I'm building a weather station with two UNOs, using NRF24l01+ radios. Communications are fine. I'm sending a struct from one to the other. The struct has three elements:

    struct weather {
      float tempData;
      float humData;
      float pressData;
    weather wData = {0, 0, 0};

I then populate the struct with values from my DHT22 Temperature Sensor and my BMP085 pressure sensor.

    float c = dht.readTemperature();
    wData.tempData = (c * 9/5) + 32;
    wData.humData = dht.readHumidity();

    sensors_event_t event;
    wData.pressData = (0.0295 * event.pressure);

Now I send it via the NRF24L01+.

    radio.write(&wData, sizeof(wData);

On the receiving Uno I have this.

    struct weather {
      float tempData;
      float humData;
      float pressData;

    weather wData = {0, 0, 0};

    radio.read( &wData, sizeof(wData) );

I get results similar to this:

    Temp = 75.43
    Hum = 35.76
    Press = 0.00

The first two are correct. The final one is not. I can change the order and the first two will always be correct, but the last element is always 0.00. For example:

   Hum = 35.76
   Press = 30.14
   Temp = 0.00

I know I'm missing something here with my code but I can't find it. Anyone have some suggestions?

  • read() and write() return a bool, did you try to get it and print it to Serial, just to check both functions consider everything's OK? That might help.
    – jfpoilpret
    Feb 25, 2014 at 5:59
  • Also, did you check that the weather on the emiiter contained a non-0 pressure before write()?
    – jfpoilpret
    Feb 25, 2014 at 6:05
  • The line with radio.write(... is missing a closing parenthesis.
    – jfpoilpret
    Feb 25, 2014 at 6:25
  • read() and write() return 1. Missed the closing parenthesis while moving code here but it's there in my code.
    – brorobw
    Feb 25, 2014 at 14:57
  • 1
    After taking a look at the RF24 library source code, I wonder if you have changed the payload size before read or write. To know about it, you should Serial.println() of radio.getPayloadSize() in both sketches; normally that should be 32 by default, but I wonder about it...
    – jfpoilpret
    Feb 25, 2014 at 19:50

1 Answer 1


Modified my struct by changing the first two elements from float to int.

struct weather {
  int tempData;
  int humData;
  float pressData;
weather wData;

Everything transmits fine now. I don't really need the precision of a float for temperature and humidity. However, I would still like to find the problem. Could there be some problem with the size of three floats versus two ints and a float? Serial.print(sizeof(wData)) is 12 when all elements are floats and 8 when using two int and a float. My understanding is the NRF24L01+ has a transmit and receive buffer of 32 bytes.

  • Have you tried the opposite, ie putting 4 floats instead of just 2? Maybe transmission is done by pack of 8 bytes?
    – jfpoilpret
    Feb 25, 2014 at 18:06
  • Have not tried that. Will try soon and report back.
    – brorobw
    Feb 26, 2014 at 3:33
  • Did you try testing with a known strings of different lengths instead of your struct.
    – Craig
    Feb 28, 2014 at 20:53
  • Have note tried anything else as of yet. Work and life getting in the way!
    – brorobw
    Mar 1, 2014 at 23:53

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