(solved) Measuring voltage where should be none. UNO

Development Tools IDE 1.x
1

Hi,

I started testing my Arduino Uno with the temperature sensor tutorial vom ladyada Temperature sensor tutorial - Using the TMP36 / LM35. The Serial monitor showed me some strange values differing arround 2°C between 3 to 6 seconds.
So I decided to check whats wrong... I disconnected the UNO completly from the sensor an my serial monitor is still giving me values between 0 an 1024

Temp reading = 271 - 0.87 volts
37.33 degress C
99.20 degress F
Temp reading = 271 - 0.87 volts
37.33 degress C
99.20 degress F
Temp reading = 272 - 0.88 volts
37.66 degress C
99.78 degress F
Temp reading = 272 - 0.88 volts
37.66 degress C
99.78 degress F

thats an example what results I get when there are no wires at all connected to the arduino.

Whats the reason for that? And how to fix the Problem?

my Arduino software is running on a Win7 64-Bit machine.

Thanks for your Help - I am wondering how this can happen...

Greetings Voelsi

2

Insure the voltage on the A/D pin is zero. I hear they will float pretty good if not connected to anything.
A 2.2k resistor from that pin to ground will insure that.

Read it again.

3

Did you try using the 3V3 reference mentioned in the last part of the tutorial? It is less noisy and may improve your precision.

As for removing the senor, you will always read values. The INPUTs on the microcontroller are high impedance. Unless you pull the input high or low with a resistor you will read random values. While the pin is floating move your finger over the header's pins on the botton of the Arduino. The values will change by quite a bit.

4

Yes, I tried the 3V3 reference method. Thanks James.

@SurferTim: The A/D pin voltage is zero. I used a 1K resistor to pull the voltage down - works fine. I will purchase a 2.2K resistor and try it again. For the beginning the measurement is ok. Thanks to you!

5

A 1K resistor should have done it. Don't get a 2.2K.

6

Does your code and hardware match? In other words are you sure you connected to the correct pin?

7

A similar thing is happening with me.
I was trying to test a TSOP1738 IR Sensor by mapping out values on the Serial monitor. The analogout value is a constant at around 40 (which incidentally, doesn't respond to IR from remotes). Now if i remove everything, i get a value of around 350 consistently. Does this mean my sensor is gone?

8

A reading of 40 on the analog pin with the device connected would indicate the device output is LOW.
40 = .2v

That device does not supply a lot of current (1.5ma), so no resistors to GND or V+.

No other light sources?