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.
1-wire Barometer Circuit Diagram
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.
The output is fed to the 1-Wire DS2438 A/D.
The overall gain needed is about (3.25 - 1.25) / (4.25 - 3.79) = 4.4 at sea level, and (3.25 - 1.25) / (2.77 - 2.45) = 6.25 at 10,000 ft. The available overall gain is about 12.6 (i.e. 0.68 * 2.16 * 8.58).
Note
That the MPX4115 feeds R1 through a jumper. This will allow easy change of input voltage from a source than the MPX4115 for calibration.
Mounting the DS2438
The DS2438 is a surface mount device. As such it is so small that ordinary IC mounting techniques will not work. To allow this device to be used on a common IC perforated construction board, the DS2438 must be mounted on a DIP IC socket. It can then be plugged into a socket on the barometer board. Following is a technique of mounting the DS2438.
It would appear that Digi-Key has an adapter you can buy rather than build this. Their catalog lists Digi-Key part: A724-ND as an adapter for 8-pin SOIC to 8-pin DIP for $6.97. I do not know for a fact that this part will work.
Board Layout
Mounting the DS2438
The DS2438 is a surface mount device. As such it is so small that ordinary IC mounting techniques will not work. To allow this device to be used on a common IC perforated construction board, the DS2438 must be mounted on a DIP IC socket. It can then be plugged into a socket on the barometer board. Following is a technique of mounting the DS2438.
It would appear that Digi-Key has an adapter you can buy rather than build this. Their catalog lists Digi-Key part: A724-ND as an adapter for 8-pin SOIC to 8-pin DIP for $6.97. I do not know for a fact that this part will work.
Board Layout
A suggested parts layout for Version 1.1.
Top view of the constructed board -- Version 1.0.
Bottom view. Board is flipped vertically -- Version 1.0.
Parts List
- Component PC Board -- Radio Shack 276-149
- Dual 4 Pin (or 6 Pin) PC Mount Modular Jack
- 2.1mm DC Power Connector
- 8 pin soldertail DIP socket
- 16 pin wirewrap or soldertail DIP socket
- DS2438 -- It appears that only a surface mount package is available
- Motorola MPX4115 silicon pressure sensor
NOTE: No pin numbers are on the schematic because the pin numbers vary for the different packages - LM358A opamp
- 1N5817 Schottky Diode -- or any power diode (rectifier) with 15 (or more) reverse voltage
- LM78L05 5 Volt regulator
- 10 turn potentiometers -- 5K, 10K
- 1/4 watt resistors - 2.2K, 3-3.3K, 10K, 15K, 47K, 100K
- Capacitors -- 2.2uf 35V, 1.0uf 15V tantalum, 4.7uF 15V tantalum
Post a Comment