Using Raspberry Pi to Receive Float from Arduino using NRF24L01

Question

Good day. I've been learning on how to transfer data from Arduino to Raspberry Pi wirelessly using NRF24L01 based on the following reference: Raspberry Pi 3 Tutorial 14 – Wireless Pi to Arduino Communication with NRF24L01+.

The reason why I want to do this is to log temperature and humidity data wirelessly using DHT22 sensors.

The Arduino code is shown below:

//SendReceive.ino

#include<SPI.h>
#include<RF24.h>

// CE, CSN pins
RF24 radio(9, 10);

void setup(void){
    while(!Serial);
    Serial.begin(9600);

    radio.begin();
    radio.setPALevel(RF24_PA_MAX);
    radio.setChannel(0x76);
    radio.openWritingPipe(0xF0F0F0F0E1LL);
    const uint64_t pipe = (0xE8E8F0F0E1LL);
    radio.openReadingPipe(1, pipe);

    radio.enableDynamicPayloads();
    radio.powerUp();

}

void loop(void){
    radio.startListening();
    Serial.println("Starting loop. Radio on.");
    char receivedMessage[32] = {0};
    if(radio.available()){
        radio.read(receivedMessage, sizeof(receivedMessage));
        Serial.println(receivedMessage);
        Serial.println("Turning off the radio.");
        radio.stopListening();

        String stringMessage(receivedMessage);

        if(stringMessage == "GETSTRING"){
            Serial.println("Looks like they want a string!");
            const char text[] = "Yo wassup, haha";
            radio.write(text, sizeof(text));
            Serial.println("We sent our message.");
        }
    }
    delay(100);

}

Meanwhile, the Raspberry Pi code is shown below:

import RPi.GPIO as GPIO
from lib_nrf24 import NRF24
import time
import spidev

GPIO.setmode(GPIO.BCM)

pipes = [[0xE8, 0xE8, 0xF0, 0xF0, 0xE1], [0xF0, 0xF0, 0xF0, 0xF0, 0xE1]]

radio = NRF24(GPIO, spidev.SpiDev())
radio.begin(0, 17)

radio.setPayloadSize(32)
radio.setChannel(0x76)
radio.setDataRate(NRF24.BR_1MBPS)
radio.setPALevel(NRF24.PA_MIN)

radio.setAutoAck(True)
radio.enableDynamicPayloads()
radio.enableAckPayload()

radio.openWritingPipe(pipes[0])
radio.openReadingPipe(1, pipes[1])
radio.printDetails()
# radio.startListening()

message = list("GETSTRING")
while len(message) < 32:
    message.append(0)

while(1):
    start = time.time()
    radio.write(message)
    print("Sent the message: {}".format(message))
    radio.startListening()

    while not radio.available(0):
        time.sleep(1 / 100)
        if time.time() - start > 2:
            print("Timed out.")
            break

    receivedMessage = []
    radio.read(receivedMessage, radio.getDynamicPayloadSize())
    print("Received: {}".format(receivedMessage))

    print("Translating the receivedMessage into unicode characters")
    string = ""
    for n in receivedMessage:
        # Decode into standard unicode set
        if (n >= 32 and n <= 126):
            string += chr(n)
    print("Out received message decodes to: {}".format(string))

    radio.stopListening()
    time.sleep(1)

Based on the Arduino code above, the code that shows the data that is transmitted is shown below:

const char text[] = "Yo wassup, haha";

Based on the Raspberry code above, the codes that decode the received data from Arduino are shown below:

for n in receivedMessage:
    # Decode into standard unicode set
    if (n >= 32 and n <= 126):
        string += chr(n)

However, these decoding code only works if I transmit string or integer from Arduino to Raspberry Pi. It doesn't work if I transmit float. Since DHT22 records temperature and humidity up until 1 decimal point, it is required for me to transmit float. Can anyone here please teach me how to decode the float values?

Thank you very much.

Answer 1

If you say you only need one digit, why not multiplying your value with 10 (with rounding), transmit it as a 4 byte (unsigned?) integer, and divide by 10 after receiving.

You only need to check if you have little or big endian conversion needed (swapping MSB/LSB 2 byte or single byte). But you see this very fast if you print the values in hex format.