LED board schematic

I did a series of posts about the LED boards that I designed for lighting and strobes (starting with Summer project in 2014), but I just realized in going over the posts that I never posted the schematic nor explained how the board worked. I had to dig out the Eagle files on my old laptop to find the schematic, then redo it in Scheme-It to make it look reasonable.

Schematic for the LED board. This design is version 8, and is the one I actually had made.

The resistor R1 is a current-sensing resistor, measuring the current through the nFET Q2 and the LED. When the current is high enough, the voltage on the base of Q1 becomes large enough to turn on the NPN transistor Q1, lowering the gate voltage on Q2 and starting to turn the nFET off.  When the current is low, Q1 is off and the resistor R2 pulls the gate voltage for Q2 up, turning on the nFET more strongly. The Schottky diode D1 is just there to protect the LED from large reverse voltages if the board is hooked up backwards.

Power is dissipated in 4 places: the LED itself, Q2, R1, and D1. Based on the measurements in LED board I-vs-V curve,  the current is limited to about 118mA, so the voltage on the base is about 0.555 V when Q1 starts to turn on and R1 dissipates about 65mW. There is some current going through R2 and Q1 that doesn’t go through the LED (probably about 1–2mA), depending on the voltage applied to the whole board, but it is only about 1% of the total current, so I’ll ignore it—we may have another 30mW dissipated in R2.

I probably should measure the voltages across R1, on the base of Q1, on the gate of Q2, on the drain of Q2,  and across D1 to get detailed information about where the power is really being dissipated.  I think that the voltage across the LED should be about 6V when the board is fully on, and the voltage drop across the Schottky diode should be small (maybe 0.1V for 12mW).  For board voltages before the current limitation cuts in, almost all the power goes to the LED.  At higher voltages, the extra voltage drop and power dissipation is all in Q2, with around 700mW dissipated in Q2 when the board voltage is 6V higher than where the current limitation starts and 700mW delivered to the LED. That’s about as high as I’d be willing to go for continuous lighting, even with a heat-sink on the board.

I should be able to make the measurements fairly easily with the Analog Discovery 2, and I might do so later this week. One thing I’m curious about is whether the drop in current with higher board temperature is due to changes in the characteristics of the NPN transistor or the nFET.  The nFET is dissipating most of the power, so its junction temperature is probably changing much more, though the LED and the nFET together warm up the whole board, so the NPN transistor is getting warm also.

Grading done!

I’ve just finished about 3 weeks of all-day 7-days-a-week grading, and I’ve filed my grades for the quarter.  I’m now ready for a 6-month break before I start grading again.

This past week has been particularly stressful, as our water heater failed (again) on Tuesday, and I had to find a decent plumber as well as making doing with cold showers for a week.  The new water heater should be installed tomorrow.

I now have time to think about what I’ll try to get done this summer and fall, both personally and professionally.  Here is a partial to-do list, in no particular order:

