Saturday, March 14, 2015

Battery Operated Temperature Sensor - Still learning about the TMP36

The previous entry on this project is here <link>

After I found out about the open emitter problem in reading a TMP36 <link> and tried it out for a longer period of time I noticed there were still some glitches in the readings. Don't misunderstand, these were tiny and represented no real problem, but I wanted to settle it down as much as possible.

Yes, obsessive, compulsive about this I am.

I recorded a ton of readings from the chip and looked them over. The problem seems to come from simple outlier readings that happen from time to time.  Every sensor has outlier readings, it's just part of reality, and there are methods of fixing this problem. Naturally, I took the easy way out. I simply read the sensor 15 times in a row into an array, sort them, and average the middle five. This way the outliers wind up at the bottom or top of an array and get excluded from the sort. Here's the code:

#define READSIZE 15

float readTemp2(){
  int readings[READSIZE];
  int reading=0;
  for (int i = 0; i < READSIZE; i++){
    readings[i] = analogRead(tmpInput);
  // Now sort the list to put the outliers at the beginning and end
  sort(readings, READSIZE);
  // grab the middle 5 and average them
  for (int i = READSIZE / 2 - 2; i < READSIZE / 2 + 3 ; i++){
    reading +=(readings[i]);
  reading /= 5;
  //reading = analogRead(tmpInput);
  float voltage =  (reading * 1.1) / 1024;
  float tempC = (voltage - 0.5) * 100;
  float tempF = (tempC * 9.0 / 5.0) + 32.0;

void sort(int a[], int size) {
  for(int i=0; i<(size-1); i++) {
    for(int o=0; o<(size-(i+1)); o++) {
      if(a[o] > a[o+1]) {
        int t = a[o];
        a[o] = a[o+1];
        a[o+1] = t;

This smoothed the reading right out and made for a very nice graph. In the fragment of readings charted below you can see how well this works:

This little piece of data is when I had the sensor outside, brought it in the warmer house, then took it back outside. Notice the shape of the curve? Classic ramp up and back down similar to the response shape of an inductor. This is the thermal resistance of the plastic case on the sensor, wires, etc, and is a far cry from the erratic reading I used to get.

The TMP36 turn out to be a really nice little device when used correctly with my new filtering. I'll eventually look at removing the power from the sensor when I put the Arduino to sleep, but I don't expect much of an increase in battery life since the quiescent current of the sensor is claimed at 50uA. Every little bit helps since I plan on running these things forever.

The next entry on this project is here <link>


  1. Just in case your array filled with garbage, you could add a check at the end to see if it's less than say 100C, greater than -5, and return the previous value if not. I've managed to send 1000C to Wunderground before now.

    1. Ha, good point. That would be so annoying.