Gas station without pumps

2016 April 16

Santa Cruz Mini Maker Faire went well

The first Santa Cruz Mini Maker Faire seemed to go well.  I did not get to see much of it, since I was busy at my booth most of the day, though I did get a break for lunch while my assistant Henry manned the booth, and I made a quick tour of the exhibits during that break, to see what was there, though with no time to chat with other exhibitors.

I understand that about 1800 people bought tickets to the Mini Maker Faire, which probably means there were over 2000 people on-site, including volunteers and makers.  I hope the food vendors did OK—I ate at the Ate3One truck, since I never have before, but my opinion afterwards was that CruzNGourmet and Zameen have better food (both of those trucks are frequently on campus, and I’ve eat at each several times).

My day went pretty well, though I had one annoying problem, having to do with my pulse monitor display. When I set up the booth Friday evening, the pulse monitor was not working, and I thought that the phototransistor had somehow been broken in the rough ride in the bike trailer, so I brought the pulse monitor home, replaced the phototransistor and tested in thoroughly.  Everything worked great, so I packed it more carefully for transport in the morning.

When I got everything set up Saturday morning, I found I had no electricity, though the electricity had worked fine the night before.  After I finally tracked down a staff member with the authority to do anything about it, he suggested unplugging the other stuff plugged in and switching outlets.  I turned out that the only problem was that the outlets were so old and worn out that they no longer gripped plugs properly—taping the extension cord to the outlet box so that the weight of the cord didn’t pull out the plug fixed the power problem.

Once I had power, I tested the pulse monitor, and it failed again!  I used the oscilloscope to debug the problem, and found that the first stage transimpedance amplifier was saturating—there was too much light in the room, and even shading the pulse monitor didn’t help. By then, my assistant for the day (and my group tutor for the class on campus), Henry, had arrived and gotten the parking permit on his car, so I raced home on my bike to get resistors, capacitors, op amp chips, multimeters, hookup wire,and clip leads to try to rebuild the pulse monitor from scratch on the bread board.

When I got back to Gateway School, I tried a simple fix before rebuilding everything—I added a pair of clip leads to the board so that I could add a smaller resistor in parallel with the feedback resistor in the transimpedance amplifier, reducing the gain by a factor of about 30.  This reduced gain kept the first stage from saturating, and the pulse monitor worked fine.  Rather than rebuild the amplifier, I just left the pair of clip leads and the resistor in place all day—they caused no problem despite many people trying out the pulse monitor.

I think that I want to redesign the pulse monitor with a logarithmic first stage, so that it will be insensitive to ambient light over several decades of light.  That should be an easy fix, but I’ll have to test it to make sure it works. I don’t think I’ll have time this weekend or next to do that, but I’ll add it to my to-do list.

I’ll need to think about whether to include having a logarithmic response in the textbook—that is certainly more advanced than what I currently include (just a transimpedance amplifier), which is already pushing students a bit.  A transimpedance amplifier is a pretty common component in bioelectronics, so I really want to leave one in the course.  I’m not sure a logarithmic amplifier is important enough or simple enough to include at this level (I don’t currently cover the non-linearity of diodes).

 

Here is the booth display with my assistant, Henry. I was permitted to use painter's tape to attach the banner to the whiteboard.

Here is the booth display with my assistant, Henry. I was permitted to use painter’s tape to attach the banner to the whiteboard.

The magenta laptop on right (which my family refers to as the “Barbie laptop”) was a used Windows laptop that I bought for testing out PteroDAQ installation on Windows. It was set up with PteroDAQ running all day, recording a voltage from a pressure sensor and a frequency from a hysteresis oscillator (as a capacitance touch center).

Just to the left of that was a fairly bright stroboscope, using 20 of my constant-current LED boards. To its left is my laptop, displaying the current draft of my book. Behind (and above) the laptop is my desk lamp, which uses the same electronic hardware as the stroboscope, though with only 6 LED boards, not 20.

In front of the laptop is the pulse monitor, which includes a TFT display in an improvised foamcore stand. I used just a half block for the pulse sensor, relying on ambient light (sunlight and the desk lamp) for illuminating the finger.

To the left of the pulse monitor was a stack of business cards for my book and sheets of paper with my email address and URLs for this blog and the book.  I should have included the PteroDAQ URL as well, but I had forgotten to do so. I did tell a lot of people how to find PteroDAQ from the navigation bar of my blog, but putting it on the handout would have been better. Ah well, something to fix next year (if Gateway is crazy enough to do another Mini Maker Faire, which I hope they are).

I also had all my bare PC boards that I had designed and not populated, plus my two Hexmotor H-bridge boards, behind the business cards. One of the amplifier prototyping boards was displaying in the Panavise that I use for soldering.