• Review senior portfolios for about 50 graduating seniors.
• Rewrite the Applied Analog Electronics textbook.  I have about 161 to-do notes left in the book from teaching the last two quarters—some from student comments, some from observations made while grading.
• Read the Student Evaluation of Teaching forms for both quarters and think about how to improve the class based on them.  This will probably require a beer or two, as I know that some of the students really hated the class (based on anonymous comments on Piazza).  I’ll wait on that until my stress level has gone down a bit, or I won’t be receptive to even the good ideas.
• Design a senior project involving testing hearing aids—perhaps contacting faculty at the hearing-aid research center at DTU.  Maybe visit DTU in Copenhagen?
• Visit my dad in Boulder.
• Get a new range hood installed (I promised this to my wife last summer).
• Get a new refrigerator (the old one is rusty and the interior light doesn’t work)
• Get a new desktop computer and monitor—perhaps a Mac mini?
• Remove the ivy and blackberry vines in the backyard (that is a never-ending project, as the vines have covered about 50′ by 20′ to almost head height)
• Clean solar panels
• Fix my desk lamp (the one I made)—the copper tubing has suffered from metal fatigue, partly as a result of the cat playing with it and bending it over.  I’m trying to decide between remaking the copper supports (out of copper tubing again) or soldering on copper pieces to splint the fatigued part.
• Mow the front lawn (easy! I can do it in an hour or two next weekend)
• Mow the back lawn (probably impossible)
• Sort all my old screws, bolts, and nuts by size and put them in accessible storage boxes.
• Clear the breakfast-room floor of electronics, magazines, catalogs, … that have accumulated while I was grading.
• Clear the bedroom floor of hardware, books, magazines, and stuff that has accumulated over the last decade.  But where will I put it all?
• Replace the soap dishes in the bathroom (I like the design of one that has cracked, but I can’t find another like it).
• Hire someone to haul the truck load of debris on my driveway to the dump.
• Get an architect to design wheelchair access to my house (I don’t need it, and hope I never will, but I’d rather it were in place before I need it).
• Get a new gate designed, built, and installed on the driveway.  The old one rotted away, so I’m thinking of going with concrete pillars and a steel gate this time, instead of redwood.
• Clean solar panels.
• Clear leaves, twigs, and dirt out of gutters.
• Install path lights, if I can find any that look decent.
• Get my annual eye exam (6 months overdue).
• Get a physical therapist or sports-medicine specialist to advise me how to run without exacerbating my hip osteoarthritis.
• Join a gym and learn to use fitness equipment (for example, I’d like to learn to run on a tilted treadmill, so that I can do a stress echocardiogram test without fearing for my balance).
• Repaint the garage door (scrape, sand, prime, paint)
• Paint the book room door.
• Fix or replace garage-door opener.
• Spot sand and apply Danish oil to wood floor where old finish has worn away.
• Do some robotics—perhaps continuing work on the bot I started for the mechatronics class, perhaps a balance bot, perhaps making a board for precisely positioning the gear motors that I have, perhaps a drawbot, perhaps a true digital clock with mechanical digits.
• Find something useful to make with my 3D printer.
• Do some weaving—I’ve not woven in over a decade, but I still have a lot of yarn and looms taking up a big chunk of the house.
• Figure out what charities (or political organizations) to give more money to.
• Look for something interesting to do with other people once I retire (most of my hobbies are solitary).
• Brew a batch of mead (I’ve not made any since the Loma Prieta earthquake in 1989).
• Improve documentation for PteroDAQ.
• Port PteroDAQ to new processors?
• Temperature-compensated VCO.
• VCO using one op amp and one FET (is it even possible?)
• Torque-measuring rig for small gear motors.  Both stall torque and torque vs. speed.

I’m sure I’ve left a dozen things off that list, but there is no way I’ll get even half of it done in the next six months anyway.

Santa Cruz Mini Maker Faire 2017

Today I spent about 10 hours on the 2017 Santa Cruz Mini Maker Faire.  The hours for the Faire were 10–5, but I spent some time setting up and tearing down afterwards, so I left the house around 8:30 a.m. and had the bike trailer unpacked and everything back in the house by about 6:30 p.m.  I figure that I spent only about 10 hours earlier on setup for this Faire: applying for the Faire, setting out all the displays and testing them at home, preparing new blurbs for my book and blog, making table signs telling people how to use the interactive parts of the display, blogging about the Faire, and doing load-in last night.  That is a lot less than last year, as I was able to reuse a lot of the design from last year.

Here is the table display I ended up with:

The bare corner at the front left was reserved for the students in my freshman design course who were coming to display their muscle-controlled robot arm, but they decided to set up in back (you can see one of the lead students in the background).

I had four interactive displays (from left to right):

