Switch to surface-mount pressure sensor

[jasonwebb]

The current pressure sensor (MPX12GP) is not as easy to fit into a manufacturable design as I had hoped. It has straight leads and a long footprint, requiring either a hacky right-angle daughterboard (very 80's) or for the pins to be bent 90 degrees to fit onto board. In testing I've found the leads to be extremely easy to break, and I'd rather not add the fabrication and assembly costs of a daughterboard. A different sensor is therefore needed.

Currently considering the MPXV7007GP, which is surface-mount and has similar specifications to the MPX12GP. Additionally this new sensor outputs a 5V signal directly, which means it can be directly interfaced with an ADC pin of the ATMega32u4 without any instrumentation amplifier.

• [X] Order new sensor
• [x] Test using lab equipment to verify it works as expected
• [x] Create and add custom part to PCB design and schematic
• [x] Order new boards for prototyping

[jasonwebb]

Just tested the new sensor and found it to work exactly as expected! Ready to go ahead and integrate it into the PCB design.

[jasonwebb]

I created an Eagle part today for the MPXV7007GP sensor and added it to the PCB design. I also added some decoupling capacitors and an low-pass RC filter as recommended by the manufacturer, as well as some board layout tips to reduce noise.

I'm going sit on this design for a few days and see if I can think of anything more to do. If not, I'll go ahead and order some new PCBs.

[jasonwebb]

I ordered some new boards from OSHPark, along with parts from Digikey, and tried to assemble a working prototype.

I'm able to upload code to the ATMega32u4 using a USBtinyISP over the 2x3 ICSP interface. I'm also able to get the red LED (sip indicator) to blink using the Blink sketch.

However, I'm not able to get the blue LED (puff) indicator or the USB port to work.

• The blue LED is now burned out, but in previous testing I was able to power it manually using my multimeter's LED checker function. Earlier I noticed that the power coming over ICSP was not quite 5V, and the blue LED in particular has a higher forward voltage (3.3V) than the others, so I suspected that the light would work if I powered the device using the USB port (which I couldn't get working anyway).
• The USB port turned out to be really difficult to solder by hand, and I just couldn't figure out a good method for getting it soldered well. I tried my handheld iron with a fine(ish) tip as well as a hot air rework station, but had difficulties with both tools. I think my best bet is to invest in a cheap SMD reflow oven like the T962a and some stencils from OSH Stencils.