Rebuild of my broken 433mhz Cresta/hideki UV-Sensor

  • Hello,

    I just started with arduino and mysensors and I would like to share my first steps (re)building my broken uv-sensor.
    I bought the orignal TS704 UV sensor about two years ago. A few weeks ago it stopped sending acurate UV readings. On lots of placesa on the internet there are complaints abouts this sensor and the relative short lifetime. Main reason I think is water leakage in the sensor. I didn't want to buy a new one because the problem would be back within 2 years and they are realy expensive at the moment (about €70).
    Then I found the mysensors site in my quest to fix my broken device and my first sensor project was born!
    I ordered the pieces I needed and started to build.
    First the gateway, connected to domoticz which was easy as I connected it directly to the raspberry pi using the pigatewayserial software. Antenna directly connected to the Pi so no Arduino board needed!.
    Changed tty TTY_NAME := /dev/ttyMySensorsGateway to tty
    TTY_NAME := /dev/ttyUSB30 in the makefile because Domoticz cannot use ttyMySensorsGateway.
    After that It was easy to add the serialgateway to Domoticz!


    my list:
    Arduino pro mini 8mb 3v
    UVM-30a UV sensor
    NRF24L01+ 2.4GHz Wireless Transceiver
    TS704 UV sensor housing including 2xAA batery compartment
    Used the default sketch with pin A0 instead of pin 3 (

    On my breadboard I had things up and running quite easy but the readings were strange. Even at night there was UV light measured. The problem was a mismatch between the intructions on this website about the out pin to use and the one mentioned in the sketch. Sketch showed pin A0 as OUT but website showed pin 3 as OUT.
    @hek has confirmed that A0 is the correct pin. Website will be updated. (is updated now)
    I was mislead because with nothing connected to pin A0 it still gives you output data.
    It's also possible to use 3V for the UVM-30A instead of only 5volt I found this in the spec sheet.

    So the stepup from 3 to 5v is not needed I think?

    To get things in the housing I had to desolder the main piece from the UV sensor. Things have gone wrong from here because after that the sensor did not work anymore. Probably fried the sensor?! New one is on its way.
    So I must think of another way to get the sensor in place at the top of the housing. There not enough place so I must be creative and not destroying another sensor!

    I also would like to share a short piece of code I found to test the uv sensors output. This way you don't need to attach all the radio wires to see if the sensor is working.


    This Sample code is for testing the UV Sensor .

    OUT-Analog pin 0

    void setup()
    Serial.begin(9600);// open serial port, set the baud rate to 9600 bps
    void loop()
    int sensorValue;
    sensorValue = analogRead(0);//connect UV sensors to Analog 0
    Serial.println(sensorValue);//print the value to serial

    The pieces on the table:

    New sensor arrived and bought a 15W soldering iron.

    Puting things together and make some room for the sensor board




    After everthing was put in place the sensor went outside for testing.
    For reference I also used a nearby UV sensor via Weather Underground.


    At first everything looked fine but it seems that the measurements from my sensor are to low.
    I removed the sensor and did some testing with the above sketch to measure the output voltage on 3.3v and 5v but the results were the samen and to low.
    Does anyone know what can cause this low measurement ? checked the wires. checked the sketch. replaced the arduino board.
    Another broken sensor?

  • @edweather

    Great post thanks for sharing!

    I have also two of these which are not functioning anymore. Did you find a solution to the low values after all?

  • Hardware Contributor

    Nice one! Good to have in the summe here when the kids are outside... I will build one as well.
    Thank you for sharing!

  • Hero Member

    I had plans to "MySensor" my Oregon scientific UV Index sensor when it fails ( It seems like it us quite common that they fail within 2 years ) but the darn thing just keeps working :sunglasses: been working for 5 years now...

  • Hello @Mediacj

    No I did not find a cause or sollution for the problem. After three broken sensors I gave up.
    I will give it a try next summer!


