RFM69HCW Hookup Guide Datasheet by SparkFun Electronics

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Page 1 of 19 RFM69HCW Hooku
RFM69HCW Hookup Guide
Introducing the RFM69
The RFM69HCW is an inexpensive and versatile radio module. You can
use it to send text or binary data between two or hundreds of modules. It’s
perfect for building inexpensive short-range wireless networks for home
automation, citizen science, and more.
The RFM69HCW comes in two flavors of frequency, the 915 MHz version
and the 434 MHz version.
Interface and Example Code
The RFM69HCW can’t do anything by itself; it needs to be connected to a
microcontroller such as an Arduino. The RFM69HCW uses a four-wire
Synchronous Peripheral Interface (SPI) plus an interrupt line. Most
microcontrollers, including the Arduino, offer an SPI interface.
Felix Rusu of LowPowerLab has written an excellent Arduino library for the
RFM69 that handles the details of setting up the module and sending and
receiving data. This guide will cover interfacing the RFM69HCW to an
Arduino microcontroller using this library.
If you’re using a different microcontroller, the information here plus the
datasheet and the library source code should help get you up and running.
(If you write example code for another system, we’ll be glad to add it to the
code repository).
Required Materials
In this tutorial we’ll show you how to get two modules talking to each other,
but keep in mind that you can use more than two modules in your projects.
Here’s what you’ll need:
Two RFM69HCW modules (with matching frequencies):
915 MHz (WRL-12775)
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434 MHz (WRL-12823)
You’ll need two Arduinos. You can use the RFM69HCW with any
Arduino, but the 3.3V Arduino Pro is a good choice because the
RFM69HCW is also a 3.3V part. We’re using the “Beefy 3” FTDI
board, because the standard FTDI board can’t supply enough current
to run the RFM69:
2 x Arduino Pro 328 3.3V
2 x Female Header Pack
2 x “Beefy 3” FTDI board
2 x USB Mini-B Cable
We’ll connect these to the Arduinos using headers and jumper wires:
2 x Break Away Headers - Straight
2 x Jumper Wires 6" M/F, pack of 10
If you’d rather use 5V Arduinos you can, but you will need a Logic
Level Converter for each 5V Arduino:
2 x Bidirectional Logic Level Converter
You’ll need some wire to make antennas, and a couple of LEDs
(optional) that we’ll make blink when data is sent or received:
About 1 foot of general-purpose wire (e.g. AWG22 Hook-Up
Wire), or you can use a jumper wire in a pinch.
2 x general-purpose LEDs (e.g. Basic Red LED)
If you don’t already have them, you will also need soldering tools and
Suggested Reading
We recommend you be familiar with the following topics before working with
the RFM69HCW. If you’d like to brush up on any of these things, follow the
links and come back when you’re done.
How to Solder
What is an Arduino
Installing the Arduino IDE
Using Github (to retrieve the example code and library)
Installing an Arduino Library
Serial Peripheral Interface (SPI)
Hardware Overview
The RFM69HCW transmits in the ISM (Industry Scientific and Medical)
band, a set of frequencies set aside for low-power, short-range, license-free
SparkFun sells two versions of the RFM69HCW, a 915 MHz version and a
434 MHz version. These frequencies are legal in different areas: very
roughly, 915 MHz is for use in the Americas and Australia, and the 434
MHz version is for use in Europe, Asia and Africa. The actual regulations
are a bit of a patchwork, so check your local regulations for other areas.
Note: Because of the low power and short range, the limited use of
either frequency isn’t likely to be an issue. But if you’re planning on
creating a commercial product or deploying a large number of
modules, be sure you’re using the correct frequency for your location.
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How far will the signal reach? Outside with few obstructions, you should be
able to get a solid link for hundreds of meters. Indoors we’ve seen it work
for over 50 meters through multiple walls.
The RFM69HCW is capable of transmitting at up to 100 mW and up to 300
kb/s, but you can change both of those values to fit your application. For
example, you can maximize range by increasing the transmit power and
reducing the data rate. Or you can reduce both for short-range wireless
sensor networks that sip battery power.
Board Pinout
The RFM69HCW Breakout Board gives you access to all the pins on the
RFM69HCW, but in the majority of cases you’ll only need seven of them.
