Modular and portable APRS platform: a miniaturized FSK modem and APRS system in a compact and extensible 50×35mm form factor, the size of a box of matches.
Matchbox is made of two boards, with a Li-Po battery sandwitched in between. The boards are connected using two flex cable connectors. Each board can be used standalone.
I’ve always liked the idea of small, inexpensive digipeaters that you could throw in a car and set up anywhere during an emergency. Imagine a truck deploying hundreds of them across an affected area. If they’re cheap enough that recovery isn’t a concern, and smart so that they don’t create congestion, it changes how we think about emergency communications. Projects like Meshtastic have shown how far modern features can be pushed into very low-cost hardware, and the same principle applies here. The goal is a device that’s affordable enough to be nearly disposable, but still flexible, modern, and genuinely useful in the real world. APRS today is often too expensive once you factor in demodulator robustness, size, and power. I’d like to see APRS succeed by becoming more democratized, accessible and widely adopted.
Second, I wanted a place to integrate the C++ APRS code I’ve been building over the years — a tracker, digipeater, and other libraries. Having a single platform you can hold in your hands that brings all of these pieces together makes it easier to develop, test, and see how they fit into the bigger picture.
The modem mainboard is based on the Nino TNC design and firmware. NinoTNC firmware and circuit copyright (C) Nino Carrillo (https://ninotnc.com).
The APRS system daughterboard adds standalone tracking and digipeating capabilities, with BLE/BT Classic access to the modem and Wi-Fi connectivity. The daughterboard controls the entire platform.
A 24-pin FFC expansion connector exposes modem mode selection, two serial ports (one to the TNC, one to USB), and status signals such as DCD and Packet Good.
Power is provided through a USB-C port via a low-noise LDO regulator with reverse-current protection. This design allows seamless switchover between USB and external 3.3 V supplies.
- Modular Two-Board Design
- Both boards measure 50 × 35 mm
- Connected with two FFC connectors for high-density, low-profile interconnect
- Li-Po battery is sandwiched between the boards and allows handheld operation
- Mainboard (Modem)
- Based on the Nino TNC design and firmware
- Provides full FSK modem and KISS TNC functionality
- Programmable via ICSP through the FFC connector (compatible with PICkit 5)
- Flexible supply options
- USB-C and 3.5mm TRRS audio jack
- Daughterboard (APRS system)
- Powered by ESP32 MCU — controls the entire platform
- Adds standalone tracker, digipeater, and igate functionality
- Digipeater and tracker are implemented using the libaprsroute and libaprstrack libraries, which are fully testable
- Small form factor: 50x35mm board size
- 0.8% larger than the Pico APRS v4 mainboard
- 37% smaller than the Mobilinkd TNC4 mainboard
- 63% smaller than the original Nino TNC mainboard
- Mainboard - modem
- Power
- Low noise linear regulator with reverse current protection
- Supports 4–20V input, up to 500mA output
- Flexible power options: USB power can be bypassed (FFC2), supplied externally (via FFC2 from 4-20V), or the board can be powered externally by a 3.3V supply (FFC1)
- Interfaces
- USB VBUS and CC1/CC2 available for optional USB-PD negotiation
- Up to 3.2A from USB PD
- Voltages higher than 5V can be negotiated and supplied to an off board regulator via FFC2
- Two UARTs (USB interface, TNC interface)
- Can be connected together by onboard resistors in standalone operation
- Can be router off the board via FFC1
DCDandPacket Goodstatus signals available on FFC1
- USB VBUS and CC1/CC2 available for optional USB-PD negotiation
- Standalone operation
- The mainboard can be configured to be used standalone without a daughter board
- Internal solder pads or resistors connect the USB serial interface to the TNC serial port
- Placeholder resistors configure the TNC digital modes
- Audio I/O