  • I dont think it's a Sensor issue, rather a sketch issue....
    I bought 4 UV sensors, and also experiencing this.
    It could be that we need to multiply the value we send by 10 ? Or we need to do some magic after the analogRead call

    From what i understand the analogRead returns a value between 0-1023

    1023 stands for the maximum voltage that can be read on this pin, and equals VREF
    [Source](5 volts (on 5V Arduino boards) or 3.3 volts (on 3.3V Arduino boards))
    So if we use 3.3V on the arduino, then 1023 stands for 3.3V

    We go back from the digital value (0-1023) to voltage with:

    int sensorValue = analogRead(A0);
    float voltage= sensorValue * (3.3 / 1023.0);

    The above sketch is using mV, so we need to multiple the voltage by 1000

    float voltage= sensorValue * (3.3 / 1023.0) * 1000.0;

  • Hardware Contributor

    How is the UV sensor / electronic best protected from rain and weather?
    Some sort of plastic lid? Or this will remove UV rays?

    I guess you want to point the sensor straight up where the rain comes from :)

  • I use a plastic case like:

    I now use the following code to read the uv value:

    #define         READ_SAMPLE_INTERVAL         (50)    //define how many samples you are going to take in normal operation
    #define         READ_SAMPLE_TIMES            (5)     //define the time interal(in milisecond) between each samples in  
    #define         VREF 5.0f
    float last_UVIndex = 0;
    uint16_t uvIndexValue [12] = { 50, 227, 318, 408, 503, 606, 696, 795, 881, 976, 1079, 1170}; 
    float UVRead()
      int i;
      float rs=0;
      for (i=0;i<READ_SAMPLE_TIMES;i++) {
        rs += analogRead(UV_PIN);
      rs = rs/READ_SAMPLE_TIMES;
      return rs;  
    float GetUVIndexFromVoltage(float voltage)
      float UV=0;
      int i;
      for (i = 0; i < 12; i++)
        if (voltage <= uvIndexValue[i]) 
          UV = float(i);
      //calculate 1 decimal if possible
      if (i>0) {
        float vRange=float(uvIndexValue[i]-uvIndexValue[i-1]);
        float vCalc=voltage-uvIndexValue[i-1];
      return UV; 
    void sendUVIRMeasurements(bool force)
      bool tx=force;
      float sensorValue = UVRead();
      float voltage= sensorValue * (VREF / 1023.0f) * 1000.0f; //mV
      if (voltage>1170)
      //Serial.print("UV Analog reading: ");
      float UVIndex=GetUVIndexFromVoltage(voltage);
      float diffUV = abs(last_UVIndex - UVIndex);
    #ifdef MY_DEBUG
      Serial.print(F("diffUV :"));Serial.println(diffUV);
      if (diffUV > 0.1) tx = true;
      if (!tx)
      Serial.print("UVI: ");
      last_UVIndex = UVIndex;
      Serial.print("UV: ");
      Serial.println(UVIndex, 2);
    //  send(msgUV.set(UVIndex,2));

  • @GizMoCuz Nice to see that my old post suddenly comes to live! Thanks for sharing your sketch. It was one of my first builds so it could be the sketch but I also had strange readings using the sample code.
    Be cautious with the box you're using. Screws get rusty quick and water leakage will occur!
    I have ordered a GYML8511-Uv-sensor-module to try but I will try the old sensor first with your script!

  • Hardware Contributor

    @GizMoCuz - Thanks for your box link. I have ordered one!
    @edweather - Ill post some pictures when im done to keep it alive here!

  • Hardware Contributor

    My UV sensors is complete!
    (I have not recieved any case yet - so using a electronic box and a plastic bag :+1: )


    Its a dual UV and light sensor (LM393 Light Sensor), running om my EasyPCB and 2xAA batteries.

    First I had some problems with power consumption about 1500uA. This was caused by the light sensor and i removed all leds and what I could with the digital part. My sollution was to power the light sensor from D3 and put it high and then low before sleep. Now im down to 60uA. Also had some interferance with my booster and that worked out great by adding a 0.1cap between out and gnd.

