This is a design for simple wiring barometer. This circuit has been a goal for make a simple reliable 1-Wire barometer with reasonable pressure resolution and one which could be constructed without a specialized printed circuit board by a hobbyist. This is the figure of the circuit.
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. This is more than a 1-Wire network can provide without an elaborate circuit to store parasitic power from the 1-Wire network for short burst of current for pressure measurements. 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 op amp circuit. In effect the sensor voltage is referenced to the power supply, not ground as desired.
To allow for this high voltage, a voltage divider is used to bring the voltage down to the active range of 5 volt op amps -- this has a gain of 0.68. This output is fed to an op amp 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 op amp, thereby allowing the adjustment of the output voltage offset to the A/D converter. This in turn is fed to an op amp 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.
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