On the far left of the table is my Kikusui oscilloscope and two function generators, set up to generate Lissajous figures.  I let kids play with the frequencies of the function generators, take their pulse with the pulse monitor, and play with the pressure sensor and the capacitive touch sensor.

My booth was not the most popular of the Faire by any means (certainly the R2 Makers Club in the next booth was more popular), but I was kept busy all day and I talked with a lot of people who seemed genuinely interested in what I was doing, both with the UCSC course and as a hobbyist.

2013 January 14

Weekend work

Filed under: Circuits course — gasstationwithoutpumps @ 23:12
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I just realized that I blogged about today’s class, but not about what I spent my entire weekend doing: writing up the lab handout for the hysteresis lab. I decided to swap the hysteresis lab (which had been in week 6, after the op amp lab) with the sampling and aliasing lab (which had been in week 4).  This isn’t because the hysteresis lab is simpler, but because I knew what I wanted students to do for it.

I spent the entire weekend redoing the lab, rewriting the software for the Arduino that converts the period of the hysteresis oscillator into an on/off decision, and writing up the tutorial on hysteresis along with the procedural instructions for the lab. I’m kind of pleased that I used hysteresis in the software as well, illustrating that the concept is a general one, not specific to the implementation with a Schmitt trigger. The software does an auto-detect of the pulse widths whenever the board is reset (since I don’t know what frequency the students will aim for), but that auto-detect is not as robust as I would like.  I decided not to mess with it, since I wanted code that was simple enough for students to read (and there is no programming prerequisite for this course).

Ah—I just figured out why the auto-detect is not robust!  The reset of the Arduino is slow, and the auto-detect adds another 1/3 of a second to that, so I was often not waiting long enough before touching the sensor, contaminating the auto-detect of the “untouched” frequency with some of the “touched” frequency.  I added a triple flash of the LED at the end of the autodetect, so that students can know not to touch the sensor after a reset until they see the triple flash.  I’ve updated the code and the lab handout on the web site to reflect this change.

I decided not to write up instructions on how to solder, as there are hundreds of perfectly good tutorials about soldering on the web, and students are unlikely to pick a bad tutorial by accident.  We now have soldering in labs 4, 9, and 10.

I tried soldering up one of the boards myself on Sunday, to make sure that they came out ok. They do, and the soldering is fairly easy.  There are a few places where beginners are likely to make solder bridges, but I did include solder suckers in the tool kits for them, so corrections should be easy.

The capacitive touch sensor, the hysteresis oscillator board, and an Arduino board. I included this picture (at higher resolution) in the lab handout, but carefully covered up the color codes on the resistor, as choosing the resistor and capacitor values is the main design challenge of the lab.

The capacitive touch sensor, the hysteresis oscillator board, and an Arduino board. I included this picture (at higher resolution) in the lab handout, but carefully covered up the color codes on the resistor, as choosing the resistor and capacitor values is the main design challenge of the lab.

The capacitance touch sensor in the picture is made out of aluminum foil and packing tape. One of the pre-lab exercises is to estimate finger-touch capacitance from the thickness of the tape, the dielectric constant of the polypropylene tape, and the contact area. One of the post-lab exercises is to estimate the capacitance from the frequency shift of the touch. I tried doing this and got reasonably similar results (given how much variation there is in the capacitance based on how firmly you press your finger against the sensor, increasing the area of the contact).

Panavise Jr.

Panavise Jr. that I started using this weekend—a much nicer board holder. I got this picture from one of the companies that sells it (I forget which one).

PCB holder from the web—the one I have is this model, but is rusty and has some dings in the ring that holds the lens. I got this picture off the web, but I forget from where (I carelessly copied it without citation).

PCB holder from the web—the one I used to use is this model, but is rusty and has some dings in the ring that holds the lens. I got this picture off the web, but I forget from where (I carelessly copied it without citation).

One thing we don’t have in the lab, though are board holders for soldering. I wonder whether we can borrow them from other labs for these three labs, or whether there are few enough around the labs that we couldn’t get enough anyway. I just bought myself a Panavise Jr., which is much easier to use than the old alligator-clip-and-swivels that I used to use.

The hysteresis oscillator board is easy enough to solder on the benchtop without a holder, but the more densely packed instrumentation-amp protoboards for weeks 9 and 10 will be much easier with a holder.

The next handout that needs to be written is for lab 5, the op-amp lab. That one should be fairly straightforward, and then I’ll have to get back to worrying about the sampling lab. I’m beginning to think that the sampling/aliasing lab should be a combination of the usual demo of aliasing (which students do need to play with) and the design of a simple RC-voltage divider as a low-pass filter. I think that with some small changes to the data logger (allowing down-sampling of some of the analog inputs), we can do the demo on the Arduinos using slow waveforms from the signal generator. I want to come up with some meaningful signals to use as inputs, though.

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