    Having some issues with measuring batteries and analogread. There is a correct reading on the input on the atmega328 pin (about 0.9v) but still get -80% when using analogread. Must be a faulty atmega chip.

    My code: (Combined of @GizMoCuz and MySensors)

    Code removed - please use code below

  • Trying to get the GYML8511 sensor working with the example code from sparkfun.
    Must be me again but the thing gives me strange outputs.

    utput: 1023ML8511 output: 455 / ML8511 voltage: 1.47 / UV Intensity (mW/cm^2): 3.96
    output: 1023ML8511 output: 688 / ML8511 voltage: 2.22 / UV Intensity (mW/cm^2): 10.19
    output: 1023ML8511 output: 1023 / ML8511 voltage: 3.30 / UV Intensity (mW/cm^2): 19.14
    output: 1023ML8511 output: 546 / ML8511 voltage: 1.76 / UV Intensity (mW/cm^2): 6.39
    output: 1023ML8511 output: 568 / ML8511 voltage: 1.83 / UV Intensity (mW/cm^2): 6.98
    output: 1023ML8511 output: 969 / ML8511 voltage: 3.13 / UV Intensity (mW/cm^2): 17.70
    output: 1023ML8511 output: 842 / ML8511 voltage: 2.72 / UV Intensity (mW/cm^2): 14.30
    output: 1023ML8511 output: 488 / ML8511 voltage: 1.57 / UV Intensity (mW/cm^2): 4.84
    output: 1023ML8511 output: 748 / ML8511 voltage: 2.41 / UV Intensity (mW/cm^2): 11.79
    output: 1023ML8511 output: 1017 / ML8511 voltage: 3.28 / UV Intensity (mW/cm^2): 18.98
    output: 1023ML8511 output: 500 / ML8511 voltage: 1.61 / UV Intensity (mW/cm^2): 5.16
    output: 1023ML8511 output: 629 / ML8511 voltage: 2.03 / UV Intensity (mW/cm^2): 8.61
    output: 1023ML8511 output: 1017 / ML8511 voltage: 3.28 / UV Intensity (mW/cm^2): 18.98
    output: 1023ML8511 output: 645 / ML8511 voltage: 2.08 / UV Intensity (mW/cm^2): 9.04
    output: 1023ML8511 output: 502 / ML8511 voltage: 1.62 / UV Intensity (mW/cm^2): 5.22
    output: 1023ML8511 output: 936 / ML8511 voltage: 3.02 / UV Intensity (mW/cm^2): 16.82
    output: 1023ML8511 output: 950 / ML8511 voltage: 3.06 / UV Intensity (mW/cm^2): 17.19
    output: 1023ML8511 output: 448 / ML8511 voltage: 1.45 / UV Intensity (mW/cm^2): 3.77
    output: 1023ML8511 output: 711 / ML8511 voltage: 2.29 / UV Intensity (mW/cm^2): 10.80
    output: 1023ML8511 output: 1023 / ML8511 voltage: 3.30 / UV Intensity (mW/cm^2): 19.14
    output: 1023ML8511 output: 499 / ML8511 voltage: 1.61 / UV Intensity (mW/cm^2): 5.14
    output: 1023ML8511 output: 607 / ML8511 voltage: 1.96 / UV Intensity (mW/cm^2): 8.02
    output: 1023ML8511 output: 1014 / ML8511 voltage: 3.27 / UV Intensity (mW/cm^2): 18.90
    output: 1023ML8511 output: 639 / ML8511 voltage: 2.06 / UV Intensity (mW/cm^2): 8.88
    output: 1023ML8511 output: 541 / ML8511 voltage: 1.75 / UV Intensity (mW/cm^2): 6.26
    output: 1023ML8511 output: 888 / ML8511 voltage: 2.86 / UV Intensity (mW/cm^2): 15.53
    output: 1023ML8511 output: 903 / ML8511 voltage: 2.91 / UV Intensity (mW/cm^2): 15.94
    output: 1023ML8511 output: 516 / ML8511 voltage: 1.66 / UV Intensity (mW/cm^2): 5.59
    output: 1023ML8511 output: 738 / ML8511 voltage: 2.38 / UV Intensity (mW/cm^2): 11.52
    output: 1023ML8511 output: 1007 / ML8511 voltage: 3.25 / UV Intensity (mW/cm^2): 18.72
    output: 1023ML8511 output: 531 / ML8511 voltage: 1.71 / UV Intensity (mW/cm^2): 5.99
    output: 1023ML8511 output: 656 / ML8511 voltage: 2.12 / UV Intensity (mW/cm^2): 9.33
    output: 1023ML8511 output: 1020 / ML8511 voltage: 3.29 / UV Intensity (mW/cm^2): 19.06
    output: 1023ML8511 output: 635 / ML8511 voltage: 2.05 / UV Intensity (mW/cm^2): 8.77
    output: 1023ML8511 output: 542 / ML8511 voltage: 1.75 / UV Intensity (mW/cm^2): 6.28
    output: 1023ML8511 output: 878 / ML8511 voltage: 2.83 / UV Intensity (mW/cm^2): 15.27
    output: 1023ML8511 output: 915 / ML8511 voltage: 2.95 / UV Intensity (mW/cm^2): 16.26
    output: 1023ML8511 output: 529 / ML8511 voltage: 1.71 / UV Intensity (mW/cm^2): 5.94
    output: 1023ML8511 output: 733 / ML8511 voltage: 2.36 / UV Intensity (mW/cm^2): 11.39
    output: 1023ML8511 output: 1009 / ML8511 voltage: 3.25 / UV Intensity (mW/cm^2): 18.77
    output: 1023ML8511 output: 534 / ML8511 voltage: 1.72 / UV Intensity (mW/cm^2): 6.07
    output: 1023ML8511 output: 676 / ML8511 voltage: 2.18 / UV Intensity (mW/cm^2): 9.87
    output: 1023ML8511 output: 1021 / ML8511 voltage: 3.29 / UV Intensity (mW/cm^2): 19.09

