3 replies to this topic

[donnmo]

I've attached an LM35 to both a Netduino and a Netduino Plus. The readings fluctuate unexpectedly on both boards.

 

The attached screenshot shows three columns of data: The first is the value retrieved from the AnalogInput.Reading method; the second is the temperature in degrees Celsius; the third is the temperature in degrees Fahrenheit.

 

These readings were taken when the LM35 was sitting on my desk in a room that's showing 75 degrees Fahrenheit on a digital thermometer in the same room. (The readings are within an acceptable range--but--I don't understand the fluctuations.)

 

Has anyone else observed this with the LM35?

 

Thanks!

--Donn

Attached Files

•  lm35_output.PNG   32.95KB   2 downloads

[stotech]

I've never tried an LM35 but I've had similar fun time with a TMP36.

 

Assuming they work on a similar principle, what your actually seeing is fluctuations in the input or reference voltage. This could be caused by any number of things in your hardware setup. Like what type of power supply you are using.

 

My first guess would be to try decoupling the power fluctuations using small ceramic capacitors. If you had access to a scope you could check just how steady that voltage is at various places. Another thing you can do is take a series of readings and work out the average to take out any freak line fluctuations. I've got a little code i use for getting readings randomly off my analogue pins. I know it's not perfect and it's not fast but it works for me.

class MyAnalogPort    {        private static object Analog_Lock = new Object();        public static int Read(int port, double ratio = 1023, bool invert = true, int sample_size = 5, bool dispose = true)        {            lock (Analog_Lock)            {                SecretLabs.NETMF.Hardware.AnalogInput sample_port = new SecretLabs.NETMF.Hardware.AnalogInput(Pins.GPIO_PIN_A0);                if (port == 1) { sample_port.Dispose(); sample_port = new SecretLabs.NETMF.Hardware.AnalogInput(Pins.GPIO_PIN_A1); }                if (port == 2) { sample_port.Dispose(); sample_port = new SecretLabs.NETMF.Hardware.AnalogInput(Pins.GPIO_PIN_A2); }                if (port == 3) { sample_port.Dispose(); sample_port = new SecretLabs.NETMF.Hardware.AnalogInput(Pins.GPIO_PIN_A3); }                if (port == 4) { sample_port.Dispose(); sample_port = new SecretLabs.NETMF.Hardware.AnalogInput(Pins.GPIO_PIN_A4); }                if (port == 5) { sample_port.Dispose(); sample_port = new SecretLabs.NETMF.Hardware.AnalogInput(Pins.GPIO_PIN_A5); }                int sample = 0;                int[] read = new int[sample_size];                for (int Counter = 0; Counter < sample_size; ++Counter)                {                    read[Counter] = sample_port.Read();                    Thread.Sleep(Counter);                }                sample = Convert.ToInt32(AverageElements(read).ToString());                double input = (ratio / 1023) * sample;                if (invert) { input = ratio - input; }                sample = (int)System.Math.Round(input);                sample_port.Dispose();                return sample;            }        }    }public static double AverageElements(int[] arr)        {            lock (Calc_Lock)            {                int sum = 0;                for (int Counter = 0; Counter < arr.Length; Counter++)                {                    sum += arr[Counter];                }                return sum / arr.Length; ;            }        }

It takes 5 readings by default and not at the same time distance apart. Just in case there is a set frequency of interference.

 

Hope that helps.

 

Grant.

[donnmo]

Grant: Many thanks for the reply. (Appreciate it!)

 

I'll probably implement the software "smoothing" -- similar to what you'd done. (I'd noticed other postings since my original post and it appears that the issue is fairly common and the workaround is to take an average of the readings.)

 

--Donn

[dcimma]

Not sure if you solved your problem, but the LM35 can oscillate pretty bad sometimes.  I have a few of them on 30 ft runs around my shop monitoring solar system temps.  Look at the LM35 data sheet, there are a few suggestions like a bypass cap and series resistor if you have long runs and high capacitance.  I ended up using a 10uf cap across the output to solve my erratic readings problem.