Accelerometer built in voltage divider?

Hey guys, I recently got my 3 axis accelerometer, the MMA7361 from sparkfun (http://www.sparkfun.com/products/9652). Problem is that i’ve been using the arduino uno’s digital pins to turn off the sleep. (by setting the pin to HIGH) but i’m afraid i’m going to fry my board cause it only takes max 3.6 volts.

That is a concern. I would use a resistor in series and a 3.3V zener diode from the sleep pin to ground (the resistor sits between the zener diode’s cathode and the Arduino pin). A resistor value of 1k-10k is reasonable I think.


The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

That's what level changers are for... http://www.sparkfun.com/products/8745

Last time I went to the creatroninc I got a half dozen as they are a Canadian re-seller of Sparkfun stuff...

Thanks for the reply!

@RuggedCircuits ; I didn't have any zener diodes around but I added a 10k resistor from the sleep to the digital pin. The results from serial monitor are the same but somehow a bit more responsive. Is that what it's suppose to do. The reading will be more accurate with a lower voltage pin?

@WillR; Thanks for giving me the link. It's cheap and small cause i'm using a protoshield. The problem is I don't know where to put the inputs and the outputs to change the logic level.

Look for recent threads on the BMA180…

But all I did with that unit is wire the 3.3 v to inputs I wanted to stay high. If you want to control them it should be obvious where to put the leads… One side has a HI voltage the other side has a LO voltage.

If you are only sending to the accelerometer you can use the voltage divider called a receiver… (RXI) Put the Arduino output on the HI lead of RXI and the Accelerometer side on the LO side of RXI. Or you can look up the schematic and and get two resistors of CLOSE to the same value or the same RATIO. Don’t forget to hook up the GND to both sides and the 5V and the 3.3 V to the appropriate leads if you use the level changer.

The TXD is for a lead that goes both ways – like SDA and SCL if you use the BMA 180 – a much better accelerometer.

I use a level changer because I have the BMP085 (Pressure/Temp) and the BMA180 feeding off the 3.3 V output leads of the level changer (same one). On the other side of the level changer I have a DS1307 clock – feeding off the same signal – but at a 5V level…

I didn't have any zener diodes around but I added a 10k resistor from the sleep to the digital pin. The results from serial monitor are the same but somehow a bit more responsive. Is that what it's suppose to do. The reading will be more accurate with a lower voltage pin?

It's possible that without the resistor you were stressing the part and it was responding appropriately. The 10k resistor will limit the overcurrent due to the overvoltage (5V>3.3V) so that may be helping. As mentioned, instead of using a 5V zener you could also use a resistor divider to knock down the 5V to 3.3V on this pin (say, 3.3k and 4.7k).

-- The Aussie Shield: breakout all 28 pins to quick-connect terminals