    Checked wiring again and again, even replaced wires and nano pro.

    On my way to Aliexpress to buy a uvm30a

    resources used:

    0_1469283551311_20160723_141457345_iOS.jpg image url)

  • @sundberg84 You mention that you have an ATmega328.
    In your sketch setup() function you set the reference voltage value:
    According to the Arduino Language Reference pages, [](link url)
    , on an ATmega328 that will set the analog reference value to 1.1V.
    Any 10-bit sample taken by that ATmega328 will spread 1.1V out over 1024 values. In your code, you do the calculation using 3.3V as the AREF. That will produce 3 times smaller samplevalues as intended.

    I see quite some people using code derived from the UV-example-sketch.
    The code is a bit tricky, and in my opinion: not correct.

    *) In your own code, you convert samplevalues to mV values: Very good. That should be done. The example does not do that.

    *) The piece of code that returns the UV-index does this:

    for (i=0; i<12; i++)
      if (voltage <= uvIndexValue[i]) {
        UV=float(i); break;

    It returns a value that is 1 index too high. (try it: 317 mV will return UV=2).
    But later a fractional part of the UV-index is calculated, which corrects the initial (too high) value:

    //calculate 1 decimal if possible
    if (i > 0) {
      float vRange = float(uvIndexValue[i] - uvIndexValue[i - 1]);
      float vCalc = voltage - uvIndexValue[i - 1];
      UV += (1.0f / vRange) * vCalc - 1.0f;

    At first glance it looks like the value UV will even get bigger (UV+=something), but it turns out that always a negative number is added to UV.
    (1.0f / vRange) * vCalc - 1.0f is always negative. So something is subtracted from the initial value UV.
    It's a bit counter intuitive...

