Different result if connecting 9V?

To learn, you need to read the datasheet on the sensor and think what changes with the switch from USB to 9 volt battery.

Clem

Hi Jose,

Where are you attaching the 9 volts?

John

Per John's comment, I'm assuming you're using a 9V barrel adapter.

A couple of points:

1.     You have to get enough power for the Arduino, the LED, the TMP36, etc.  I never use carbon-zinc batteries or cells (usually labeled "Heavy-Duty").  I prefer Lithium batteries or cells as they can output high current, when needed; but alkaline batteries/cells should be OK.  Check the battery: not just the voltage, but how much current it can output.  Preferably use a new battery.

2.     The type of problem you're seeing is almost always due to insufficient power.

3.     Since it's working properly with USB power (original USB 2.0 specifies 5V at 500mA) that should tell you what you need to replicate for total max power.

Hi Brian,

I have also found that even with batteries it is usually a good idea to have a decoupling capacitor across the power feed.

John

Hi Jose,

Your hookup looks good. As Brian has pointed out there is no reason why it will not work if the power source is solid and hooked to the DC input jack or to the Ground and Vin terminals of the Arduino header. I would also put a 0.1 uF cap across the 9 Volts near the Arduino. That book by Jeremy is excellent and I had a lot of fun working through it a while back.

John

Thank you for the answer. I have checked with another board with a LCD and a DTH 11 sensor and the difference is 2 degrees centigrade between USB and 9v. What I can't understand is why. I thought the board provided 5v when connected to either 9V or USB.  I have checked the battery and it is providing 6.54v.

Sensors are very dependant on the voltage provided. Change the voltage, they need to be recalibrated. Try a separate independent power source on the rail to the sensor that is 5V. repeat the experiment and report back the results.

Clem

I think you'll find that the 5v line is less than 5v while being powered from USB.

Clem is correct that it will need recalibrating.

I tend to always use a Dallas One Wire DS18B20 (or similar) to avoid this.

Mark

Do you have a reasonably accurate DMM - if so try measuring some voltages around you circuit and you will be able to work out what is changing and affecting the result - it might then be simple to deal with the problem. If you post the circuit you have I can suggest places to measure.

MK

The TMP36 output is 10mV per degree C and is designed to be 750mV at 25C.

The sensor is specced for -40 to +125, which means a voltage range at the output of 0.1V to 1.75V. That range is designed to allow operation off of a low voltage supply like 3.3V or less, but it does mean that in this case you are only using about a third of the A/Ds range.

I'm not really familiar with the Uno, but it looks like the A/D is 10-bit and is operating ratiometrically [taking the supply as its reference voltage]. Let's try calculating the reading for 25 degrees C (ie an input of 0.75V) for two different supply voltages [I've just arbitrarily chosen plus or minus 4% from a nominal 5V]

supply 5.2V, reading 148  (0.75 / 5.2V) * 1024
supply 4.8V, reading 160  (0.75 / 4.8V) * 1024

Each step from the A/D is equivalent to approximately 1/2 a degree, so that [quite modest] change in the supply constitutes about 6 degrees C in the calculated temperature. Since you are seeing about 2 degrees, it means your supply change is less than that.

Point is, the design is very sensitive to the supply voltage.

If you want it to work accurately regardless of the supply voltage then there are several options:

1) use the internal 1.1V reference of the ATMEGA328. That will be accurate and won't vary with the supply voltage. Limits you to 60 degrees C if you connect the sensor directly. You could get the full range if you use a potential divider on the sensor output.

2) provide your own reference and set the processor to use that rather than the supply. A 2.5V reference into AREF would be appropriate. [It might be that the Uno software would conflict with having an external reference, so have a look around and see if this is generally done or not. If you are still not sure, put a resistor between the reference output and the AREF pin so that you don't have two outputs driving each other directly and look at the voltages either end to see if there is any difference before your own code runs and sets up the processor to use the external reference.]

3) A final alternative would be to amplify the output from the sensor so that it makes better use of the A/D's input range. The effect of any supply change would then decrease in proportion. So if you amplified it 3 times, the effect from the supply change would be a third. If you don't need the full -40 to +125 degree range, you could amplify it even more with just the range you were interested in giving a full 0 to +5V at the A/D pin.

All would require you to change the way the software calculates the temperature from the result- so only try it if you are confident programming, though I'm sure you'd get a lot of help from people here if you wanted to give it a go.

And the TMP36 could do with a decoupling capacitor across its supply pins.