# Remote Air Pressure Sensing with XBee

First off, I'll give an overview of what I want to do, and then I'll ask my questions.

• I would like to remotely view and capture air pressure values (in the range of 0-45 psi) from a mechanical pressure gauge.
• I would like to plumb a rubber line (with clamp) onto a sensor like http://www.omega.com/Pressure/pdf/PX40.pdf or http://www.omega.com/Pressure/pdf/PX70.pdf.
• I would then like to send my signal output to the Analog input of an XBee S2 radio that I'm powering on the same bus as the sensor.
• Finally, I would have another XBee S2 radio and Arduino Uno on my side to capture and process the signal.
• I would write some application to visualize and log the air pressure on the laptop side.

So, questions-

• I figured I could create a power bus on the sensor side by making a spot for 9V batteries and just putting a resistor on the bus to knock it down to 5V; this seems simple enough, but am I missing something?
• The sensors output a 0 - 4.5 VDC signal, but I heard the S2 XBee radio analog inputs can only handle ~1.2 VDC; how would I convert the levels?
• Am I missing any other generalities?

Feel free to comment on the overall plan, I'm not much for Electronics and open to suggestions.

I figured I could create a power bus on the sensor side by making a spot for 9V batteries and just putting a resistor on the bus to knock it down to 5V; this seems simple enough, but am I missing something?

Yes, you really can't use a single resistor to drop power like that, the voltage will vary with the load. Anyway it's just dodgy. Use a regulator. Even better don't use a 9V battery as they aren't good at supplying highish loads, maybe 3xAA to get 4v5 and don't bother regulating at all.

but I heard the S2 XBee radio analog inputs can only handle ~1.2 VDC; how would I convert the levels?

Use a voltage divider.

Rob

Thanks for the response!

I'll continue with the silly questions for now-

• Does 4v5 mean 4.5V or 4V/5Ah?
• With the Voltage Divider, would you just dump the non-XBee side back onto to ground bus?
• One question I forgot to ask before: as the whole sensor side will be in a little portable enclosure attached to a handheld device, would I just send the ground side of the bus to a small metal plate or similar?

Thanks again.

4v5 == 4.5v, it's a common shorthand.

5v --- resistor --- resistor --- GND ^ less than 5v

Google "voltage divider", there'll be plenty of explanations.

I don't really understand the metal plate question, in general you just connect the GNDs of all devices together with no need for shielding.

Rob

I get how Voltage Dividers work, both questions are related to the same concern- where do I send the ground circuit? For example, sensor systems in cars have a common ground which will be a terminal bolted to the chassis (a big piece of metal to ground with). In my little circuit, where do I ground the thing?

Sorry for all the amateur questions.

You probably don't, it's a stand-alone gadget using batteries. Connect the sensor GND to the Xbee GND and the GND of anything else in your box including the batteries.

Cars use the chassis as a GND (earth return) because it's there and it halves the number of wires required. But in your case the PCB traces and/or other wires do the same job.

Rob

Ahh fair enough, there's the basics that I was missing.

Well I've got the sensor on order so I'll get to testing when it shows up, and then there will be more questions from there.

Thanks for the help thus far.