Here’s an overview of the pins we’ll be using:
Name Function
3.3V Power 3.3V power
supply (at
least 130 mA,
see below)
G /
Ground Power ground
Name Function
O /
In Slave
Data from
I /
Slave In
Data from
C /
Clock signal
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S /
Select signal
0 /
I/O 0
Received data
ready interrupt
signal from
Name Function
A /
Antenna Wire
antenna, see
the next page
for details
G /
Ground Antenna
ground (same
as power
(You can use
either G pin
adjacent to
the antenna
The unused pins labeled 1 to 5 have functions that we won’t be using. See
the RFM69HCW datasheet for more information.
Power requirements
The RFM69HCW will run on voltages from 1.8V to 3.6V, and can draw up
to 130mA of current when it’s transmitting.
IMPORTANT! Do not power your RFM69 with 5V or connect the data
lines directly to a 5V microprocessor. Doing so will damage the device.
The easiest way to use this board is to connect it directly to a 3.3V Arduino
such as the 3.3V Arduino Pro. However, you can connect it to a 5V Arduino
if you use a logic-level translator. See the diagram in the Hardware
Connections section.
In this tutorial we’ll use two 3.3V Arduino Pros powered by the “Beefy 3”
FTDI basics. We’re using the Beefy 3 FTDI because it can provide up to
500mA, as opposed to the standard FTDI which can only provide 50mA.
TIP: If you don’t have Beefy 3 FTDI boards, you can ensure that you
have enough current by connecting your Arduino Pros to an external
power supply such as a Wall Wart or Lipo Battery.
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PROTIP: If your power supply can’t provide 130mA, your Arduino may
reset or lock up when you try to transmit. In these cases, a large
capacitor placed between power and ground on the RFM69HCW
board should help. This capacitor will store power while the board isn’t
transmitting and release it when it’s needed.
Incidentally, the RFM69HCW uses 16mA when it’s in receive mode, and it
has several sleep options that use less than that.
Hardware Connections
Connecting the RFM69HCW to an Arduino
There are lots of ways to connect boards together. In this tutorial, we’re
going to solder Male Breakaway Headers to the RFM69HCW board, solder
Female Headers to the Arduino Pro, and use M/F Jumper Wires to connect
everything together. Of course, you can use whatever wiring methods you
Step 1: Solder male header onto the RFM69HCW:
Break off one 8-pin length of Male Breakaway Header, and solder it to the
“O I C S R 0 G 3.3V” side of the RFM69HCW board. You can solder it to
the top or bottom, your choice.
Note: The board is laid out so that all the interface connections you
need are on one side of the board, so you should only solder headers
or wires to the “O I C S R 0 G 3.3V” side of the board. Leave the
opposite side empty for your antenna.
Repeat for the second RFM69HCW.
Step 2: Solder female headers onto the Arduino Pro:
Solder the four Female Headers to the Arduino Pro. They should stick out
of the top of the board.
Repeat for the second Arduino Pro.
Step 3: Use M/F jumper wires to connect the RFM69HCW to the
Arduino Pro:
Different Arduino models use different pins for the SPI port. See the
following table for the proper connections:
Mega 32U4
O / MISO 12 or ICSP-1 50 or
I / MOSI 11 or ICSP-4 51 or
C / SCK 13 or ICSP-3 52 or
S / NSS 10 53 10
0 / DIO0 2 2 3
3.3V 3.3V (labeled "VCC" on Pro/Mini)
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A / ANT See the next section for antenna information
If you use the ICSP header for your SPI connection, note the pin
Wiring Diagrams
3.3V Arduinos
As we’ve mentioned, it’s easiest to connect the RFM69HCW directly to a
3.3V Arduino such as the Arduino Pro:
Be sure that you’re powering your Arduino with a “Beefy 3” FTDI board, a
wall-wart, or a Lipo battery. A normal FTDI Basic won’t be able to supply
enough current to run the RFM69HCW.
A 3.3V Arduino Pro Mini is also a great choice, since both boards are very
5V Arduinos
You can connect the RFM69HCW to a 5V Arduino, such as our RedBoard,
if you use a logic level translator between them. The translator translates
the Arduino’s 5V signals to 3.3V signals, which won’t damage the
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5V Arduinos have both a 5V and a 3.3V supply. Here we’re using the 5V
supply (purple wire) to power the high-voltage side of the logic level
translator, and the 3.3V supply (red wire) to power both the RFM69HCW
and the low-voltage side of the logic level translator. Remember to never
connect 5V to the RFM69HCW.
