Monthly Archives: September 2013

Vinegar Egg Science Trick

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Vinegar Egg On PlateMy son read that if you put a raw egg in vinegar, the shell will dissolve.  So we got out a bowl and tried it.

It was fun!  The reaction bubbles out CO2 which collects on the shell.  The bubbles at the bottom gently roll the egg. That’s handy since you don’t have to keep coming in to rotate the egg or stir, and it’s fun to watch.  We left the egg to toss and turn all night.

The next morning we rushed down to see what had happened.   The egg now had only two receding polar caps of calcium carbonate,  presumably because the rolling action affected those ends the least.  A quick rinse and our vinegar egg was 100% shell free.

I was surprised at how delightful the shell-less egg is.  So natural and familiar, but so unlike an egg.

I shot a video of it rolling in vinegar and of the final result.

Harbor Freight Drill Hacking Fun

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Drill Cracked OpenA long time ago, I got a Harbor Freight drill driver that was on sale for less then twenty bucks. I knew not to expect too much from the drill, but I thought that a motor,motor-controller, gear box, battery, and battery charger was a pretty awesome bunch of stuff for that price. The drill didn’t see much use.  At one point I took it to work and fashioned a spiraling hypo disc for it from office  supplies.  So I guess it did help me with some technical discussions at work, but for years the drill sat idle. A few months ago, my son wanted me to build a Van De Graaff generator. I was originally going to go with an AC motor, but my boxes of motors had very few fast AC motors. Lots of stepper/servo motors, lots of DC motors, and AC motors with gear heads, but basically no high speed AC motors. I read an Instructable about Van De Graaff generators that said you should be careful not to have sensitive electronics plugged into the same line as the generator! I don’t really know if that’s true. But if I went cordless, Pioneer might be able to take the generator to school, and we wouldn’t have to worry about randomly killing electronics around the house. I remembered that old drill, fished it out, and charged up the battery.  Much to my surprise, it was still functional.

motorTriggerAndMosfetI knew I’d have to ditch the gear head in order to get more speed, so I ripped the thing apart. I took off the gear head, and the motor seemed to still have plenty of power, which was encouraging. I opened up the trigger assembly to see what was going on inside. I popped the case open, and  every wire immediately fell out. I happened to be holding the wires in order, so I groped around one-handed to get out my camera to document the wire order. Thankfully, I later found that I’d already taken enough photos to document the wiring, but it was an exciting moment. The controller uses a cleaver set of pinch connectors that release the wires as soon as the case opens. Fortunately, the trigger assembly actually has decent labels for all the connections.

Motor Control Cracked OpenI didn’t dissect the circuit that extensively, but there’s a very nice IRFZ44N N channel VMOS transistor that does all the switching, and it even has it’s own internal protection diodes, which means the circuit is really simple. The trigger switch is actually quite complex and cool. It does different tasks depending on its position. When the trigger is fully out, the switch shorts out the motor for quick stopping. When the trigger is pulled in all the way, the switch bypasses the  power transistor so it’s not doing any work. When the switch is in the middle range, the trigger is controlling the wiper for a linear potentiometer that’s controlling the speed of the motor. There’s a little 8-pin chip that is taking that pot’s voltage and producing a pulse-width modulated signal that’s driving the gate of the power transistor.

Motor Control RewiredIn an old Bosch drill that I fixed, the PWM was being provided by an actual 555. This little 8-pin chip was not labeled, but I kind of doubt a  555 can drive the kind of gate capacitance that Mosfet has. So who knows, but it doesn’t matter for this project. I thought, “Oh, I can force the switch into that middle range, then wire up an external pot for a nice knob-speed control” I cut the on-board potentiometer with a Dremel cutting disc and soldered leads to the three points of interest. Then I drilled a hole out the side for the wires to exit.  I also had to take my grinder and do incredible violence to the sliding-switch assembly that had been holding the wiper to make room for my big soldered wires. The switch was never going to work the way it used to.  I just had to hope it was going to work at all.

I started reassembling, and I realized that one of the tiny clips from that cleaver wire-clipping mechanism was missing! I spent 10 minutes hunting around on my cluttered work tables and grimy shop floor to no avail. I had just given up when (as I was walking out of the garage) I stepped and heard a funny scrape.  There, under my shoe, was the missing clip! *phew*

Pot Wired UpThe project was saved! Be careful not to lose those little guys. I re-assembled the trigger assembly without the trigger. The original internal pot was about 700k, so I plumbed in a 1Meg pot and decided to give it a try. It worked! At the low end, the motor would whine and not turn, but I knew I could wire in a fixed resistor so we could set the min PWM duty cycle that would make the motor and belt turn.  The knob would be the perfect speed control. Now the only problem was that I no longer had an on/off switch. I really wanted a switch integrated into the pot, but when I went to the surplus store all the pots with integrated switches where log-tapered pots.

Two Pot Back PlatesI assume that’s because most people use an integrated switch when they want ‘off-on volume’ style controls and our perception of audio volume is logarithmic.  What to do? I didn’t  want to have to special order some pot just for this hack. So I decided to build a Franken-Pot (TM). I bought a pot with a switch, pried it open, and swapped resistive elements with a different 1meg linear pot. The surplus pot was clearly of much nicer construction then the cruddy RadioShack 1meg pot. The biggest difference was that the center contact ring on the surplus pot was raised up, but I bent its wiper blades out a bit more, and they were able to make contact with the other pot’s center ring just fine. I had to do a little bit of filing and grinding to make the bits fit snugly, but once I had the tabs bent back down, no one would be able to tell it wasn’t just a stock pot.

