Jig for drilling Lego bricks

For the pulse-monitor lab, I have students drill ⅛” holes through 1×2 black Lego bricks (see Lego as LED holder and Lego as LED holder revisited), to make holders for 3mm phototransistors and 3mm LEDs. I hauled my drill press up to campus in my bike trailer last year to have students drill their own bricks, but I was not happy with the plywood jig that I had put together at the last minute—it worked, but there was a tendency for the bricks to jack-knife when the drill press vice was closed, because I had a sliding piece to clamp the brick.

I redesigned the jig yesterday and came up with a much simpler and (I think) more reliable design:

This side view shows the jig cut out of a piece of polypropylene cutting board that had been retired from kitchen use as too beat up. The bottom edge is straight, and the top edge has a rectangular cutout exactly the width of a Lego brick. (The top edge is not flat, because it was cut from a part of the cutting board that had a handle.)

Here is a top view of the jig in the drill press vice. In classroom use, I’ll probably use some painters’ tape to hold the jig to the non-moving jaw of the vice.

To use the jig, open up the vice a little bit, snap the brick into place, and tighten the vice gently to hold the brick securely. The cutting board is just a little thinner than the height of the brick, so the clamping force of the vice is entirely on the brick.

I may add these photos to my book.

My son returned to college yesterday

I’ve been wandering around the house today, cleaning things up and generally being a bit adrift, in part because my son has returned to UCSB (where he will be a senior in computer science this year) on Friday.

His trip back was a little different from what was planned.  It started out as planned, with him catching the 7:55 a.m. Highway 17  Express bus to Diridon station in San Jose, where he planned to wait for the Coast Starlight down to Santa Barbara.  We noted before he left that the Coast Starlight was running about 3.5 hours behind schedule (a common occurrence—hence the nickname the Coast Starlate).  When he tried to check his luggage at Diridon station they recommended that he change to the 4790 Thruway bus to San Luis Obispo and take the 790 Pacific Surfliner the rest of the way, because the Coast Starlight was running so late.   He did that, though he much prefers the comfort of trains to buses, and it turned out to be a good move.  He could get out at Goleta, rather than Santa Barbara, cutting out about 10 miles of ground transport at that end.  He ended up getting to his new apartment in Isla Vista about the same time that the Coast Starlight left Salinas, so he saved over 6 hours (the Coast Starlight never made up the delay—by the time it got to Santa Barbara it was 4.5 hours late).  I don’t know whether he’ll take the Pacific Surfliner in future, or even try the Greyhound (which is even faster, as there is a direct bus between Santa Cruz and Santa Barbara)—it depends on his willingness to trade off comfort for speed, as the Coast Starlight is a very comfortable way to travel, even if it is ridiculously slow.

One thing I did today was to box up 52 pounds of stuff (mostly clothing, but also bedding, some electronics, and dishes) to ship to him in Isla Vista (via UPS ground, about $40).  He only took about 75 pounds of luggage with him, because of the Amtrak 50-pound limit on single items for checked luggage (though the Thruway bus+Pacific Surfliner switch meant that all his luggage ended up being carry-on).  Because he is living in an unfurnished apartment this year, he had already ordered furniture (a bed and mattress, anyway) from Amazon, and his roommate had been there to receive it, so he knew he had a bed waiting for him (though he probably had to assemble it).

You’d think that by the 4th year, I’d be used to having him go away to college, but the transition each fall is still a little unsettling—I’ll miss our technical conversations.  Oh well, within a couple of weeks I’ll have his bedroom set up as a workshop again, with the drill press and scroll saw back on the table, and the stuff he left scattered on the table packed away in boxes.

I’ll need the workshop this fall, as I need to make more lab setups for my course (I’ll have lab sections of 50 students, so we’ll need 25 lab stations, instead of just 12).  I’ll also be sitting in on the Mechatronics course at UCSC, which has always sounded like a lot of fun, but which will probably be close to a full-time job for a person to do alone instead of in a 3-person team. My sabbatical this fall will be spent on the Mechatronics course, continuing revisions to my book, and building the lab setups for winter and spring.

Spike in views on Monday

I had double my normal readership on Monday 18 Sept 2017, which surprised me as I had not put out any posts recently.  I checked the statistics and found that the bump was mainly from one post: Engineering vs. Science, which got 173 views on Monday and 185 all week.  The referrals were from classroom.google.com, so apparently someone required reading the post for a class—I’ve no idea who, as the classroom links are somewhat anonymous (I have links, but would need to be in the class to follow them).

Because of that class assignment, the  Engineering vs. Science post is my most viewed site this week and this month, though not for the quarter—three others still beat it:

• Making WAV files from C programs
• Algorithmic vs. Computational thinking
• Where you get your BS in CS matters

It always surprises me which of my posts become the most popular.  The top 10 for the past year were

Home page / Archives	20,144
Where you get your BS in CS matters	1,775
Making WAV files from C programs	1,655
Tools and parts list for Applied Electronics W2017 and S2017	1,387
Algorithmic vs. Computational thinking	986
Sum of probabilities in log-prob space	706
How many AP courses are too many?	687
Journals for high school researchers	565
Getting text from Amazon’s “Look Inside”	547
Pressure sensor with air pump	435

The “home page” hits are large because all new posts read by subscribers get counted there. The next 9 are all posts with original content by me (unlike some of my all-time most-viewed posts, which were mainly pointers to other things on the web).