PROTIP: If you’re using jumper wires to connect everything together,
you can make handy “Y” jumper wires by cutting and splicing two
regular jumper wires together.
External Status LED
One more thing: the example code we’ll get to in a few pages blinks a LED
to indicate when data is sent or received.
Unfortunately we can’t use the Arduino’s built-in LED for this, because that
port (D13) is also used by the SPI port’s SCK line. We use the SPI port to
communicate with the RFM69HCW, so this LED will be on a lot of the time.
To make up for this, the example code is written so you can stick any old
LED into your Arduino, with the long (positive) lead in D9, and the short
(negative) lead in D8.
This step is optional, and not required for the normal operation of the
(Normally we would want to include a current-limiting resistor in series with
the LED to prevent it from being burned out. But for simplicity, we’ll rely on
the current-limiting built into the Arduino’s I/O ports, along with the fact that
we’ll only be turning it on for very brief periods to indicate packets sent or
The Antenna
IMPORTANT: You MUST attach an antenna to the board. Aside from
not working without an antenna, transmitters can be damaged if they
transmit without an antenna present.
Creating an antenna is not as hard as it may sound. The simplest antenna
is a wire cut to the proper length and soldered to the “A / ANT” pin. The
length depends on the frequency of your board and is marked on the
bottom of the board for your convenience:
Frequency Length
915 MHz 3.07" (3
+ 1/16")
434 MHz 6.47" (6
+ 1/2")
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FYI: This is called a quarter-wave antenna because the length is one-
quarter of a full wavelength at the transmission frequency.
PROTIP: Cut the antenna wire slightly longer to give yourself room to
strip one end and solder it to your board. You can always trim it to the
proper length after you solder it down.
Antenna Placement
For antenna placement you have a few options:
Single Antenna
The simplest option is to solder a properly-sized antenna wire to the “A” /
“ANT” header hole (the one with the circle around it), and stick it straight up.
That’s it!
PROTIP: If you can, keep all of the antennas in your network as
vertical as possible. The radiation pattern from a vertical antenna is
shaped like a donut laying on a table, with the strongest signal out to
the sides and the weakest signal above and below (where the hole is).
Keeping your antennas vertical will maximize the horizontal distance
that your signal will reach.
IMPORTANT: Radio waves will pass through plastic, but will be
blocked by metal. Your radio module and antenna will work within a
plastic box, but not a metal one.
If you need the antenna to be shorter to fit within a plastic box, you can
bend or coil the antenna (this will adversely affect the range and
directionality), but don’t change the length unless you know what you’re
If you’re using a metal box, you should pass the antenna through a hole
and extend it outside the box (see below). Also try to keep your radio
module and antenna away from large metal objects, as the metal will
disrupt the transmission.
You will get good results from this basic antenna, but if you want to
maximize your range, consider the following options as well:
Add a Dipole
Antennas love to have a ground underneath them. The RFM69HCW board
contains a ground, but, the larger the ground, the better. A dipole is an
antenna that has an equal length of grounded wire extending below the
board. This creates a virtual ground that will increase the gain of the
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To make a dipole, cut a second wire the same length as your antenna,
solder it to one of the “G” / “GND” header holes adjacent to the antenna,
and point it in the opposite direction of the antenna (straight down instead
of up).
Keep both wires straight, in free air, and away from large metal objects if
Remote Mounting
You can mount the antenna a short distance away from the board by
(ideally) using 50-ohm coaxial cable, or (less ideally) by running a ground
wire next to it. At the point where your antenna begins, simply end the
coaxial shield / ground wire, or connect it to your ground plane (see below)
or dipole. (The unshielded portion of your antenna should be ¼-wave long
as discussed above).
Using a Ground Plane
Because antennas love to have a ground underneath them, the best
antennas stick up above a well-grounded surface. If you’re using a metal
box,or mounting the antenna on a vehicle roof, you can extend the antenna
through and above the metal surface, as long as the surface is grounded.
(Ensure that the antenna conductor doesn’t short out against the ground
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plane where it passes through it.) If you don’t have a handy metal surface
you can create workable ground planes with a grid of copper tape or foil,
radial grounded wires, etc.
