Radio Shield

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Assembled Radio Shield

The Radio Shield is an add-on kit for the Arduino development board that provides AX.25 packet radio send and receive capability, a prototyping area, and an HD44780-compatible LCD interface.

Packets are sent and received in AX.25 UI frames at 1200 baud. This allows operation on the VHF APRS network.

Radio Shield 2

The Radio Shield 2 replaces the original Radio Shield and comes pre-assembled, except for the pin headers that connect it to the Arduino. The interface is unchanged. Further documentation will be posted soon.


Radio Shield Schematic

Parts list:

Part Number Value Notes
R1,R5,R10 10k brown-black-orange
R4 10m brown-black-blue
R3,R12,R13 680 blue-gray-brown
R7 6.8k blue-gray-red
R9 240k red-yellow-yellow
R6 2.2k red-red-red (optional, for HT PTT mode)
R2 2.2k red-red-red
Q1 2N7000
Y1 29.4912 MHz crystal
C4,C5 18pF
C1,C3,C6 0.1uF
C2 10uF negative lead toward C2 marking
D14 Red/Green LED Install with longest lead toward board edge


  1. Check parts list to ensure you have all necessary parts.
  2. Locate part locations on PCB next to their silk screen labels.
  3. Begin by inserting lowest profile parts and soldering into place first, working up to the taller pieces.
  4. Install parts in the following order easiest construction:
    1. Resistors (R1-7, R9-13) - R6 is *not* needed for Kenwood and Chinese import radios, do not install if it is not needed!
    2. Capacitors (C1, C3-6) - not C2 yet
    3. Crystal (Y1) - be sure to fold crystal over as seen in photo.
    4. IC socket - don't install the IC yet
    5. Push button (P1)
    6. LED - longest lead toward board edge
    7. Taller Capcitor (C2)
    8. Transistor (Q1) - note the orientation on the silk screen
    9. Component side pin headers (be sure to remove jumpers under after soldering)
    10. Arduino side pin headers
    11. Insert U3 into socket
    12. Solder on data cable paying attention to the pinout for your particular radio
  5. Program Arduino with sample code from below, connect radio and insert shield to Arduino. Ensure that jumpers are on RUN side of pin header.

Radio Interfacing

Four interface lines are provided at one edge of the board for connecting a radio:

Pin Function
GND Ground
TX Transmit audio (radio mic)
RX Receive audio (radio speaker)
PTT Push-to-talk (active low)

For hand-held radios that don't have a separate PTT line (most makes and models except for Kenwoods and Chinese imports) install the provided 2.2k resistor (red-red-red color code) in position R6.


Keep both JP7 jumpers in RUN position for communication with Arduino. Use PROG position for serial pass-through to shield. Remove jumpers (or the entire shield) when programming the Arduino board.

All serial communication with the Radio Shield is at 4800 baud. Use Serial.begin(4800); to achieve this.

Configuration parameters are stored in flash memory, therefore only need to be called once. ( i.e. in setup() )

All commands start with a single character and end with a CR/LF.

LCD Commands

Command LCD Function
W<text> Write to LCD
G<line,column> Go to LCD line (0 or 1) and column
C LCD clear
H LCD home
B<n> LCD contrast, 0-1023

Note that pin 16 of the LCD interface does not connect to any other circuitry on the PCB. Some LCD panels may require this pin to be connected to ground. If your display's backlight does not turn on when power is supplied to the Arduino and RadioShield, you may need to ground this pin - check your LCD's datasheet to be sure.

Radio Commands

Command Function
 !<text> Send plain text packet
@<text> Send Morse code text
M<callsign> Set my callsign (returns 1 for success)
S Status. Returns 1.
V Reports version. Added in firmware version 1.1.
L<0-255> Set TX audio level (returns 1 for success)
D<0-255> Set TX delay in 1/120 sec intervals (returns 1 for success)
P<path> Set AX.25 path (e.g., WIDE1-1,WIDE2-1)

Received Packet Format

Received packets are output with source and destination call, followed by packet contents and CR/LF, e.g.:


Sample Applications

The following blocks of code are provided as a starting place for your experimentation.

LCD Counter

This code sample displays a counter on the LCD.

int counter = 0;

void setup()                    // run once, when the sketch starts
  delay(3);                     // Allow time for RadioShield setup

void loop()                     // run over and over again
  Serial.print("C\r\n");        // Clear LCD
  delay(10);                     // Clear command takes some time to complete
  Serial.print("WCount");       // Send text to LCD
  delay(8000);                  // Wait a bit

Display Incoming Packets

This demo receives incoming packets and displays them to the LCD. Note that there is no line length limiting - displayed packets may wrap strangely depending on the size of your display.

char inbyte = 0;                 // Received byte
char buf[260];                   // Incoming data buffer
int buflen = 0;                  // Length of buffered ata

void setup()                     // Runs at startup
  Serial.begin(4800);            // RadioShield runs at 4800 baud
  delay(3);                      // Allow time for RadioShield setup
  Serial.println("B100");        // Set display contrast
  Serial.println("C");           // Clear screen
  delay(10);                     // Can take a bit to clear the LCD, so wait
  Serial.println("WWaiting");    // 'Waiting' message stays until we receive something

void loop()                      // Runs constantly after startup
  while (Serial.available() > 0) // Check for an incoming byte on the serial port
    inbyte =;      // Get the byte
    if (inbyte == '\n')          // Check for end of line
      Serial.println("C");       // Clear screen
      delay(10);                 // Can take a bit to clear the LCD, so wait
      buflen = 0;
    else if (inbyte > 31 && buflen < 260)  // Only record printable characters
      buf[buflen++] = inbyte;
      buf[buflen] = 0;

Transmit Packets

This demo will transmit a simple test message about every 30 seconds, with an incrementing counter for each message sent.

int counter = 1;

void setup()
  Serial.begin(4800);                      //OPEN SERIAL LINE AT 4800
  Serial.print("MW1AW\r\n");               //SET YOUR CALLSIGN HERE, HERE YOU SEE W1AW
  Serial.print("PWIDE1-1,WIDE2-1\r\n");    //SET DIGIPATH HERE

void loop()
  Serial.print("!>This is RadioShield test message #");     //BEGIN MESSAGE BUT DON'T SEND YET...
  Serial.print(counter);                                    //  ...CONCATENATE VALUE OF count TO OUTPUT...
  Serial.print("\r\n");                                     //    ...SEND CR/LF TO COMPLETE AND TRANSMIT PACKET.
  delay(30000);                                             //30,000ms = 30sec

Note, the delay() is not from the end of the APRS packet, but from the time the transmit command (!...\r\n) was issued. If the APRS transmit takes 1 second to transmit and you are delaying 30sec, the next packet will transmit 29seconds after the previous. This is rarely an issue, just something to keep in mind if you're watching the clock.

Revision History

Firmware revision 1.1 - Released 4/23/2013. Fixed buffer overrun bug in (P)ath command that could cause configuration to become garbled and possibly set permanently to 300 baud mode. Added (V)ersion command.

Useful Links

For more information on creating correctly formed APRS packets, visit the following links: