My son’s first PC board

In Towards automatic measurement of conductivity of saline solution, I complained about not being able to use the KL25Z board, because my son was using it.  What he was doing with was building his first prototype for the light gloves project:

Here is his first PC board design, populated and mounted on the Freedom KL25Z board. The 5cm×5cm board (the cheap size from Iteadstudio) is a bit smaller than the KL25Z board is wide, so it only plugs in on one side (there is a screw acting as a spacer to keep it from being a cantilever). He has not yet mounted the Bluetooth module.

The prototype board has many differences from the final design: no battery, no battery charger, no buck/boost regulator, no flash memory, no processor, screw terminals instead of jacks—even the LED driver chip is different, since the chip he plans to use is only available as a surface-mount device. But there is enough of the design here to start demoing and writing code.  They are hoping to keep the final board below 5cm×5cm, so as to get low PC board prices even in very small production runs.  That will mean all surface-mount parts, so I think I’ll have to get a hot-air rework tool so that they can assemble a prototype—I’ve been thinking that I might want one for myself to play with surface mount designs, so this isn’t really a hardship.

My son still owes me some money for buying him the PC board run, the screw terminals, the Bluetooth module and some heat-shrink tubing. It is a bit annoying that he isn’t old enough to get his own Visa card, so that he can do his shopping without me as an intermediary. (We’re not talking big bucks here—we’ve spent more on pizza for him when they work through dinner than they’ve spent on all parts combined.)

I’m pleased that he got his first PC board working on the first attempt—he did the design entirely on his own, though he did ask my advice about things like via sizes and how fat to make the wires. Since there can be moderately high currents for the LED driver, I recommended that he make the ground and power lines as fat as he could, and he decided to do a flood for each. The board looks quite nice:

The top view of the board with the screw terminal to be mounted on the top and sides, the header on the lower left, and the Bluetooth module on lower right. The hole near the top right is for the screw that acts as a spacer.

This is what the glove looks like with the five RGB LEDs lit up (I understand that the final design will have more LEDs—but the through-hole driver chip has limited pinout). They don’t have the user interface written yet, so the lights were set up by a quick-and-dirty Python script talking to the KL25Z board over a USB cable (which is also supplying power).

They have not implemented programmable flashing yet, but the pulse-width modulation (PWM) frequency is set very low here (much lower than what they intend to use in the final design), so that one gets a stroboscopic effect even with steady light settings, just from the PWM. That’s not my son in the picture, but the high-school student who started the project—my son has done most of the electronics and programming, but did not originate the idea.

The two teens spent a big chunk of the day wiring up the LEDs and writing a small test program, as they want to demo the glove tomorrow for the back-to-school event. It may also be an enticement for teens to join an Arduino/microcontroller club—look at the cool stuff you can learn to make!

Another view of the prototype light glove in action.

Once they got the demo working, they invited over a third member of the team to do some brainstorming about what else needs to be done (and how they’ll do it). It looks like they’ll be talking half the night.

Since it is clear that my son will be spending a lot of time on this engineering project this year, we decided to make it part of his official school work.  In addition to the engineering design work, he’ll also do some a paper for his econ course (on pricing the components and manufacturing, and setting a retail price for the gloves), and papers for a tech writing course.

His first tech writing assignment is to write up a description of the color space he decided to use for representation of colors in the program, and why he chose that color space out of the dozens available.  He spent a week thinking about color spaces anyway, before settling on a commonly chosen one—so writing up that reasoning for the other members of his team will be a good writing exercise.

Random thoughts on circuits labs

DNA melting

I spent some time yesterday thinking about whether we could do optical detection of DNA (particularly some variant of the DNA melting lab from MIT—see also the Fall 2008 class handouts).  I noted in the 2008 handouts that they were using a blue LED array driven by a 0.29A current source (made from an LM317T voltage regulator, a rather inefficient method). The wiki page uses a regulated 5V supply and a 25Ω series resistor, which would be around 60–70mA for a typical forward drop of 3.2–3.5V in a blue LED.  That’s still a pretty powerful light source for an LED. They say they are using LZ1-00B200, which has a 3.6V forward voltage, but can handle a full amp of current, so is more LED than is needed.