Connecting a Commercial Antenna
If you’d like to connect a commercially-available antenna, you can solder an
SMA Connector to the RFM69HCW board. Position the connector over the
three headers marked G / A / G, and solder on both sides to ensure a good
IMPORTANT: Note that the above connector is SMA, not RP-SMA
(“Reverse Polarity”). Ensure that your antenna has a true male SMA
connector and was designed for the radio frequency of your
RFM69HCW (915MHz or 430MHz).
Other Antenna Types
The quarter-wave antenna is simple and effective, but, if you need
additional range, there are other antenna designs like the Yagi that are far
more directional, which is useful if you can point your antennas towards
each other.
We’re not antenna experts, so this is where we’ll leave you. But if you’re
interested in this topic, consider getting involved in amateur radio. It’s a
great way to learn about antenna design and many other things:
How it Works
“How does… how does it work?”
“I know not, my Liege”
“Consult the Book of Armaments!”
-Monty Python and the Holy Grail
The information below will help you better understand how your
RFM69HCW module works, which will help you use it more effectively in
your projects. However, if you’re itching to start using it right away, you can
jump straight to Running the Example Code.
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To save power and let multiple modules share the same frequency, the
RFM69HCW doesn’t transmit continuously. Instead, it sends short bursts of
data called packets. You can think of these as short messages sent from
one RFM69 to another.
Packets can be sent very quickly and repeatedly, giving the illusion of a
continuous connection. And, because the transmitter is on only for the
fraction of a second it takes to transmit each packet, many modules can
share the same frequency.
Each packet can contain up to 61 bytes of data. This is the actual message
that you’re sending from one node to another. This data can be text
characters or binary numbers; as far as the RFM69 is concerned, it’s just a
block of data. If you need to send more than 61 bytes, just send more
The format of the message (what it contains) is entirely up to you. Normally
you’ll come up with a format for your specific application (e.g. “three
decimal numbers separated by spaces”), and write your code so that both
the sender and receiver expect the same format.
In order for a message to get from one RFM69 to another, each module
(often called a node) must be given its own address. Addresses consist of
two numbers: the network number, which is like the town in which all the
nodes live, and the node number which is like an individual street address.
Each of these numbers can range from 0 to 255.
You get to (and must!) choose these numbers. Even if you just have two
nodes, you’ll need to give them address numbers in your code. The
address is set by your code each time the RFM69HCW is powered up; it
doesn’t remember the address when it is powered off.
Network Number
The network number should be the same for all the RFM69HCWs that you
want talking to each other. Each network will be isolated from all the others
- modules in network 0 won’t be affected by anything going on in network 1,
etc. You can have up to 256 networks operating in the same area,
numbered 0 to 255.
Node Number
The node number should be different for all your RFM69HCWs (on the
same network). These are the individual addresses that are used to e.g.
send a message from node 2 to node 3. You can have up to 255 nodes in a
network, numbered 0 to 254.
Broadcast Address
Sharp-eyed readers may have noticed that node addresses only go to 254,
not 255. That’s because 255 is a special “broadcast” address; if you send a
message to address 255, all of the nodes on that network will receive it.
“Promiscuous Mode”
Apart from the broadcast address, messages sent between nodes are
usually “private.” Nodes will normally ignore messages that aren’t
addressed to them.
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However, you can turn on something called “promiscuous mode,” which
tells the node to receive every message that it hears on the network. This
can be useful for debugging, routing messages to and from the radio
network, etc, but note that even promiscuous mode won’t let you hear
messages sent on a different network.
When you send a message, you have the choice of sending it “in the blind,”
hoping that the receiver will pick it up, or you can request an
acknowledgement (usually shortened to “ACK”) that the message was
received. In fact, the RFM69 library can automatically resend a message
until it either receives an acknowledgement from the receiver (success!) or
it has resent it a specific number of times without a response (fail).
Whether you use acknowledgements or not depends on your application.
You may want to use acknowlegements if the data you’re sending is very
important or if you want to be sure that both ends of a link are functional.
But, some applications work better in the blind, such as high-bandwidth
data where a lost message isn’t a big deal, beacons that multiple modules
may receive, one-way links using highly-directional antennas, etc. It’s up to
There’s one catch: even though it’s easy to request an acknowledgement
when you send a message, the receiver will not automatically send an
acknowlegement. Your code must check received messages for
acknowledgement requests and send acknowledge messages manually.
See the example code for details.