    I've seen other people's projects where they removed the fractional/decimal part from their code (unlike your code). Their sensors will therefore always be interpreted with a full 1 UV-index value too high.

    *) Correct me if i'm wrong, but the example code calculates a UV-index with a one-decimal precision. But the value sent to the gateway, is sent in 2 decimals. Wouldn't it be enough to do:

    send(msgUV.set(UVindex, 1));

    Here are bits of my own code. I use the Arduino map() function to calculate the decimal for me.
    Perhaps others find it easier to read too.:

    const int UV_threshold[12] = {50, 227, 318, 408, 503, 606, 696, 795, 881, 976, 1079, 1170}; // in units of mV
    const float SAMPLE_TO_MV = (1.1*1000)/1024;  // mV per sample-resolution
    uint16_t UV_index;        // the UV index
    uint16_t UV_index_f;      // the fractional part of the UV index
    // determine the UV index
    if (UV_value < UV_threshold[0]) {
      // too low value  or 
      // invalid value (sensor is wrongly connected? => always 0)
      UV_index = 0;
      UV_index_f = 0;
    } else {
      for (UV_index=11; UV_index>0; UV_index--)
        if (UV_value >= UV_threshold[UV_index]) break;
      // calculate fractional part of the UV-index
      if (UV_index == 11) {
        // already at the maximum level;
        // Displaying a fraction is meaningless
        UV_index_f = 0;
      } else {
        UV_index_f = map(UV_value, UV_threshold[UV_index],UV_threshold[UV_index+1], 0,9);
        // one decimal, so a number ranging 0 to 9
    float UV_index_float = UV_index + (UV_index_f/10.0);

  • Hardware Contributor

    @core_c - looks good! Thank you. Coding isnt my strong side so appreciated you looked at it!

  • @core_c
    Thanks for sharing! It was my experience to that the sample values were to low.

    Is it possible to share your complete version?

  • I stripped the code to only include the UV measurement.
    Hopefully it helps you out a bit.