We can get a blue LED for under $2 that can handle 50mA continuously (LTL911CBKS5), though it has a forward voltage of typically 4.3V.  In surface mount for $1.14, we could get MLEBLU-A1-0000-000T01, which has a dominant wavelength of 465–485nm (depending on bin code) and a luminous flux of 10.7 lm at 150mA (forward voltage 3.2V).  I estimate the LED MIT mentions produces about 6lm at that current.  The expensive part of the illumination is not the LED, but the focusing lenses to concentrate the light and the optical filtering needed to keep the excitation wavelength from being detected by the photodiode.

I was thinking that it would be cool to use a laser as an excitation source, rather than an LED, since then no lenses or filter would be needed on the source—just a blocking filter on the photoreceptor. Unfortunately, blue lasers are very expensive.  What are cheap are the blue-violet lasers at 405nm, since the laser diodes are made in quantity for BluRay players. (Amazon has 405nm laser pointers for under $10 with shipping.) Unfortunately the usual fluorescent dyes used for DNA melting measurements (SYBR Green, LC Green Plus, EvaGreen) are not excited at 405nm, and need excitation wavelengths in the range 440nm–470nm).  I’ve been wondering whether one of the 405nm-sensitive dyes used in flow cytometry (like Sytox Blue dead cell stain) could be used.  But I’ve not found a double-stranded DNA dye for sale that is easily excited at 405nm (even Sytox Blue is way down in sensitivity from its peak), so laser excitation seems to be out—the excitation wavelengths needed for standard dyes require fairly expensive lasers.  The benchtop lasers usually used in labs and flow cytometry equipment are priced in the “if-you-have-to-ask” price range.  Buying enough copies for a student lab is more than this lab is worth.

I still don’t see a way to make the DNA melting curve project work within our course.  Even MIT gives up half a semester to this lab, and we don’t have that much time (nor that caliber of students, on average).

Soldering project

I want the students to learn to solder (at least through-hole parts, not necessarily surface-mount). I don’t want to do the traditional blinky-light soldering practice, so I’ve been looking for a place in the course where it makes sense to require soldering, rather than wiring up a breadboard.

Breadboards have problems with loose wires, so the more complex the circuit, the more problems a breadboard causes.  Breadboards also have problems connecting to wires that have to leave the breadboard—particularly wires to moving objects.  This suggests that the EKG/EMG circuit would be the most appropriate as a soldering project, as it is fairly complicated and the long wires to the Ag/AgCl gel electrodes can cause a lot of problems with loose connections (my first check on debugging is to wiggle the header pins for those wires).

But I want the students to be doing some designing for the EKG circuit, not just soldering up a predetermined circuit, so I’m thinking of designing an instrumentation-amp protoboard, which has an ina126 instrumentation amp and an MCP6002 dual op amp chip, with power pins wired up and a place for the Rgain resistor and bypass capacitors, but everything else in a breadboard-like configuration, so that resistors, capacitors, and jumper wires could be added.   Off-board connections could be done with screw terminals to make sturdy connections.

My son and I came up with the further idea of adding an optional LED output, to make a blinky-light-EKG device.  I think that the approximately 1.5V, 30msec pulse that I was seeing for the R segment of the EKG would be enough to make a visible flash—I’ll have to try it out on my breadboard.  I tried it today, but I was only seeing 0.5V pulses today (poorer contact with the electrodes?), and I had to raise the 3.9kΩ feedback resistor to 10kΩ to increase the gain of the final stage., which was enough to get weak flashes from an LED with a 100Ω series resistor.  Because the op amp has limited output current (±23mA short circuit), I felt it fairly safe to put the LED directly between the op amp output and the Vref signal, which gives a good flash even with a green LED.

The lower voltage that I got this time (until I raised the gain) makes it clear that if I do make an EKG protoboard, it should have room for some trim pots for adjusting the final gain.