As noted above, “promiscuous mode” will let you silently listen in on all the
conversations going on in a given network. If you’re sending sensitive
information, this may not be good news to you since someone else may be
listening to your conversations. Fortunately, the RFM69 can protect your
communications using AES (Advanced Encryption Standard) encryption,
which is extremely secure.
Encryption can be turned on or off by a library command (see the examples
for details). You’ll also need to make sure that both ends of your link are
using the same 16-byte key. Only those RFM69s using exactly the same
key will be able to decode the messages.
PROTIP: Similarly, anyone who *has* the key will be able to decode
your messages, so be sure to keep the key secret! In particular,
remember that it will be saved along with your source code, so erase
the key before saving it, or keep your source code secret as well.
Running the Example Code
Felix Rusu of LowPowerLab has written an excellent Arduino library for the
RFM69 that handles the details of setting up the module and sending and
receiving data. This guide will cover interfacing the RFM69HCW to an
Arduino microcontroller using this library.
If you’re using a different microcontroller, the information here plus the
datasheet and the RFM69 library source code should help get you up and
running. (If you write example code for another system, we’ll be glad to add
it to our code repository).
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Note that the author of the library keeps the most up-to-date version at
github.com/LowPowerLab/RFM69. SparkFun has created their own
repository for the RFM69HCW Breakout Board that includes a copy of this
library plus hardware schematics, example code, etc. at
Installing the RFM69 Library
Hopefully you’ve done a bit of Arduino programming and know how to
install a library. If not, take a look at our tutorials on the process:
Installing the Arduino IDE
Using Github (to retrieve the example code and library)
Installing an Arduino Library
NOTE: This library is not found in the Arduino Library Manger currently, so
you’ll have to download the library and add it using the Add .ZIP Library…
option found under the Sketch $rarr; Include Library menu.
The basic process is:
1. Download a copy of the repository:
2. Open the above archive, go into the “Libraries/Arduino” folder, copy
the “RFM69” folder, and add the folder via the Library Manager or
paste it into your “Arduino/libraries” folder.
3. Restart your Arduino IDE.
Plug in the Hardware
In this tutorial we’re going to set up two Arduino / RFM69 nodes and get
them to communicate with each other.
If you haven’t yet, do the wiring steps on the Hardware Connections page
of this tutorial.
At this point, you should have two matching Arduino plus RFM69HCW
nodes. We’ll need to figure out the serial ports associated with these nodes.
Plug one USB cable into your computer. A new COM port number should
be added to the Arduino IDE’s “Tools/Port” list. Write it down. Now plug in
the other USB cable from the second node. Another COM port number
should appear. Write that one down as well.
Customize the Code for Each Node
Before you upload the code to your Arduino, you’ll need to customize it
for each node. Use the code below, and make the followinf changes for
each separate node.
Node 1
Let’s set it up for your FIRST node:
1. Near the top of the code, look for #defineNETWORKID , and change
the value to 0. This will be the network all your nodes are part of, so it
should be the same for all your nodes. (You can of course make this
any number from 0 to 255, as long as it’s the same for all your
2. Now look for the #defineMYNODEID line, and change the value to 1.
That will be this node’s address.
3. Look for the #defineTONODEID line, and change the value to 2. That
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will be the other node’s address; the one you’ll be talking to.
4. Below these lines will be a section for defining the radio frequency of
your RFM69HCW board. Uncomment the line corresponding to your
board’s frequency, and make sure the others are commented (have //
in front of them. If you forget it, the frequency is marked on the
bottom of your RFM69HCW board).
5. If you want to use encryption, change the #defineENCRYPT value to
true , and put a 16-character string of your choice into the
ENCRYPTKEY value. This key must be the same for all nodes on your
network. Keep it a secret!
6. Finally, if you wish to use acknowledgements, set USEACK to true .
If not, set it to false . Use the same setting for all your nodes.
Now upload the sketch to your FIRST node. Remember that you should set
the “Tools/Port” menu to the COM port you wrote down earlier for the
FIRST node, and if you’re using a 3.3V Arduino Pro as we recommend, you
should set the “Tools/Board” menu to “Arduino Pro or Pro Mini” and
“Tools/Processor” to “ATmega328 (3.3V, 8MHz)
Node 2
Time to modify the sketch for the SECOND node.
1. Go back up to the #defineMYNODEID line, and change the number
to 2. That will be this node’s address.