• A pair of function generators and an oscilloscope showing Lissajous figures.  I changed this from last year, as I did not use the FG085 function generator this year, but one of the function generators from the Analog Discovery 2.  I still used the Elenco FG500, despite the very low quality of its waveforms, because it has a knob that is easy for kids to turn, and is easy to reset if they mess it up (unlike the jamming buttons on the FG085).  I did not use the second function generator on the Analog Discovery 2, as I did not want kids playing with just a software interface (and a rather complicated one at that).  It might even be worthwhile for me to build a simple audio sine-wave oscillator with a single big knob over the summer, so that I can have something for kids to play with that is fairly robust and that can’t be easily set into a weird state.  I could even do two, just for Lissajous figures, though having one fixed oscillator worked well this time.  I had the Analog Discovery 2 oscilloscope showing on the laptop next to the old Kikusui CRT oscilloscope, showing both the waveforms and the XY plot, so that I could explain to adults what was happening with the Lissajous figures and about the differences between classic oscilloscopes and modern USB-based ones.
  A lot of people asked me about the Analog Discovery 2, which I was very enthusiastic about—Digilent should be giving me a commission! (They aren’t, although I’m sure I’m responsible for at least half a dozen sales for them, and a lot more if we go ahead with our plan to use them in place of bench equipment in my class next year.)
• In front of the laptop showing the Lissajous figures, I had a standalone optical pulse monitor using the log-transimpedance amplifier and the TFT LCD display.  Using the log-transimpedance amplifier worked well, as did using a lego brick to block light to the sides and back of the phototransistor.  A lot of people have trouble holding their hands still enough to get good readings (particularly children), so it would be good to have some sort of clip instead of resting a finger over the phototransistor.  I’ve tried making clips in the past, but I’m not good at mechanical design, and I’ve always ended up with either a clamp that is too tight (cutting off circulation and getting no reading) or too loose (falling off).  Ideally, I’d want a pressure between systolic and diastolic pressure, so about 12kPa (90mmHg).  People did like the use of Lego as a support, though—it provided a familiar element in the strange world of electronics.
• To the right of the pulse monitor was a pressure sensor.  I had a mechanical gauge and the electronic sensor both connected to a piece of soft silicone tubing taped to the table top.  Kids pressed on the tubing to get an increase in pressure, visible on the gauge (about 20–60 mmHg) and on graph PteroDAQ was running on the little laptop (which we refer to as the “Barbie” laptop, because of its color and small size).  I explained to kids that the tubing was like the tubing stretched across roads sometimes to count cars, with a pressure sensor that recorded each pulse as wheels compressed the tubing.  (For some of the old-timers, I reminded them of when gas stations used to use a similar system.)
  PteroDAQ worked well for this setup, running all day at 20 samples per second without a glitch.  The only problem was occasional display sleep from the laptop, fixable by touching the touch pad.
• At the far right end of the table, I had a phototransistor which kids could shadow with their hands, with the result visible on another channel on PteroDAQ.  This was a last-minute change, as I was getting very unreliable results from the capacitive touch sensor when I tested it out last night.  The capacitive touch sensor worked fine at my house, but in the kindergarten room at Gateway I has a different electrical environment, and it would not work unless I grounded myself. Rather than fuss with the touch sensor, I made a new table sign and put in a light sensor instead.
  I might want to experiment this summer with different ways of making touch plates—trying to get one that doesn’t rely on the toucher being grounded.  My initial thought is that if I have two conductors that are not too close together, but which would both be close to a finger if the touch plate is touched, then I may be able to get more reliable sensing.  I could try some wire-and-tape prototypes and maybe make PC boards with different conductor layouts.  (OSH Park‘s pricing scheme would be good for such tiny boards).

I also had my laptop displaying my book; some quarter-page blurbs with URLs for my book, PteroDAQ, and this blog; my 20-LED strobe; my desk lamp; and a PanaVise displaying one of the amplifier prototyping boards.

I’d like to think of a more exciting project for kids to play with next year—perhaps something I could build over the summer.  Readers, any suggestions?

In addition to my display, some of the freshmen from my freshman design seminar class demonstrated their EMG-controlled robot arm (which uses the MeArm kit):

The students built a MeArm from a kit, then programmed a Teensy board to respond to muscle signals amplified by amplifiers designed by other students in the class. The combined project had two channels: one for controlling the forward-backward position of the arm (using the biceps), the other for controlling the gripper (using muscles in the forearm). With practice, people could pick up a light object with the robot arm.

The scheduling of the Mini Maker Faire was not ideal this year, as it conflicted with the Tech Challenge, Santa Cruz County Math contest, the California Invention Convention, and the Gem and Mineral Show, all of which draw from the same audience as the Mini Maker Faire.

