2x10 inch piece of stainless steel bar stock, the kind of thing you'd use if you were machining something on a metal lathe. They only machining I did, though, was to drill and tap some holes for a hook. And I'll have you know, I only broke 1 tap! It's still there, in the bottom of the main weight. The tensioning weight is the same idea, but exactly half scale.

The chain I'm using is the same sort of thing that's used for dog tags, #6 ball chain. You can make it into loops with a special connector piece. I've tried running the clock with that connector piece running through the pulleys, and it works, but we can do better. With a needle nosed pliers and some patience, you can form the chain into an endless loop, with one of the balls only slightly mangled. There is also a $100 specialized tool you can get to do a better job. I've made a 3d-printed version of this tool for much less, and it works fine - I used it to form the loop you see in the video.

Assembling this one was a bit difficult. On the previous version, the bearings fit loosely, so this time I added 0.2 mm of interference to the fit. That was way too much, though, so I needed to do a lot of sanding. This made the shafts not quite true, which gave the gears a "preferred" orientation, so I needed to sand even more so the shafts fit loosely. Also, the frame flexes considerably under the total 4.5 kg weight. I used some wire to brace it, but even still, the bearings would work themselves loose until I superglued them in place. The motion work was also a bit fragile, and I ended up gluing together the parts that are supposed to slide when setting the clock. (You can still set it by removing the pendulum and letting it run freely.) After all that gluing, though, it's been holding together fine (knock on wood).

Next version I make, I want to do away with the ball bearings entirely. (See my previous post for some thoughts about that.) The goal is, send a model in, get a working clock out, with almost no assembly.

Friction

I've been experimenting with 3D printed mechanical clocks, and I really want to print one that is fully assembled. Just take it out of the machine, dust it off, hang some weights, and have it work. But to make an efficient clock, you need to minimize friction - how hard is this? To find out, I've measured the friction of a plain bearing made in three of the materials that Shapeways offers.

Here is my experimental setup:



The horizontal beam is attached to a 5 mm shaft, and it can pivot by sliding in the holes on the upper and lower arms. By hanging a large load weight from the lower arm, measuring how much weight at what distance will make the beam move, I can estimate the friction in the bearing.

The results: Alumide was best with 0.14, followed by Strong and Flexible at 0.17, and Frosted Detail at 0.30. With a drop of oil, the Alumide got 0.07, the Strong and Flexible got 0.11, and the Frosted Detail got 0.20. I tried loads as high as 1kg and as low as 100g, and in all cases it was linear.

The alumide showed an interesting effect - it started out at a considerably higher friction, but after spinning it for a minute or so under a 1kg load, the friction was much reduced. I suspect that's because it starts with a rough surface, but after a short time, it polishes itself, and you have a somewhat smooth aluminum bearing surface.

Both the Alumide and the Strong and Flexible had a "textured" feel to their motion. Especially after the oil, the alumide was a little inconsistent in its motion - a weight would make it go a short distance and stop, but if you pushed it past that it would move further. The Frosted Detail, especially under higher loads, had a grabby feeling to it - similar to when a sliding a wet finger makes a squeaking noise, but lower frequency. The oil didn't help this much.

The oil I used was "Liquid Bearings" synthetic clock oil. I applied one drop to each of the 4 bearing surfaces, and waited an hour or so. The Strong and Flexible and Alumide plastics are both very porous, though, so I'm curious how long the oil will last. I'd also like to try other lubricants, such as silicone grease or graphite powder.

Version 1 of my clock used printed bearings, but it wouldn't run due to gear clearance issues. Version 2 and 3 both used ball bearings. The ball bearings work well, and probably have a much lower coefficient of friction. But they're expensive, annoying to assemble, and don't tolerate any misalignment. For heavily loaded joints, the ball bearings probably win, but in lightly loaded joints, the constant drag of the grease may be more than the linear drag of a plain bearing. Anyway, my calculations (and some experiments) show it should be feasible to make a clock with only plain 3d-printed bearings, so I'm gong to try that next.

My first 3D-printed clock

For the past few months I've been working on a 3d-printed clock.  I want to challenge myself to make something functional and even reasonably accurate, within the constraints of 3d printing.  Here's the first working prototype.


 This video is actually a few weeks old - stay tuned for the second prototype, and some experiments.

The first version had a few issues.  The pallets came very close to one of the gears, and would rub unless I ran the gear slightly out of its bearing.  The ball chain pulley wasn't quite as good as I'd hoped, and it needed a very large tensioning weight to avoid slippage.  And there was no way to wind the clock without running it backwards or slipping the chain by hand.  The next version fixes all those problems.

Propeller Results

Well, they don't fly, but other than that they work...

