Barometer with Sensor Motorola MPX4115

Posted on Nov 16, 2012

This design uses a Motorola MPX4115 Silicon Pressure Sensor, a Dallas Semiconductor DS2438 Smart Battery Monitor (to perform 1-Wire analog to digital conversion), an operational amplifier, a voltage regulator, a diode, and several resistors and capacitors. The circuit requires an additional power source other than that of the 1-Wire network. The MPX4115 requires about 7 ma of current. For barometric pressures the MPX4115 output voltage ranges from about 4.25 to 3.79 volts at sea level, and about 2.77 to 2.45 volts at 10,000 feet. Most of this range is above the active voltage range of a 5 volt opamp circuit. In effect the sensor voltage is referenced to the power supply, not ground as desired.

Barometer with Sensor Motorola MPX4115
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The resolution of the barometric pressure is somewhat greater than ideal. With a pressure range of 28.0 to 31.0 inHg the resolution is about 0.015 inHg. If a reduced pressure range of, say, 28.7 to 30.7 is tolerable a resolution of 0.01 can be achieved. To allow for this high voltage, a voltage divider is used to bring the voltage down to the active range of 5 volt opamps -- this has a gain of 0.68. This output is fed to an opamp stage, U1B, which has a gain of approximately 2.16. This stage has an adjustable voltage input which is added to the barometric sensor output within the opamp, thereby allowing the adjustment of the output voltage offset to the A/D converter. This in turn is fed to an opamp stage with a gain of U1A, capable of a gain range of 1/1 to about 8.58/1. The 10-turn potentiometers (pots) control the gain and offset. R3 controls the gain of U1A and R4 controls the offset of the output voltage. To see if the DS2438 is alive, connect the barometer to your 1-Wire adapter and run the iButton viewer. If it is alive you will see its 64 bit address (ID). Now to see if the barometer is alive -- if you did not set the 10K pot (R3), set it in the middle of the resistance range. With the barometer still connected to the 1-Wire adapter, in a MSDOS window run: baroCalibrate 3.25 1.25 31.0 28.0 baroCalibrate should show the adapter and COM port followed by a DS2438 device ID. It will give you a chance to select...

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