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Topic: Voltages and the adxl/scp1000 examples (Read 975 times) previous topic - next topic

tbabb

Hi,

What's up with the examples involving the adxl33x accelerometer and the scp1000 pressure sensor? The diagrams show their i/o pins plugged directly into what appears to be a duemilanove (5V), but those devices' specs say they only accept up to 3.3v. Wouldn't this damage the components?

Before I checked more thoroughly, I tried the diagrammed setup with the ADXL. It worked, but the reading was extremely noisy. After reducing the supply voltage, my results got much better. Should the examples be changed? It seems like this is a bad thing to teach beginners (like me) to do.

-t

mpeuser

#1
Aug 25, 2010, 04:57 pm Last Edit: Aug 25, 2010, 04:58 pm by mpeuser Reason: 1
Difficult to know about which of the many versions and variants of documentation and products you are talking (SPI? I2C?)

Nevertheless it is important to understand how to connect 5 and 3.3 volt devices properly. There is a most excellent and recommendable article from SparkFun here - I myself couldn't hardly do it better ;-)

http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=65

tbabb

Quote
Difficult to know about which of the many versions and variants of documentation and products you are talking (SPI? I2C?)


Here:
http://arduino.cc/en/Tutorial/BarometricPressureSensor
and here:
http://arduino.cc/en/Tutorial/ADXL3xx

mpeuser

#3
Aug 26, 2010, 08:41 am Last Edit: Aug 26, 2010, 08:42 am by mpeuser Reason: 1
I see.
There is a host of ADXs but I think most will work with any voltage between 3 and 5 volts, so this is just fine.

The BrometricSensorTutorial is not so good. It would be better to use serial resistors as explained in the SparkFun article I linked to above.

The reason why all those designs work and people are unaware of hidden issues is, because only short pulses (of 5V) will be on the lines. This is "stress" - so to speak - for the 3 volt device, but not necessarily fatal...

In any case it must be avoided to wire a constant current of >200uA of 5 volts to a 3 volt device. 22k to 47k resistors will provide ample protection

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