Materials

This is the most encouraging part.  I had the propellers printed with two materials, Shapeways' "White, Strong, and Flexible (polished)", and their "White Detail".  Both were able to spin (at about 8500 rpm) without any issues, and both were very well balanced -- only one needed a slight bend to track level.  The vibration seemed lower than with my other props, though the rpm was lower as well.

I tested hitting the spinning blades against a wooden beam.  The WSF blades were impossible to break like this!  (They seem slightly more flexible, and a bit gummier, than the molded blades, which helps them here.)  I didn't even notice leading edge damage.  The "White Detail" was a lot more brittle, though... this would happen on almost every impact:

My model had a 0.9 mm diameter hole, to fit snugly over the motor's 1 mm shaft.  In WSF, the hole fused shut, so I had to drill it out with a 0.7 mm diamond bit.  (The hole must have guided the bit, though, because it was still balanced despite my non-precision approach to drilling.)  In "White Detail", however, the hole was too large, and one of the four props would fly off under even its modest 5 grams of thrust.

Aerodynamics

This was disappointing.  My quadcopter weighs 60 grams, but all four of these props only provided 20 or 30 grams of thrust.  (For comparison, the pre-made purple props, with too small a diameter, were able to lift it, barely.)  They also only got up to 8500 rpm, as opposed to 13500 rpm for the purple ones.

The first problem is that the blades are stalling.  I had just eyeballed the angle of attack, which blended linearly from 30° at 25% radius to 10° at the tip, for a geometric advance ratio of 72%  However, after doing the math on the induced velocity I need to hover, I really need about 29% no-slip advance ratio, which ends up being thirty-something geometric depending on angle of attack.

Next problem is aspect ratio.  There's a bit of a tradeoff between chord length and angle of attack, but I've heard that at low Reynolds numbers (I'm around 20,000), it's better to have longer chords flying at lower angles of attack and lower lift coefficients.  Most of the commercial props have an aspect ratio between 3 and 4, but mine was 6, so I'll lower that next time.

Finally, airfoil shape.  Before, I just had a slab with a thick leading edge:

But going forward, I'll try something more like this:

That's a NACA 4415 airfoil, modified to never be thinner than 0.5 mm.  (An 0.5 mm diameter capsule plus the NACA airfoil with the max thickness reduced appropriately.)  I've also made lots of improvements in the .scad file, including cylindrical coordinates, a smoother surface, better hub fillet, some and some integration of performance data.

Next Steps

I'm missing a lot of data: I know how much thrust I need, but I don't know the efficiency vs. rpm curve of my motors, and I don't have good data on how the airfoils will perform.  So the next step is to order up some more props with various angles of attack, and to see how they perform.

3D Printed Propellers

Above you can see my new 3D printed propellers, before and after.  Best part?  Only $4.06 for the lot!

For my TomCopter project (a 12-cm quadcopter where the circuit board doubles as the frame), the hardest part to source is propellers.  There are lots of plastic propellers available for RC aircraft, but very few of them come in both left- and right-handed versions, and very few are as small as I need (< 73mm diameter).  The best I had found online were "Air Hogs MINI STORM LAUNCHER Propeller" replacements on eBay.  They were too small, though, and were designed as pushers, so I had to drill through the hub and mount them backwards.

Then I found OpenSCAD, which is a script-based CAD system.  I'd tried to design propellers before in Blender and several GUI-based CAD programs, but I wouldn't be able to iterate -- once I finished the propeller, if I decided I needed a different diameter or airfoil pitch or something, it'd be very hard to tweak.  With OpenSCAD, I was able to make all the magic numbers tunable, so I can make a propeller of any shape and size just by hitting "compile".

There are some limitations of the language, unfortunately.  The extrude is not as flexible as I'd like (I can't parameterize it), so I had to build it out of short extruded segments.  You can see the stairstepping in the 3d model.  Also, since there isn't a way to append to arrays, I can't specify the airfoil cross section the way I'd like.  (For this experiment I went with a very simple linear cross section.)  Finally, since I had to take the union of hundreds of short extrudes, it ended up very slow to compile.

Shapeways did the 3D printing.  (I also looked at Ponoko, but Shapeways was cheaper.)  I had it made in several materials, but "White, Strong, and Flexible" worked best - more on that later.  Shapeways was also a little behind on shipping it to me, and accidentally polished one of the sets, but no biggie.  The best part: since the propellers are so small, they are actually cheaper than most molded-plastic ones online!  Only $4.06 per set.

Next up, some tests to see how they performed...

Welcome.

Here are two unusual things I do in my spare time: work on random engineering projects, and learn about random things.  This blog is a place to talk about those projects, in case others find them interesting or useful, and a place to vent any random thoughts about science, technology, or philosophy.

The blog's name asks a question: what technology can one twenty-something guy make, using only a bunch of random knowledge from Wikipedia and elsewhere on the net, a background in computer science and game programming, and too much free time?