Using Raspberry Pi to Receive Float from Arduino using NRF24L01

Projects Programming
1

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) &lt; 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 &gt; 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 &gt;= 32 and n &lt;= 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 &gt;= 32 and n &lt;= 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.

2

I would just use dtostrf() and send a float string.

3

I don't have an RPi but I wonder if the problem is that the RPi encodes floats in different format to the Arduino?

A simple solution might be to send the float as two integers - one for the left of the decimal point and one for the right of the decimal point

Or, perhaps, just multiply the float by 100 and send the integer part.

...R

4

Thank you very much for your reply. Please give me some time to try your suggestion.

5

Somebody suggested me to convert the float to string in Arduino, then transmit the string from Arduino to Raspberry Pi throught NRF24L01, then convert back the string to float in Raspberry Pi. I think I will also try that. Below are some of my references:

Arduino - StringConstructor
https://www.arduino.cc/en/Reference/StringConstructor

Parse String to Float or Int

I will inform the result once I tried this.

6

Somebody suggested me to convert the float to string in Arduino

No one said anything about converting the float to a String, so your first link is useless.