Ultrasonic transmitter and receiver impedance measurement

In Ultrasonic rangefinder with Analog Discovery 2, I looked at the impedance of  an ultrasonic transmitter with the Analog Discovery 2, but I only modeled the transmitter as a capacitor, not modeling the resonances.

So today I collected new data, both for a transmitter and a receiver, using a 1nF C0G (1%) capacitor as the reference impedance, so that I could have clean data from a known pair.  I also looked at the transmitter+receiver as a network, and located the peaks of the signal transmission.  I was curious whether they corresponded more to transmitter or receiver resonances.

I could model the transmitter quite effectively as a capacitor with 4 LCR resonators in parallel.

I could model the receiver quite effectively as a capacitor with three LCR resonators in parallel.

The fitting was done with gnuplot, fitting one resonance at a time starting with the lowest frequency one, then refitting the previously fit parameters to tweak the fit. The radius of convergence for the fitting is pretty small—I needed to get the LC resonant frequency pretty close to correct before the fitting would converge. Increasing L makes the down-spike and up-spike closer together, and R controls how low the minimum gets, so I could get reasonable initial values (good enough to get convergence) without too much guessing, by plotting using the initial values, adjusting L to get the spacing between the spikes about right, adjusting C to get the resonant frequency right, and doing a rough guess that R is about the minimum value.

The peaks of the transmitter+receiver characteristic seem to correspond most closely to the minimum impedance points of the transmitter, which is reasonable when you consider that I’m driving the transmitter from a fixed voltage—the power is going to be V²/R, so power out is maximized when the impedance is lowest.  The one exception is the 331kHz peak, which seems to fall on the higher frequency of the two closely spaced transmitter resonances, and near the peak of receiver impedance. (Of course, only the 40kHz resonance of the transmitter or receiver actually gets used—the other resonances don’t provide nearly as much response in the transmitter+receiver pairing.)

Zooming in on the transmitter impedance for the high-frequency resonances, we can see that there are minor resonances that have not been modeled, but that the model does a good job of capturing the shape of the peaks. The peak of the transmitter+receiver response here falls on the higher-frequency resonance.

I did all my modeling with just the magnitudes of the signals, so it is interesting to see how well the model fits the phase response.

I got excellent matches to the phase response (even when I zoomed in on each peak), except for the low-frequency region, where the impedance seems to have a negative real part (phase < -90°).

I do have models for no resonance, single resonance, two resonances, and three resonances for the transmitter, as well as the four-resonance model. If a simplified model is needed, then it is better to take one of those fits, rather than omitting parts of the more complicated model, as each resonance affects the other parameters somewhat.
As a simple example, the receiver can be modeled as just a 739pF capacitor, but the LCR circuits contribute some of the capacitance, so 708pF gets used for the base capacitor of the model with the 3 resonances.

Wilder Ranch

Yesterday, my son and I took a bike ride through Wilder Ranch State Park and UCSC.  We had a fun, though somewhat warm ride.  The weather was unseasonably hot over Labor Day weekend, hitting an all-time high for Santa Cruz of 108°F.  We were promised cooler weather on Tuesday, but the temperature was well over 80°F at noon.  We waited until 1:30, when the temperature finally dropped below 80°F before leaving the house.

We headed out on the paved bike path to Wilder Ranch, up Engelman’s Loop  and Long Meadow Trail to the Chinquapin Trailhead, then across Empire Grade to UCSC property, down past the Painted Barrels (there are actually two sets of barrels—Google doesn’t map the more northerly set, which were just after we entered the woods again from Marshall Field).  On the way down from campus, we stopped at the overlook above Pogonip Park and at the UCSC farm stand, where I bought some apples, cauliflower, and flowers.  The farm stand seems to have less produce this year than in previous years—I don’t know whether this is because the farm is producing fewer varieties or that they have better marketing outlets elsewhere.

Because we were doing the ride mid-week, we saw only a few other bike riders—maybe 5 or 6 on the paved path out to Wilder Ranch, one couple on the trails in the park, a pack of middle schoolers with adult guides on the UCSC trails, and a few bike commuters on the UCSC roads.  I suspect that the Wilder Ranch trails are more populated on weekends.

A map of our route. It was 13 miles (21 km) with 1178 feet (360m) of climbing.

I used Google Maps to make a route map of the route we took, which was harder than I expected.  At first I just dragged around intermediate points using Google directions, but Google kept throwing out the route.  Then my son pointed out that I could use the “+” button in directions to grow the route incrementally, though that required  couple of tries to work also, as there is a small limit on the number of points you can add, so I had to be very choosy about which points I added.

The weather was really a bit too warm for strenuous exercise, but we had cloud cover for much of the ride, so it wasn’t as bad as it could have been (certainly not as bad as last weekend would have been).  Today might have been a better choice, as the hot weather seems to have ended and we’re back to more normal temperature swings.

Neither of us have mountain bikes—my son rides a commuter bike with narrow road tires, and I have my Vanguard long-wheelbase recumbent.  The loose gravel, deep dust, and ruts of the trails in Wilder Ranch were a little difficult for us to handle, though mountain-bike enthusiasts would have found them tame.  I had to get off and walk my bike on a few steep hills, because I fell below my minimum balance speed and couldn’t start again on the loose gravel.  I only fell once—trying to get out of a rut on a steep uphill and falling below minimum balance speed.

It would have been good to do more bike riding this summer with my son—he’s headed back to UCSB in just over 2 weeks, and we’re making a trip to Boulder to see my Dad next week, so I doubt that we’ll have time to schedule another bike ride.