Pressure sensor on a Pic

[brentonw2004]

Hello Everyone! I am trying to build a digital boost gauge for automobiles. I am using a Pic 16F876, a 16x2 HD44780 LCD, and this pressure sensor:

http://catalog.sensing.honeywell.com/datasheet.asp?FAM=Pressure&PN=24PCDFA6G

For anyone here who has helped me with the rest of my project, I have finally gotten my LCD to work with my Pic, and have gotten serial communication, so thanks! The only thing that I have to figure out is how to get the pressure sensor to work with the Pic. There are four pins on the pressure sensor. 1->+V, 2->Output+, 3->GND, 4->Output- . I am supplying it with +5V and GND. I am then sending my + & - output to the An0, and An1 port of the Pic. I checked both of their values with a meter and one was 2.53V and the other was 2.54V. This is not nearly enough of a difference to measure. I need to have one output voltage with 0v being -30 psi, and 5v being +30psi. I think I need some type of differential amplifier for my circuit. If anyone has a schematic or advice on building a differential amplifier that can let me measure both boost and vacuum pressure I would definitely appreciate it. Does anyone know of any sites with an example of using a sensor such as this? Any type of advice is welcome. Thanks!

 

[Sebi]

As i see this is also a bridge-type sensor as Motorola similar device. You need some amplifier like this:

 

[brentonw2004]

What exactly is the MC33274? I take that it is a quad differential amplifier. Where can I find one of these? Does anyone have any experience with these? Can I find one of these at RadioShack? I don't want to pay 6 dollars shipping for a 1 dollar piece. Any advice is welcome.

 

[brentonw2004]

Also, what type of voltages do I need to operate this chip? I know that most op amps work on +-18V, but i am working off of a +5v for the pic controller, or the unregulated vehicle voltage. How would I obtain the +-18V from a car power supply? Schematics are very welcome.
Thanks!
Brent

 

[brentonw2004]

Sorry about this, but I have one more question. How do you have to adjust the amount that the amplifier amplifies the signal? Sorry about my ignorance.
Thanks!

 

[Roff]

This should do what you want. I used a current source to excite the bridge, since the datasheet says that this makes it self-compensating. You can tweak the current with RV1 to calibrate the full scale pressure(basically a gain adjustment), if you have a way to calibrate it. Otherwise, just set RV1 to half scale. Adjust RV2 to set the output to half scale (+2.5v if you have a 5v reference on your A/D converter) with zero differential (atmospheric) pressure.
You should try to use 1% resistors (not the pots) for good temperature stability of offset, gain, and common-mode rejection. They are not expensive, but may be hard to find locally.