    // Print debug information (test versions), (or not; in the final version)
    #define DEBUG_PRINT     // print debug information when this line is uncommented (and the next line is commented)
    //#undef DEBUG_PRINT    // (vice versa)
    // UVM30A ultraviolet detector.
    // The sensor's VCC is connected to +5V
    // At first glance, the datasheet is not entirely making sense:
    //  "输出电压" translates to "Output voltage", with a value of: DC 0—1V.
    //  So the sensor will never output/measure the 2 highest values listed in the graph & table of the datasheet (1079 and 1170+).
    //  The datasheet explains it: "(对应 UV 指数 0-10)" translates to "(corresponding to UV index 0-10)".  Fair enough.
    //  "测试精度" translates to: "Test accuracy", with a value of: ±1 UV INDEX
    //  That is not very accurate on a range 0-10. Example: if the real UV-index is 4, you could measure 3, or 5.
    //  For that reason we take an average of multiple samples, to get a better measurement.
    // The sensor's (analog) output is connected to the Arduino Nano's at pin 14 (A0)
    #define UV_PIN 14
    // The list of UV index thresholds (in mV).
    const int UV_threshold[12] = {50, 227, 318, 408, 503, 606, 696, 795, 881, 976, 1079, 1170}; // in units of mV
    // The read value is a 10-bit value, ranging from 0 to 2^10-1 (0 to 1023).
    // The reference voltage on the input pin is set to 1.1V.
    // That voltage is spread out over the 1024 possible values of a sample.
    // So our achieved resolution equals (1.1 Volts / 1024) = 0.00107421875 Volt.
    // The UVM30A sensor datasheet lists all UV-index values according to measured milli-Volts.
    const float SAMPLE_TO_MV = (1.1*1000)/1024;  // mV per sample-resolution
    uint16_t UV_value; 
    uint16_t UV_index;        // the UV index
    uint16_t UV_index_f;      // the fractional part of the UV index
    // Compiler directives for averiging the UV-sensor readings
    // (Change the values of UV_AVERAGE_T & UV_AVERAGE_N according to your own taste).
    #define UV_AVERAGE_T 4000 // Sample interval duration in milliseconds..  (1 <= T <= 60000)
    #define UV_AVERAGE_N 10   // ..During that interval, N samples are taken, and averaged.    (1 <= N <= 100, must be <>0)
    #if UV_AVERAGE_T < 1      // Sanity check. It must be dummy proof
    #define UV_AVERAGE_T 1
    #if UV_AVERAGE_N < 1      // Sanity check. It must be dummy proof
    #define UV_AVERAGE_N 1    // This value must be <>0 at all times, because we divide by it
    // calculate once, use many times in the loop()
    const float UV_AVERAGE_N_RP = 1.0 / UV_AVERAGE_N;
    const uint32_t UV_AVERAGE_D = UV_AVERAGE_T * UV_AVERAGE_N_RP;
    // Process a UV measurement:
    // read an average value from the UV detector.
    // The average consists of N samples taken during a T milliseconds interval.
    void processUV() {
      // Set the reference voltage for sampling
      // For our ATmega328 Nano: INTERNAL = 1.1V, DEFAULT = 5V
      // After using analogReference(), the first few samples may not be accurate (according to the Arduino language reference),
      // and that is why we read a few dummy samples before starting the actual measurement.
      // NOTE: If you change the next statement, beware to adjust the value of SAMPLE_TO_MV too.
      for (int i=0; i<10; i++) UV_value = analogRead(UV_PIN);  // ignore the possibly inaccurate samplevalues.
      #ifdef DEBUG_PRINT
        Serial.print("UV raw values: [");  
      uint32_t average = 0;
      for (int i=0; i<UV_AVERAGE_N; i++) {
        #ifdef DEBUG_PRINT
          UV_value = analogRead(UV_PIN);
          Serial.print(UV_value);  Serial.print(" ");
          average += UV_value;
          average += analogRead(UV_PIN);
      UV_value = average * UV_AVERAGE_N_RP;
      #ifdef DEBUG_PRINT
        Serial.print("]    avg: ");  Serial.print(UV_value);
      // We must convert sample-values into mV-values before we look up the UV-index.
      UV_value *= SAMPLE_TO_MV;
      #ifdef DEBUG_PRINT
        Serial.print("     mV: ");  Serial.print(UV_value);
      // determine the UV index
      if (UV_value < UV_threshold[0]) {
        // too low value  or  invalid value (in case the sensor is wrongly connected it always returns 0)
        UV_index = 0;
        UV_index_f = 0;
      } else {
        for (UV_index=11; UV_index>0; UV_index--) {
          if (UV_value >= UV_threshold[UV_index]) break;
        // calculate fractional part of the UV-index
        if (UV_index == 11) {
          // already at the maximum level; Displaying a fraction is meaningless
          UV_index_f = 0;
        } else {
          UV_index_f = map(UV_value, UV_threshold[UV_index],UV_threshold[UV_index+1], 0,9);  // one decimal, so a number ranging 0 to 9
      float UV_index_float = UV_index + (UV_index_f*0.1);  // that is the same as /10
      #ifdef DEBUG_PRINT
        Serial.print("     UV index: ");  Serial.println(UV_index_float);
    void setup() {
      // By default the Arduino sets all pins as input at startup,
      // so there is no need to explicitly set the pinMode
      pinMode(UV_PIN, INPUT);
      #ifdef DEBUG_PRINT
        // debug purposes
    void loop() {
      // UV measurement
      // just some delay
      for (int i=0; i<5; i++) delay(1000);

    I edited my post.
    The reason is that i'm using an Arduino Uno & Arduino Yún in my own project.
    The Nano's pin numbering is quite different: pin named A0 is numbered 14.
    If You copy/paste the code, and run it, it would look something like this:

    UV raw values: [351 353 339 343 349 349 334 346 338 347 ]    avg: 344     mV: 369     UV index: 2.50
    UV raw values: [344 344 344 348 343 341 349 346 345 349 ]    avg: 345     mV: 370     UV index: 2.50
    UV raw values: [348 345 309 343 342 346 315 346 343 346 ]    avg: 338     mV: 363     UV index: 2.40
    UV raw values: [330 317 343 340 343 343 341 338 342 341 ]    avg: 337     mV: 362     UV index: 2.40
    UV raw values: [338 337 312 337 335 335 336 299 342 338 ]    avg: 330     mV: 354     UV index: 2.30

  • Hardware Contributor

    @core_c - If I change the analogReference(INTERNAL); will this change all readings on all analog pins? Im using this becuse of a voltage divider that returns the battery status.

    Because if thats true - I think we need to inform this at our battery build instruction. Using this is very common with battery powered sensors and if this is making error values for analog readings...

    @awi? Any imput? I know you have a great battery knowledge.

    Its getting into the season so I will update my Sensor and see if I can combine it with battery measurement as well. Thanks.

    Edit: Or I might missunderstood this completley... If the sensors return 3.3v as maxoutput, Can i still combine this sensor with the battery report using analogref(internal) or should I read the batterystatus some other way? I see you use analogReference(INTERNAL) as well using this sensor. Did I just make the code the wrong way?

  • Hero Member

    @sundberg84 The internal reference is around 1.1V. A reading of 1024 will therefore be 1.1V like @core_c mentioned in a previous posting. All analog inputs will use this reference (if not changed in the mean time ;-)) But if you have a stable LDO powering the Arduino there is nothing wrong with using the external (Vcc) reference. It's main use is avoiding external components when powering directly from batteries. (ref @Yveaux 's Vcc lib).

  • The Arduino language reference says about "analogReference()": Configures the reference voltage used for analog input (i.e. the value used as the top of the input range).

    To me that means: Any reading you make, after setting the analogReference once, will use the same reference.
    So, if you are unsure, or if you want to be sure,.. Just set the analogReference before taking any new sample (for another sensor).

    If you have ANY other sample to make, from -some- device, and you know what maximum voltage it will return (that is: the voltage that is to be sampled),.. set the analogReference to the best fit, and then make the sample. Just be sure that the analogReference value can at least hold the maximum voltage to be sampled. In other words: If you have some device that outputs (say) 2 Volts, it will be incorrect to set the analogReference to 1.1V, because the input voltage could be higher than the reference voltage (and you can never sample values that exceed 1.1V).

    Once the sample has been taken (and you have your 10-bit sample), you just have to know what that sample represents.
    If, in your code, you never set the analogReference at all, the default input signal can be up to 5V. With a 10-bit sample resolution (1024 possible values), that means: 5 Volts / 1024 steps = 0.0048828125 Volt per sample-step. You can still sample voltages of 1 Volt,.. no problem.. but over an analogReference set to 5V, the maximum samplevalue is just a fifth of what could fit in 10 bits. In other words: The precision you measure will go down by a factor of 5.

    Because i know from the datasheet that the UVM30A will always produce a maximum voltage of 1 Volt, i set the analogReference to a value that fits best. It turns out that analogReference(INTERNAL) is quite good for sampling voltages that never exceed 1 Volt.

    In your case: If your battery reports greater voltages than 1.1V, simply set the analogReference to DEFAULT (5V) before taking a sample from the battery.

    I just checked the Arduino Reference: It seems not possible to put a voltage greater than 5V on the AREF pin. I have no experience with measuring battery voltages (yet). But i know the Arduino can be powered with more than 5V.
    What i want to say is: If you would have a sensor that outputs 2.3 Volts maximum, you could put 2.3V on the AREF pin of the Arduino, set analogReference(EXTERNAL), and make samples that are prefectly suited for that sensor.

  • Hardware Contributor

    @core_c - Woaw! Thank you! I really appreciate your help, effort and great explanation here!
    This made me understand how it works and now I understand what I made wrong. I will take my UV sensor down this week and upgrade it.

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