# Gas station without pumps

## 2011 October 25

### HexMotor 2.3 and pressure-sensor boards

Top view of my second PC board. 3 copies of HexMotor 2.3 and 2 copies each of 3 different breakout boards for a pressure sensor.

I got the boards back from 4pcb.com about a week ago.

The HexMotor rev2.3 boards have several new features: LEDs for +5v and +6.25v, a reset button, 16-bit shift register instead of 8-bit, servo outputs connected to pins 13, 7, 2, 9, 10 (rather than to the pins used for PWM).  The new board should be able to do either 6 PWM motors or 4 PWM motors, 5 servos, and 2 non-modulated reversible motors.  I was going to have the robotics club solder the board today, but they did not have time.

[Note: as of 1March 2012, I have put the HexMotor Eagle design files on the web.]

I made some breakout boards for the MPXHZ6250A pressure sensors from Freescale Semiconductor,  which gave me my first taste of SMD soldering.  At least the design uses gull-wing pins, which can be hand soldered.  The breakout board that I think that the robotics club will end up using puts a pressure sensor on one side and headers for a piggyback ADXL335 breakout board on the back.  that way there only need to be one set of wires for connecting the analog inputs and power to the sensors.

That is the board I soldered a sensor to.

Top view of the breakout board with the sensor and headers soldered in place.

The pressure sensors are tiny! I found it fairly difficult to solder the  sensor to the boards, even holding it with clamping tweezers. I did eventually get everything to stick with no shorts between the 3 signal wires, but I did have some trouble with the unused copper pads delaminating from the board.  For future reference: all pads should have wires going to them (even the unused pads) to have enough surface area for good adherence and so that some of the pad is tucked under the solder mask.

Here are the solder connections on the side where none of the pins are used.

Here are the solder connections for the power and signal pins (and an SMD capacitor).

Despite the rather sloppy soldering, the pressure sensor does work.  It turns out that the port size is just the right size for Lego pneumatics components, so testing was pretty easy.

Sensor attached to Lego pump and gauge for testing.

Here are the results of calibration tests with the (probably not very accurate) Lego gauge, done by my son and me.

0 367
5 518
6 542
7 576
8 599
9 632
10 657
11 683
12 710
13 734
14 775
15 801
16 832
17 861
18 887
19 915
20 941
21 967
22 1000

The range is about right, since 22 psi plus one atmosphere is about 250kPa, which is supposed to be the high end of the sensor’s range. Also, 600″ (50′) of water is 21.67 psi, so the range from 367 to 1000 corresponds to about 50′, so the sensor should give the robotics team a resolution of about 1″ for measuring depth, as expected from the spec sheet.

The data are well fit by $\mbox{Arduino reading}= 28.57 \mbox{psi} + 371.$ The club members will have to recalibrate the pressure sensor in water, to get calibration as depth in cm. They’ll probably have to re-zero the sensor every day they use it, to compensate for atmospheric pressure, since it is an absolute pressure gauge.

1. […] We have a pressure sensor soldered to a breakout board now and it seems to work.  We tested it using Lego pneumatics. […]

Pingback by Home schooling weeks 5–8 « Gas station without pumps — 2011 October 25 @ 17:59

2. […] I had less trouble soldering the pressure gauge this time (this was my second attempt at soldering surface mount devices).  You can see in the pictures above that the results are much cleaner than in my first attempt. […]

Pingback by Sensor board for underwater ROV « Gas station without pumps — 2012 March 1 @ 09:46

3. […] controlled the solenoid valve with an Arduino and the Hexmotor motor-control board (which is really overkill for one solenoid—a single power transistor would be enough to interface […]

Pingback by Nerf gun prototype 1 « Gas station without pumps — 2012 July 3 @ 21:12

4. […] is only \$2.25 each for relay plus socket).  The relay can be controlled by half an H-bridge of the Hexmotor board.  The other half of the H-bridge controlling the solenoid should be fine, as we never need to run […]

Pingback by Nerf gun progress « Gas station without pumps — 2012 July 10 @ 15:30

5. […] the pan mechanism they wanted a wheel that was runnable off the 12v battery and controlled by the HexMotor board. Initially they built something using a small 12v motor I had (a Mitsumi M38E-3SC) for which […]

Pingback by Robot wheels « Gas station without pumps — 2012 July 31 @ 22:46

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