The breakout boards for the MPX2053DP pressure sensors arrived today. They would have arrived two days ago, but I was at work and no one here heard the postman come—the packages have to be signed for, so I had to go down to the post office to day to get the package. I placed the order with ITEAD on 2012 Oct 29, they sent me e-mail saying they had shipped it on 2012 Nov 7, and it arrived 2012 Nov 23, for a total delay of 25 days. I plan to order more sets of PC boards from them this weekend: another order of the pressure-sensor boards, a revised instrumentation amp prototyping board, and a hysteresis oscillator board. If I order on 2012 Nov 24, I may get them by Dec 21, even without a special order.
I assembled one MPX2053DP breakout board. It turns out that the leads are just barely long enough to reach the PC board, if you bend them at 90° right where they change size. That is where I had planned the bend when I laid out the board, so I’m glad it worked. The M3 machine screws do a fine job of holding the sensor in place, making soldering easy. I’ll use metal M3 screws for the sensors I assemble for the lab, rather than nylon ones.
I’m using a different screw terminal than before. This one has a 0.1″ pitch, which is convenient for matching up with dual-inline packages and is more compact than the 3.5mm pitch screw terminals I was using before (I got a bunch of 3.5mm and 5.0mm pitch screw terminals in the ITEAD order, very cheap, but I’ll probably not be using them for new designs, unless I need heavier wires than the 0.1″ screw terminals will take). One minor problem with the new terminals is that you need a 1.5mm jeweler’s screwdriver—a 2.5mm screwdriver is too wide. Luckily I have a smaller one, and the set I was planning to include in the student toolkit has not only 1.5mm, but several smaller ones.
I tested the pressure sensor with the amplifier I had wired up for the MPX2300DT-1 sensor, and noticed that it was very easy to peg the sensor at either extreme—I’ve got the gain set too high for this sensor.
Note that the MPX2053 has a response of 8E-08 Vdd/P, while the MPX2300 has a response of 3.75E-8 Vdd/P, where Vdd is the supply voltage to the sensor, and P is the pressure in Pascals. So the MPX2053DP needs only half as much gain. I also noticed that the INS126 instrumentation amplifier does not have rail-to-rail output. I saw it going from 0.355 V to Vdd-0.7 V, though the spec only claims linearity from 0.8V to Vdd-0.75V typically. I should probably turn the gain down by a factor of 10 in the instrumentation amp and use a second-stage op amp with a gain between 2 and 4 to get a rail-to-rail signal. That will require unsoldering the gain resistor and wiring up a couple more resistors for the second-stage op amp. I think I’ll do it in 2 steps: first adjusting the instrumentation amp gain by replacing Rgain and seeing what range of voltage I get from my breath pressure, then adding a second stage amplifier to bring that to full scale.
I think I’ll try a first-stage gain of 102.6 (instead of 1072), which should give me a response of 8.205E-6 Vdd/P, and a full scale reading of 121.88 kPa, or 119Pa/count. With that resistor in place, I’m not pegging the output at either end, and I’m using a little over ¼ of the available range. A second-stage gain of 2 should be safe, but 3 may occasionally hit the stop. Perhaps I should aim for 50Pa/count, which would need a gain of about 2.38. The best way to do this with a non-inverting op amp using standard resistors would use (11.5+15.8)/11.5 for a gain of 2.3739 (about 0.004% lower than I want). I wrote a little Python program which can find the best approximation to any ratio using standard 1% resistor values—I didn’t try them all by hand! Unfortunately, I have the standard 10% series (though with 1% resistors), so I need to try something like (47+33)/33, for 2.4242, a 3.2% error. My gain would then be 102.56*2.4242=248.63, for a full-scale reading of 50.275kPa, or 49.1Pa/count. It would be reasonable to use 33kΩ and 47kΩ.
Using just the instrumentation amp (single stage), the lowest pressure I recorded was –19.2 kPa (–2.79psi) and the highest was 15.1kPa (2.19 psi). Using a handheld vacuum pump, I found that the low-pressure stop was about -390mm Hg (-52kPa), but that is with a most-likely inaccurate cheap dial gauge. The clipping when the vacuum pump goes beyond that is at -392 counts (-46.64kPa, if the specs and the gain computations are to be believed).
After I soldered in resistors and wires for the second stage, it bottoms out at -25.24kPa (-3.66psi), with zeros. I looked at the first-stage output on one Arduino channel and the second stage output on another, and fit a straight line to the untruncated part using gnuplot. If we fit stage2=g*(stage1-m)+m, we get a gain g=2.4351 and a midpoint offset m=511.677. The total gain is thus about 249.74, for a full-scale reading of 50.05 kPa, or 48.88 Pa/count. The midpoint is not exactly at ambient pressure: it is about 300Pa below ambient pressure. The spec calls for an offset that is <2.5% of the full-scale reading, and what I’m seeing is less than 1%, so I suspect that the offset is in the sensor, not in my circuitry. Doing another run blowing into the tube and sucking on it, the highest pressure I could manage was about 17.3kPa (2.51psi). The lowest pressure I could get by mouth was about -16.6kPa (-2.41psi).
It would probably be a good idea to calibrate the sensor with a water column, but I still need to think that through a bit more—I don’t want a flood.
Other items in the package
In the same delivery from ITEAD, I got an assortment of 25 different capacitor values (10 of each). These turn out to all have a 0.1″ (or 2.5mm) lead spacing, so I need to redesign the hysteresis oscillator board for that spacing (rather than the 0.2″ spacing I had used in the design). I had thought that there was about equal chances of 0.1″ or 0.2″ spacing, so I was not going to send out the new PC boards until I’d gotten the capacitors and checked. The assortment comes nicely sorted in 25 tiny zipper-seal bags, but the bags aren’t labeled, so the first thing I did was to get out a Sharpie and a magnifying glass so that I could read the labels on the capacitors and label the bags. I can’t order the capacitors for the students until Dec 14 (as students may still be registering for the course), so they will probably need to be ordered with express shipping.
Also in the same delivery was an assortment of 11 different bipolar transistors, 10 of each. If we were doing anything in the class with bipolars, the price is pretty good (6.2¢ each), but now that I’ve decided we’ll use FETs for the power amplifier output, there won’t be any need for bipolar transistors. I’ve not found a comparably cheap collection of FETs.