Accurately measuring temperature with Arduino

Question

I'm trying to build thermostat with Arduino. I want to power it using mobile phone battery/charger which makes system voltage quite variable. Right now I use Arduino Uno, but once it is complete I will port it to Lilypad.

First I tried to use TMP36 temperature sensor. So far it was complete failure. While the sensor itself appears to be very stable, I can't figure a way to accurately measure its voltage.

Using built-in 5v reference for analog sensors isn't working at all -- even powered from USB arduino's +5V are actually +4.8V (which shifts measured temperature by few degrees). When the board is powered from the battery, voltage drops to about 4V and measured temperature sky-rockets. I also tried to use +3.3V from the board as a reference. It seems to be more stable when the board is powered from USB, but its voltage drops when running off the battery.

Is there any other way to reliably measure sensor output voltage?

For the second stage I'm planning to use thermistors. Just ordered a couple of these 20K thermistors.

From what I understand, these should be easier to measure accurately if I build voltage divider and use V_in as reference voltage for ADC.

A couple of questions about them:

• Does it make sense to use few voltage dividers with different fixed resistor to increase accuracy?
• I can use programmable pin as V_in, and measure temperature using few different voltage levels. Though its not clear to me whether this will actually increase accuracy.

Answer 1

It seems you are aware of the problem with the reference voltage changing and if you use a device like the TMP36 (fixed 10mV/degC) there is nothing you can do other than use a voltage reference from a chip to stabilize things.

However, if you are using an RTD or a thermistor then the problem won't arise. You ADC is making a ratiometric measurement - it compares the ADC input to its reference voltage BUT, if you power the RTD or thermistor (via a suitable resistor) from the same ref voltage it won't affect readings. If the ref goes up 10% then so does the voltage into the ADC.

Answer 2

I think you should consider using a digital temperature sensor like DS18B20/ DS18S20 because it does not depend on the accuracy of your ATmega ADC to measure an anaolg signal, it uses 1-wire digital protocol to report temperature.

Refer to the following tutorials
http://playground.arduino.cc/Learning/OneWire
http://www.hobbytronics.co.uk/ds18b20-arduino

Answer 3

Your measurement will be as good as reference voltage for ADC is good. Arduino by default uses supply voltage as reference voltage, but in your case the right way to do it would be to use Aref pin of arduino. You have to get a special chip called "Voltage reference" and connect it to Aref pin, then set ADC to use the external reference in Arduino code (analogReference(EXTERNAL))

The voltage of the reference has to be selected so that the full swing of your temperature sensor will fit into referece voltage. TMP36 will output ~1.5V @ 100C, so you would have to use reference above 1.5V for measuring temperature up to 100C. You want your reference as close to the maximum voltage measured as possible to get as much resolution as possible.

Atmega328p has two internal references that can be used without any external components. One is 1.1V, another 2.56V. They usually are a bit worse accuracy than what you would get by using external dedicated component. Check Arduino documentation for analogReference and Atmega328p datasheet for internal reference accuracy.

If you really want to get nuts with different ranges, you can use several external references and switch them by using an analog switch like 74hc4051. Or you can switch between two internal references.

With thermistors you will have better results if you set up a constant current source instead using a dumb resistor. On another hand - a dumb resistor powered from a stable voltage reference would work ok.

When choosing an external reference, make sure you have enough voltage to accomodate for it's drop out voltage when powering from batteries and the batteries are flat. Vref+Vdropout < Vbat-min.

Answer 4

Not having a stable ADC reference is actually a symptom of a another problem in your circuit: you are not supplying a high enough voltage to the board. This is indicated by the 5V supply dropping to 4V and the 3.3V dropping as well.

The voltage regulator (MC33269D-5.0 IIRC) on the Arduino board has a dropout voltage of ~1.0V, therefore you need to supply it with at least 6V to get a stable 5V output. AA batteries start off at 1.5-1.6V and are almost dead at 1.1V so you must power the board with at least 6 AA batteries for a stable output over the entire battery life.

Powered correctly, you may either use the internal ADC reference or either the 5V or 3.3V lines. Since the temperature sensor varies by around 10mV per degree Celcius, you could use a voltage divider to set the reference voltage equivalent to the maximum expected sensor output voltage (e.g. for 50 degree C). This will give a more precise measurement.

If you want to use fewer than 6 AA batteries, try a DC-DC boost converter, e.g. https://www.sparkfun.com/products/10968. The linked example takes 1V - 4V and makes 5V. The output would be fed directly into the 5V pin of the Arduino, bypassing its regulator.

To get the board to run longer on batteries, put the MCU to sleep between sensor reads. The rocketscream low power library is great for this purpose. But it is only really useful when using an efficient regulator / DC-DC converter as the standard Arduino regulator uses 10mA just by itself!

Answer 5

Answer for the question Is there any other way to reliably measure sensor output voltage?

ADC uses the reference voltage for the conversion from analog to digital. So if there is a change in the reference voltage, the converted values(i.e digital value) will change. The digital value will be different for the same analog input if the reference voltage changes.

One easy option is to use the internal reference voltage inside the Arduino(i.e Atmega controller).

See the below link where a sample code is provided to use the internal ADC (Arduino function name - analogReference(DEFAULT) )

http://tronixstuff.com/2013/12/12/arduino-tutorials-chapter-22-aref-pin/

I think this will solve your issue and there no need to switch towards thermistors.