The Faire seemed to be reasonably well attended (rather slow for the first hour and half, but picking up considerably in the afternoon).  There was plenty of room for more exhibitors, so I think that organizers need to do a bit more outreach to encourage people to apply.  It would probably help if they were quicker responding to applicants (it took them over three months to respond to my application, and then only after I nudged them).

Some obvious holes in the lineup: The Museum of Art and History did not have a display, but I saw Nina Simons there, and she said that MAH definitely plans to do it next year, but the Abbott Square renovation is taking up all their time this year.   The fashionTEENS fashion show was April 21, just over a week ago, so it would have been good to get some of them to show their fashions again: either on mannequins or as a mini-show on the stage.  It might be good to get some of Santa Cruz’s luthiers or fine woodworkers to show—we have a lot of top-notch ones, and many do show stuff at Open Studios. The only displays from UCSC were mine and the Formula Slug electric race car team.

Of the local fab labs, Cabrillo College Fab Lab and Idea Fab Labs were present, but The Fábrica and the Bike Church were not.  I thought that Cabrillo did a great job of exhibiting, but Idea Fab Labs was a little too static—only the sand table was interactive.

It might be good to have Zun Zun present their Basura Batucada show (entirely on instruments made from recycled materials) and have a booth on making such instruments.  It might be hard to get Zun Zun to volunteer, but they used to be very cheap to hire (I hired them to give a show at my son’s kindergarten class 15 years ago—they were very cheap then, but I don’t know what their prices are now).

One problem my wife noted was the lack of signs on the outside of classrooms, so that people would know what was inside.  The tiny signs that the Faire provided (I think—I didn’t get one) were too small to be of any use.  It may be enough to tell makers to bring a poster-sized sign to mount.  I had my cloth banner behind my table, but a lot of the displays were hard to identify.  Instructions or information mounted on tables would also have been good—again these would have to be provided by the makers.  I did not see people carrying maps this year—they can also be helpful in getting people to find things that were tucked away in odd corners.  Not many people made it back to the second kindergarten room where FabMo and the Lace Museum activities were.

Update 2017 May 1: It turns out that there were some things I missed at the Faire.  The principal of Gateway sent me email:

… we did have 4 of the Fashion Teens exhibit their creations on the stage at 11:30—might be cool to have them put those on mannequins and have a booth next year. Also we had two more UCSC projects—Jim Whitehead and the Generative Art Studio, and Project AWEsome from the School of Engineering. We would LOVE to have more UCSC-related projects …

 

Applying for Mini Maker Faire 2017

I’m submitting an application for the Santa Cruz Mini Maker Faire 2017 (2017 April 29), since last year’s Mini-Maker Faire went well (see Santa Cruz Mini Maker Faire went well).  This year I’m getting my application in early, rather than dithering about it for months as I did last year.  I have less free time to prepare the display this year, but I have a better notion what I want to do, so it should not take long to get ready.

Last year’s banner, which I can reuse this year. I might also make a shorter one that will fit on the front of the table.

The “non-public” description of my display is straightforward:

I’ll bring a tabletop full of electronics projects, as last year (see https://gasstationwithoutpumps.wordpress.com/2016/04/16/santa-cruz-mini-maker-faire-went-well/ ).

Laptops demonstrating free software to turn cheap microprocessor boards into data-acquisition systems suitable for home labs and science-fair projects.
Homemade LED desk lamp and stroboscope.

Several of the projects will be interactive (an optical pulse-rate monitor, oscillators that can be adjusted to change Lissajous figures on an oscilloscope, …).

A few changes from last year: a more reliable pulse-monitor design and a new USB oscilloscope.

The public blurb is similar to last year’s:

See your pulse on a home-made optical pulse monitor!
Record air pressure waveforms using free PteroDAQ data acquisition software!
Play with a bright custom-design LED stroboscope!
Control fancy Lissajous patterns on an oscilloscope!

I removed mention of an EKG, because I decided that it was too much trouble to tether myself with EKG leads all day.

My “Maker bio” is a bit boring, :

Kevin Karplus has been an engineering faculty member at UCSC since 1986, but has done hobbyist electronics on-and-off since the 1960s. For the past few years he has been working on a low-cost textbook to make hands-on analog electronics accessible to a wider range of students.  Several of the projects on display are from the textbook.

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.

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.