2. Now look for the #defineTONODEID line, and change the number to
1. That will be the other node’s address; the one you’ll be talking to.
See how we swapped the MYNODEID and TONODEID numbers? This way
each node will send messages to the other one.
Upload the sketch to your SECOND node. Change the “Tools/Port” menu to
the second COM Port you wrote down from above, and upload. That’s it,
we’re done!
Load and Modify the Code
Copy the code from the below window into the Arduino IDE (be sure the
editing window is completely blank first). After you’ve installed the RFM69
library, you can also find this code in the Arduino IDE under “File /
Examples / RFM69 / SFE_RFM69HCW_example.ino”. Don’t forget to make
the changes mentioned above.
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void setup()
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void loop()
static char sendbuffer[62];
static int sendlength=0;
if (Serial.available()>0)
char input= Serial.read();
if (input!= '\r')//notacarriagereturn
sendbuffer[sendlength]= input;
if ((input== '\r')|| (sendlength== 61))//CRorbuffe
for (bytei=0;i< sendlength;i++)
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if (radio.sendWithRetry(TONODEID,sendbuffer,sendleng
else //don'tuseACK
if (radio.receiveDone())//Gotone!
for (bytei=0;i< radio.DATALEN;i++)
if (radio.ACKRequested())
void Blink(bytePIN,int DELAY_MS)
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Running the Sketche
You now have two nodes that will send messages to each other, but, to use
them, we’ll need to open two serial terminals.
One way to do this is to run two separate Arduino IDEs. You’ll have to
actually start Arduino twice - you can’t just open a “new” code window from
the first IDE.
Set one IDE to the COM port of the first node and the other to the COM port
of the second. Then, open serial monitor windows from both IDEs.
In each serial monitor window, you’ll need to set the baud rate to 9600 and
make sure the “line ending” dropdown is set to “carriage return.” (The
example code uses carriage returns as a signal to send a packet.)
Once both windows are up, you should be able to type messages in the text
entry box at the top of one window and press return to send the message to
the other window. Try it!
You can also try turning ACKs and encryption on or off, using the
“broadcast address” (255), etc.
PROTIP: Having two Arduino IDEs open can lead to confusion when
compiling and uploading code. If you load the example code into both
IDEs, save one of them as a new name to keep the compilation and
uploading separate.
You can also try this one of the many serial terminals mentioned in this
Signal Strength
You might have noticed the “RSSI” number that the example code prints
out. “RSSI” stands for “Receive Signal Strength Indicator;” it’s a
measurement of how strong the transmission was when a message was
This number is in decibels (dB), which means that when nodes are close to
each other this will be a low number like -25, and, when they’re further
away, they’ll be higher numbers like -50. Usually you’ll ignore this number,
but you could use it to estimate how far away nodes are from each other,
provide a warning when the link is becoming weak, or, in sophisticated
applications, vary the transmit strength so that you don’t waste power
Resources and Going Further
For this tutorial, we connected both of the nodes to the same computer to
make it easy to send messages from one node to another. But, remember
that when you do your own projects, your nodes might be attached to
weather stations, garage doors, Halloween decorations, drones, robots,
your cat… the sky’s the limit!
The example code has comments that explain how messages are sent and
received. Feel free to use it as a basis for the code in your projects.
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For advanced usage, you can also check out the other examples and
source code in the library. The RFM69.h and RFM69.cpp files list all of the
library functions and tell you a little bit about what they do.
For all the hardware files, check out the links below.
RFM69HCW GitHub Repository
RFM69HCW Schematic
RFM69HCW Eagle Files
RFM69HCW Datasheet
For more wireless fun, check out these other great SparkFun tutorials:
GPS Shield Hookup Guide
This tutorial shows how to get
started with the SparkFun GPS
Shield and read and parse NMEA
data with a common GPS receiver.
XBee WiFi Hookup Guide
An overview of Digi's WiFi XBees,
and a quick how-to on getting them
connected and interfacing with the
Wireless Arduino
Programming with Electric
Reprogram your Arduino from
anywhere in the world using the
Tomatoless Boots wireless
bootloader with the Electric Imp.
Are You Okay? Widget
Use an Electric Imp and
accelerometer to create an "Are You
OK" widget. A cozy piece of
technology your friend or loved one
can nudge to let you know they're
OK from half-a-world away.
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