1-Wire Barometer Project

The 1-wire weather group is always looking for new sensors and ideas that will allow them to monitor more and more elements, the 1-wire barometer is just one of these sensors. Based around the Motorola MPX4115A absolute pressure sensor, the group has been trying to design / decide on a good interface circuit for quite a long time.

The most commonly used and my personal preference is the Bray series. Both the version 1,2 and 3 designs use the same amplifier with adjustable offset and gain presets, the main difference between the three versions is that the second two versions have dual voltages, 5v and 10v with the amplifier being powered from the 10v rail, this way the amplifier has a greater output range which uses the full input range of the DS2438 ADC, subsequently giving better resoltion capabilities.

I have built and calibrated my own version 1 barometer and it has worked fine, tracking a local stations pressure quite closely.

Link to the designers page
Links to similar designs
Whilst this project was getting a foothold I designed my own version using the DS2760 instead of the DS2438, I tried this because the DS2760 had a better resolution at 5v than the DS2438 but unfortunately I ran into problems at low pressures because the DS2760 goes to sleep after an input voltage of lower than 1.25v. This wasn't a problem as the dual voltage versions soon came along, I didn't feel the need to design my own circuit as an already suitable circuit had been designed, although I have produced a version 3 pcb which has full compatibility with the 1-wire hub, plug and play. This pcb also includes a service header (J1) for calibration purposes, all you have to do is plug the calibration tool onto this header and you can then inject the minimum and maximum voltages that you would expect from the
pressure sensor output, this minimises the onboard component requirement and speeds up the calibration process.

This pcb can be used for either the version 2 or 3 bray designs by just changing a few resistor values.



The calibration of this version is the same as for the other Bray versions but instead of using a single pot or two plug in pots I'm using a small circuit that you can manually switch between the vHigh and vLow voltages making calibration easier and quicker, especially for bulk production. Details of the tool are below. Connector J1 on the tool simply plugs into J1 on the barometer board.
Calibration Tool
Board Layers. (Board Size is shown on top layer) order is All, Top, Bottom and Silkscreen.
Version 3 Finished Barometer