Arduino Uno + TMP102 sensor + AnalogWrite?

Other Hardware Sensors
1

Hi guys

I am trying to connect a TMP102 temperature sensor (see http://bildr.org/2011/01/tmp102-arduino/) to my Uno. However, instead of using the pin that says "3.3v", I am trying to use an analog pin through which I send 3.3v to the TMP102. When I do this, though, I keep getting bad data (negative values, even though it is hot where I live), or the serial output just freezes.

Any thoughts on what I may be doing wrong, and/or what is going on?

The wiring that I am using is exactly the same as the one shown above in the example form BILDR, except only for the V+ pin (which I am connecting to pin 11 [through which I am sending 3.3v], instead of the pin labeled "3.3v" in the Uno board). Note that when I keep all the same and move the V+ pin from the TMP102 to the pin labeled "3.3v" in the Uno, all works perfectly...

Thanks so much! :slight_smile: Here is the code that I am using:

#include <Wire.h> 
int tmp102 = 11;
byte res;
byte msb;
byte lsb;
int val;

void setup() {                
  Serial.begin(9600);
  Wire.begin();
  analogWrite(tmp102, 175);    // this will make pin 11 send 3.33v
}

void loop() {
    res = Wire.requestFrom(72,2); 
    if (res == 2) {
    msb = Wire.read(); // Whole degrees
    lsb = Wire.read(); // Fractional degrees
    val = ((msb) << 4);   // MSB
    val |= (lsb >> 4);    // LSB 
    Serial.print( "  Temp Celsius: ");
    Serial.println(val*0.0625);
  }
}
2 
analogWrite(tmp102, 175);    // this will make pin 11 send 3.33v

analogWrite() uses Pulse Width Modulation. It turns the output ON (+5V) for some portion of the time and OFF (0V) for some portion of the time. This is far from a steady 3.33V and probably explains the unexpected results.

You can get close to 3.33V by using a low-pass filter on the pulsed output to get an analog voltage with some ripple. I'm sure Google can help with that.

3

Hi John

thank you so much for your response. I now understand a little bit more about PWM and the effects on the 3.3v component I am trying to run.

I read several articles I found re. low pass filters (this one was informational - http://provideyourown.com/2011/analogwrite-convert-pwm-to-voltage/), but it gets very confusing very fast, especially as I looked at the online calculator ((Sample)RC Low-pass Filter Design for PWM - Result -).

So... in you & any one else's experience & knowledge, what values should I plug in the calculator to see what value of R and C I would need to connect the TMP102 using the analogWrite function?

Thanks!

4

Why do you not want to use the stable 3.3Volt pin?
It should give you lots more accurate readings.
If you decide to use a PWM to try to get 3.3Volts, then you need to know how much current is being drawn by the TMP102. Do you know?

5

Hey Jack

thank you for your response. I just learned that I can use the 3.3v pin (as you suggested) in conjunction with a digital pin & a transistor connected to it :slight_smile:

Did I mention that I love arduino & its awesome community? :wink:

Thanks again!!