- 3.5 mm TRRS jack (TXA/RXA/PTT), selectable for Mobilinkd TNC4 or Digirig compatibility
- Status
- All of the 5 LEDs on the Nino TNC are routed to display modem status
- TX (red), DCD (yellow), RX (green), TX QUEUE (blue), CRC (red)
- 0201 LEDs are used to minimize board space
- All of the 5 LEDs on the Nino TNC are routed to display modem status
- Additional configuration options: AC/DC coupling, mic/data mode, 1x/11x TX range
- Adjustable TX delay and TX deviation via adjustable miniature resistors
- Extensible interconnect for optional daughterboards (see FFC1 and FFC2 specification)
- Power consumption about 50mA at 3.3V or about 0.2W
- Power
- Daughter board - Standalone Digipeater and tracker
- Power management
- Integrated 1.5 A Li-Po linear charger
- Power-path management automatically switches between battery and USB power
- Buck-boost regulator provides stable 5V rail to the mainboard regardless of the battery voltage
- Independent power
- Low-noise LDO for daughterboard power
- Two load switches allow the MCU to power-gate the GNSS and mainboard and fully turn off both devices programatically
- Push button controller starts the system via a tactile switch with MCU soft power off
- Auto start and
always onare also supported
- Auto start and
- Fuel gauge with voltage and current measurement for accurate state of charge estimation
- Power bypass enables power from an external source like an solar MPPT regulator
- GNSS
- Integrated GNSS receiver with onboard antenna
- Retains warm-start capability via small rechargeable backup cell
- GNSS receiver data can be used by the tracker, or passed through to the USB interface
- Tracker
- APRS packet types: position, Mic-E, compressed
- Mic-E status
- With or without timestamp (DHM/HMS formats), UTC or local
- Position ambiguity
- Speed, course, altitude for both position packets and Mic-E packets
- Custom messages with UTF-8 support
- Smart-beaconing, Periodic or Manual beacon trigger
- Flexible APRS encoding and tracker library powered by libaprstrack
- Position/speed/direction/altitude and time supplied by the onboard GNSS receiver
- DCD is used to determine if the channel is busy and optionally can be used to prevent the tracker from TX over existing transmissions
- Digipeater
- Powerful fully featured and fully extensible digipeater powered by libaprsroute
- APRS routing: explicit, n-N
- Supports all types of valid n-N addresses, ex: DIGIn-N
- Preemtive digipeating: front, truncate, drop and mark
- Deduplication, trap or reject excessive hops, optional explicit address substitution, optional completed n-N address substitution
- Extensive routing disgnostics can be used to reconstruct the packet post digipeating or make intelligent decisions
- Viscous digipeating (hold time)
- DCD is used to determine if the channel is busy and optionally can be used to dynamically adjust the viscous delay to prevent the digipeater from TX over existing transmissions
- Fill-in / direct only option
- Digipeater implementation is completely standalone with no coupling to the MCU or other systems
- RX igate
- RX packets can be gated to APRS-IS
- Enabling the igate does not disable the digipeater
- Timekeeping
- Time is supplied by the onboard GNSS receiver.
- When a GPS lock is not available, time is supplied by a real-time clock. The GNSS receiver maintains the accuracy of the RTC.
- Onboard CR1025 lithium battery maintains the RTC time.
- KISS TNC
- Accessible over the USB, Bluetooth or Wi-Fi
- GNSS data can also be routed throught the same interfaces
- APRS packets can optionally be sent and received in plain text instead of AX.25
- Simple text based command interface can be used to configure the modem, tracker, digipeater, and GNSS receiver
- Digital modes can be configured programatically (M0-M3)
- The capabilities of the interface are the same whether it is USB, Wi-Fi or BT
- RXA/TXA Calibration
- TX calibration using Bessel null. The daughter board can instruct the modem to send a sweeping test signal in the TX chain.
- RX calibration allows detecting RX chain signal clipping and help set the right sound volume on the radio.
