BMP180 Voltage

by dennisyeh on Tue Feb 18, 2014 10:13 pm

Hello,

I recently bought a BMP180 Pressure Sensor, as well as an Arduino Due. Since the Due only operates at 3.3V, I wanted to make sure that I wouldn't fry it by hooking up a 5V SCL/SDA input. When I checked the voltage using a voltmeter, it seems that the SCL/SDA voltages are 4.93V for a 5V input but when I hook up VIN to the 3.3V pin on an Arduino Uno, the SCL/SDA voltages are 3.9V. Upon further investigation, the 3.3V pin seems to deliver 3.3V when not plugged into the BMP180, but when I connect the wires to the breakout board, my volt meter registers a voltage fluctuating between 3.45V and 3.65V.

Will it be okay to use the BMP180 on an Arduino Due, if the SCL/SDA voltages are at 3.9V and not 3.3?

Also, why would the input voltage increase when the board is connected? If I had to guess, I would think that there is some sort of back EMF from the voltage regulator or some other component on the breakout board. However, I find it strange that this only happens for a 3.3V input and not a 5V input.

Thanks,
Dennis
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Re: BMP180 Voltage

by adafruit_support_mike on Tue Feb 18, 2014 11:56 pm

I'm afraid I can't follow the connections as you've described them.

The BMP180 is a 3.3v chip. We've added a 3.3v regulator and a level shifter to the breakout to make it work with 5v devices. The breakout's 3Vo pin is connected to the output of the 3.3v regulator. It can deliver up to 100mA of current at 3.3v to some other circuit that needs 3.3v power. If you don't need to power other devices, you don't need a connection to the 3Vo pin at all. Just connect the supply power (3.3v or 5v) to Vin.

If you're using a Due and feeding the BMP180 3.3v power, you shouldn't see any voltage over 3.3v.
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Re: BMP180 Voltage

by dennisyeh on Wed Feb 19, 2014 1:34 am

Thanks for the quick reply Mike,

I haven't hooked the breakout board to the Due yet - I'm currently using an Arduino Uno to see what the voltages are.

3.3V on the Uno is hooked up to VIN on the BMP180,
GND on the Uno is hooked up to GND on the BMP180,
Nothing is hooked up to 3Vo on the BMP180,
A5 on the Uno is hooked up to SCL on the BMP180, and
A4 on the Uno is hooked up to SDA on the BMP180.

With the example code running, and Serial Monitor open, the voltage between VIN and GND fluctuates between 3.45V and 3.65V.
Also, the voltage between SCL and GND fluctuates between 3.75V and 3.9V.
The voltage between SDA and GND fluctuate between 3.75V and 3.9V as well.

Then, when I disconnect A5 and A4 from SCL and SDA, both read at a steady 3.24V and the voltage between VIN and GND is a steady 3.3V.

I'm not sure if the higher voltage is because I'm using an Arduino Uno with 5V I/O pins, will it be safe to hook up the breakout board to the Due if I'm only feeding the BMP180 3.3V?
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Re: BMP180 Voltage

by adafruit_support_mike on Wed Feb 19, 2014 11:48 pm

The Arduino Wire library (which handles I2C communication) has a bad habit of enabling the pull-up resistors on pins A4 and A5 by default. Technically it's a convenience because you don't have to use external pull-ups when you connect the Arduino to an I2C device. Unfortunately it means you have a current path to the Arduino's 5v rail, which is bad for 3.3v I2C devices.

The BMP180 breakout can survive that connection, but you don't want to do that with just any 3.3v I2C device.

The voltages higher than 3.3v that you saw are entirely the result of connecting the BMP180 to the Arduino's 5v pull-ups. The Due, being a 3.3v device, won't cause any problems.
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Re: BMP180 Voltage

by dennisyeh on Thu Feb 20, 2014 2:00 pm

Awesome, thank you very much for the clear explanation, Mike :D!
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