The biggest mistake beginners make is connecting the JDY-40 to Serial (Pins 0/1). This clashes with the USB programmer and crashes your uploads.

The best example uses SoftwareSerial:

#include <SoftwareSerial.h>
// Define RX (JDY-40 TX) and TX (JDY-40 RX) pins
SoftwareSerial jdy40(2, 3); // RX = pin 2, TX = pin 3
String receivedData = "";
void setup() 
Serial.begin(9600);   // For debugging via USB
jdy40.begin(9600);    // JDY-40 default baud rate
Serial.println("JDY-40 Master/Slave Ready");
// Optional: Set the module to transparent transmission (default mode)
pinMode(4, OUTPUT); // SET pin
digitalWrite(4, HIGH); // HIGH = Data mode, LOW = AT mode

void loop() 
// ----- TRANSMIT BEST PRACTICE -----
static unsigned long lastSend = 0;
if (millis() - lastSend > 2000) 
lastSend = millis();
// Best practice: Send structured, short packets
// Never send Strings larger than the buffer (max 64 bytes per packet)
jdy40.print("TEMP:");
jdy40.print(23.5);
jdy40.print(";BAT:");
jdy40.println(4.12);


// ----- RECEIVE BEST PRACTICE -----
while (jdy40.available()) 
char c = jdy40.read();
if (c == '\n') 
Serial.print("Received: ");
Serial.println(receivedData);
  // Parse your data here
  if (receivedData.startsWith("TEMP:")) 
    // Extract and act on data
receivedData = "";
 else 
  receivedData += c;

Range Test (Open Field, 0 dBm, default antenna):

Throughput: At 9600 baud, effective data rate ~900 bytes/sec after overhead. Not suitable for streaming, but adequate for sensor readings or short commands.

Power consumption: 23 mA average during continuous transmission.

To achieve the best results, do not just jumper wire this on a breadboard with long flying leads.