- Standalone
- The tracker, digipeater and igate operation are fully standalone; they run directly on the ESP32 MCU
- A computer or smartphone is not needed other than for the initial configuration
- The wired (USB) and wireless (BT, Wi-Fi) are only used for configuration (with the exception of the igate)
- The tracker and digipeater can operate fully off-grid with no wired or wireless connections, with no computers, with no smartphones or any other devices
- Only the igate requires a Wi-Fi and internet connection to connect to APRS-IS
- Wi-Fi
- Operates in client mode
- Credentials are supplied via USB or BT
- 1 x TCP client supports the igate
- 1 x TCP server supports the KISS TNC over Wi-Fi
- 1 x HTTP server supports the configuration pages
- Connectivity
- USB serial
- Both BT Classic and BLE are supported
- Wi-Fi
- Power management
The 24 pin connector used is FFC2B28-24-G, it can be used with a flat flex cable 05-24-A-0030-A-4-06-4-T.
| Pin | Label | Description |
|---|---|---|
| 1 | USB VBUS | USB bus supply |
| 2 | USB VBUS | USB bus supply (duplicate pin for higher current or alternate routing) |
| 3 | USB VBUS | USB bus supply (duplicate pin for higher current or alternate routing) |
| 4 | USB VBUS | USB bus supply (duplicate pin for higher current or alternate routing) |
| 5 | GND | Ground |
| 6 | GND | Ground |
| 7 | GND | Ground |
| 8 | GND | Ground |
| 9 | RX1 | UART1 receive input to the USB interface |
| 10 | TX1 | UART1 transmit output from the USB interface |
| 11 | RX2 | UART2 receive input to the TNC |
| 12 | TX2 | UART2 transmit output from the TNC |
| 13 | PKT | Packet received signal |
| 14 | PGED1/M0 | ICSP data / Programming Data (PGED1) or Mode 0 selection |
| 15 | PGEC1/M1 | ICSP clock / Programming Clock (PGEC1) or Mode 1 selection |
| 16 | MCLR | ICSP reset / Master Clear / Reset input for microcontroller |
| 17 | M2 | Mode 2 selection |
| 18 | M3 | Mode 3 selection |
| 19 | TEST TX | Test transmit output |
| 20 | DCD | Packet data clock detection |
| 21 | GND | Ground |
| 22 | +3.3V | +3.3V supply from the LDO; can also be used to power the board |
| 23 | Not connected / reserved | |
| 24 | Not connected / reserved |
The 12 pin connector used is FFC2B28-12-G, it can be used with a flat flex cable 05-12-A-0050-A-4-06-4-T.
FCC2 connector is optional if pins 3, 4, 5, 6, 11 and 12 are tied together using the on board resistor footprint R7 marked "PWR"
Pins 11 and 12 supplies 5V to the LDO. Higher voltage can also be supplied up to +20V
| Pin | Label | Description |
|---|---|---|
| 1 | USB CC2 | USB PD configuration |
| 2 | USB CC1 | USB PD configuration |
| 3 | USB VBUS | USB bus supply |
| 4 | USB VBUS | USB bus supply (duplicate pin for higher current) |
| 5 | USB VBUS | USB bus supply (duplicate pin for higher current) |
| 6 | USB VBUS | USB bus supply (duplicate pin for higher current) |
| 7 | GND | Ground |
| 8 | GND | Ground |
| 9 | GND | Ground |
| 10 | GND | Ground |
| 11 | +5V | LDO +5V supply, typically to be connected to USB VBUS |
| 12 | +5V | LDO +5V supply, typically to be connected to USB VBUS |
The on board LDO on the can run from as low as 4V for its maximum rated current of 500mA
Higher voltages can also be supplied to the LDO for up to +20V
The typical board consumption is about 50mA
Because the LDO has reverse current protection, the board can be powered externally from 3.3V, while simultanously powered by the USB-C port
Finally, I wanted a platform that's simple to maintain and contribute to, with enough hardware flexibility that it can be used in scenarios I haven’t even thought of yet.
Nino TNC firmware and circuit copyright Nino Carrillo (https://ninotnc.com).
Gerber files copyright Ion Todirel. The gerber files can be used for non commercial purposes. The gerber files can be used by the Nino TNC project creators and contributors for any purpose as long as the following notice is reproduced:
matchbox APRS
Copyright (C) 2025 Ion Todirel
libaprs, libaprsroute and libaprstrack copyright Ion Todirel under MIT license.
All the other assets in this repository are in the public domain.