XBee Voltage Help

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danlag
 
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XBee Voltage Help

Post by danlag »

Hello,

I'm working on a project that requires a pressure sensor be made wireless. The sensor is powered by 2 Li-ion batteries producing (7.4.volts). The internal electronics regulates that down to a working voltage of (5.0 volts).

The sensor has an analog output with a range of (0.0 - 5.0 volts) depending on pressure level.

Ultimately, I want the XBee inside the sensor to "send" the voltage readings to an XBee/Arduino hooked up to a laptop, converting the voltage readings back into pressure values and displaying them on the screen.

My questions are..

What is the maximum analog input voltage for an XBee? Does the voltage powering the XBee need to match the analog voltage coming in?
If I am limited to 3.3 volts, how would I best tackle this project?

Thank you for any help you have to offer..

Dan

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Franklin97355
 
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Re: XBee Voltage Help

Post by Franklin97355 »

If I am limited to 3.3 volts, how would I best tackle this project?
Try a voltage divider to scale the voltage to 0-3.3

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adafruit_support_bill
 
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Re: XBee Voltage Help

Post by adafruit_support_bill »

The absolute max to avoid damage is VCC (3.3v). But the max measurement range is determined by the voltage on the VREF pin. This is tied to 3.3v if you are using our adapter. Otherwise, be sure to connect that pin in your circuit.

waltr
 
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Re: XBee Voltage Help

Post by waltr »

Also, go to Digi Int's web site and download the documents for the XBee module you have. Ensure you get the correct manual for either the Series 1 or series 2 XBees since they are different. The series 1 document number is 90000982 and the series 2 doc number is 9000976. These docs have the full specs and all the commands require to configure an XBee.
It would be nice if Adafruit had a link to these documents on the product page.

Depending on which XBee series you have depends on whether you need to supply Vref to the XBee pin. The series 1 modules have Vdd on Vref while the Series 2 modules do not and the user must supply a Voltage to this pin. See the 'Pin Signals' table in the document.

danlag
 
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Joined: Wed Sep 04, 2013 9:29 pm

Re: XBee Voltage Help

Post by danlag »

Thank you very much for the information.

I'm going ahead with putting a voltage divider into the variable voltage circuit as well as the XBee power input. The other option would be to use a 3.3v voltage regulator to feed the XBee.

Thanks again for your help.

waltr
 
Posts: 306
Joined: Wed Jun 12, 2013 5:01 pm

Re: XBee Voltage Help

Post by waltr »

Powering the XBee with a resistive divider is not a good idea. The current drawn varies from receiving to transmitting. A Voltage regulator is much better to power an XBee.
A resistive divider between the 5V sensor output and the XBee's analog input will work if the resistor values meet the analog (ADC) input impedance spec (Ras in table 1-05 of the series 1 XBee doc). This input impedance is 10k Ohm. This means that the Thevin equivalence is not to exceed 10k Ohm so the parallel equivalence of the two divider resistors is not to exceed 10k Ohm.
http://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem

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