The "brain" for a Small RF repeater/relay

This code is the foundation of a simple UHF/VHF or any kind of RF repeater. It doesn't transmit or receive anything by itself, requiring two RF modules for that.

For instance, when the module 'A' receives a signal on its assigned channel, it will assert a pin, signaling the code to put the other module ('B') into transmission mode. The audio output of module 'A' is routed via a relay (NC) to the audio input of module 'B'. When the module 'A' finishes receiving the signal, the code will energize the relay, routing the audio input of the module 'B' into the filtered PWM output of the Arduino. A "tail" sound will be played. After this sound finishes, the code stops module 'B' transmission. The opposite happens when module 'B' receives a signal on its assigned channel.

There are some timers slots that can be assigned with intervals, where the inverval is expired, a sound will play to either the A or B module, or both.

All of these functions can be configured via a simple .ini file on an SD card. The sounds are stored there as well, on a FAT32 formatted card as WAV files, 8 bits/16kHz/mono.

Automagic builds

Go to https://github.com/cocus/picorepeater/actions and select PlatformIO to download the hex or elf for the latest PRs and pushes to the repo.

Requirements

• Any Arduino with an AVR M328
• An SD card adapter (with appropriate 3v3 level shifter if your Arduino is 5V)
• Visual Studio Code + PlatformIO for vscode
• Arduino Framework for AVR installed on PlatformIO

Hardware Setup

SD Card module connections:

Arduino	M328 Arduino Pin	SD Card
MISO	D12	MISO
MOSI	D11	MOSI
SCK	D13	CLK
D4	D4	/CS

RF Modules connections:

Arduino	Usage
D5	Module A PTT (TX)
D2	Module A Busy (RX)
D6	Module B PTT (TX)
D3	Module B Busy (RX)

Miscelaneous:

Arduino	Usage
D7	Blinky LED
D8	Relay control output for audio route
D9	PWM audio output, needs to be filtered

Note about D7: You'll need to remove the LED that's usually connected to the D13 pin to avoid some issues with the loading of the pin and the high speed mode of the SPI peripheral. That's why I've chosen the D7 pin for the blinky LED instead of the default D13.

Note about D8: It's encouraged to add a transistor to turn on the relay. Use a simple BJT to achieve this.

Note about D9: The PWM present on the pin D9 needs to be filtered out, I didn't get a really good performance using a RC filter, but neither I did with the RLC filter. I recommend the classic 1k and 100nF for it. But you can chose a better one. I strongly suggest you to check the amazing Filter Design and Analysis tools of OKAWA Electric Design.

For the RF modules, you'll need to connect the audio input and output to a relay as follows:

Relay Contact	RF Module connection
COM1 (common 1)	Module A audio input
NC1 (normally closed 1)	Module B audio output
NO1 (normally open 1)	Output of the PWM filter
COM2 (common 2)	Module B audio input
NC2 (normally closed 2)	Module A audio output
NO2 (normally open 2)	Output of the PWM filter

You might want to add potentiometers on each NC and NO poles of each throw of the relay so the volume can be tweaked independently for all the combinations. Use the wiper of the potentiometer as the output, one end to ground and the other ground to the NC or NO.

SD Card setup

An SD card of 1, 2, 4, 8GB should be used. It needs to be formatted in either FAT16 (FAT) or FAT32. A file called pico.ini should be placed on the SD card to make usage of the fancy features of this project. If not, this will act as a dumb relay. The structure of it should follow the standard structure of an ini file.

pico.ini Sections

All of the supported sub-sections should contain the play and out keys.

• play sets the file name to be played for the trigger of the sub-section.
• out sets the output module, can be A, B or * (both).

tail_a and tail_b Section

These sections specify the audio to play after reception ends on the A or B module. For instance:

[tail_a]
play = tail_a.wav
out = B

Will play the file tail_a.wav on module B after reception of the module A finishes.

timerX (X = 0 thru 4) Section

These sections specify the audio to play after a selected time passes. It will not play it if the modules are receiving/transmitting. The key period specifies the time to wait before playing the audio file. It's expressed in milliseconds. For instance:

[timer0]
play = play10s.wav
out = A
# 10s
period = 10000

Will play the file play10s.wav on module A every 10 seconds, if neither of the RF modules are busy.

Contributions

Please feel free to open an Issue to discuss features to be added, or bugs to be addressed. Incoming PRs are encouraged!