Donner Pot(Full disclosure: mostly, I just wanted to be able to say stock pot.)

So that was a bit of a side adventure. I wired in the fixed resistor to set the minimum PWM and wired in some connectors so the pot could be unclipped when you opened the base of the generator.  Then it HIT ME. The big mistake. I had one of those “What was I thinking?” moments. This thing was going in the base of a machine that was going to be shooting 100kv sparks around. That electronic speed control was never going to survive in there. I was going to have to go back to a simple on/off switch and this whole motor control adventure had been a kind-of-fun two afternoon waste of time! *smack forehead*

Oops. FAIL

Making Moomin Stacking Stools

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I’m a big fan of Tove Janson and her lovingly illustrated kids’ books about the Moomins.  When a coworker sent me a link to a set of Moomin-themed stacking stools that had apparently never come to market, I thought “I bet I could make a set of stools like that in one night!”   I’d been working a bunch of long hours, and a goofy one-night project seemed like it would be a nice diversion.   Should I have taken on another project instead of finishing up the Van de Graaff generator?  I don’t pretend to be fully in control of which projects I work on when.  I looked online for some cheap stacking stools, and they were soon Amazon Priming their way to my house.   I crossed my fingers that the stools would have legs that could be removed so I could fit the seats in the laser cutter.  Thankfully, they did.

Cardboard CircleI laser cut a piece of cardboard to act as an alignment jig. Then I used the laser’s raster mode to etch the designs onto the seats.   The plan was to etch the design on, then use paint and a squeegee to make the designs stand out.  I’d used that trick before when making the seals for the Retrotechnologist society with good results.

Moomintroll Before PaintAfter etching Moomintroll, I began to wonder if painting would be necessary.  He looked good, and I’d even managed to orient the wood grain so it serendipitously lined up with some of the reeds in his little patch of water.   Maybe this project was going to be even simpler then I thought!

Little My From Two AnglesPaint And Squeegee

When I etched Little My, she looked good from some angles but terrible from others.  The orientation of the wood grain had somehow made her very viewpoint dependent.  Here’s a little video of the laser etching and her Cheshire Cat magic trick.  Clearly she was going to need the paint treatment.  I finished etching them and headed home to apply the paint.  When I did, I got a nasty surprise!

Moomintroll After PaintThe stools where cheaply made; the clear coat was uneven and had lots of tiny holes which the black paint was happy to highlight!  Would I be able to make them look okay with some scrubbing and sanding?   I hoped so.  I applied paint to all but the Moomintroll stool.  He was so lovely without paint that it felt really risky to slather him in black, but I didn’t want him looking different and getting all self conscious.  So I took the plunge and smeared on the black paint. Then I went in the house to scrub my blackened hands and go to bed.  All the while wondering if this project could be salvaged.  Had I been working on a  one-night disaster?

So much for only taking one night

Snufkin Paint FixesThe next morning, I tried various tricks to see what could save these stools.   I tried sanding with a flat block and 220-grit paper.  Finally it seemed like the best results  where achieved by just scrubbing like mad with a wet scrubby sponge.   Snufkin’s stool had the worst surface, but since his scene was at night, the splotches didn’t seem that out of place.  I had to apply some paint fixes after some over-zealous sanding.  I applied the paint in ways that added to the figures so any mismatch with the background would look intentional, like shadows and folds instead of random patches.  I used the red sable brush I saved up for when I was in high school.  It’s still my go-to brush, and it has seen a LOT of use.

Four Stool Tops On DeckThankfully Moomintroll’s clear coat seemed to be the best of the lot, so he didn’t have too many bad specks.  I was also able to take a more painterly approach to cleaning the beach scene stool.  I scrubbed the dune crests more so they’d be lighter and even left some of the dark surface paint in place on parts of the ocean.

I got them to a point where I was no longer embarrassed about the look, and then I brushed on a quick clear coat of Minwax polyurethane semigloss.  I use that finish on a lot of my projects, so I already had some on hand.  It’s tough and fairly forgiving.

Screw On LegsAfter they dried, I put the legs back on.   You can see the little felt patches that keep the stools from scratching each other when they’re all stacked up.    
A long time ago, I bought a giant pack of 160 self adhesive felt pads with foam tape on one side. They’ve come in very handy on a lot of projects.   They’re on the bottom of the Van de Graaff generator, and I knew I’d have to put some on the stool feet.


Felt FeetThe pads were a bit too big for the stools, so I got out the razor knife and started trimming them to size.  Once those were on, it was time to try the stools  out!  First for slacking, and then for stacking.

I guess when all’s said and done, I wasn’t satisfied with the results of this project.  The black specks that no one else seems to notice really bother me.  The stools aren’t super comfortable, and we didn’t strictly need more seating around the house. I can’t declare it a resounding success, but it was quick, and they may come in handy.  Who knows.

Stool Test With PioneerStool Test SimonfinalStoolStack