LoRa APRS Tracker

N7WDZ-21: a battery-powered LoRa APRS tracker in a waterproof case, with a plug-in solar panel for longer trips

Yellow waterproof case with a short antenna, held by magnets on a silver car roof, with a small solar panel plugged into its side — Ready to roll: the tracker and its solar panel held on the car roof by magnets

APRS trackers send their GPS position over the radio every so often, so their travels show up on maps like aprs.fi. This one does it with LoRa, a long-range, very low-power radio mode, on the 433 MHz (70 cm) band. Its packets are picked up by LoRa iGates, including my own N7WDZ-25, and passed on to the APRS internet network.

Inside the yellow case is a LILYGO LoRa32 board running the free CA2RXU LoRa APRS Tracker firmware, a GPS module, a two-way booster that raises the transmit power to about 2.3 watts, and a 10,000 mAh battery. The firmware uses smart beaconing: it sends more often while driving and on every turn, and only now and then while parked, which keeps the track accurate without wasting battery. For multi-day trips a small 6-volt solar panel plugs into a waterproof port on the side.

The firmware holds three profiles, each with its own SSID and its own beacon settings: fast, medium and slow. The fast profile gives the most detailed track, and the slow one saves the most battery on long trips. I can switch between them from the tracker's menu with the select button. Right now it's running as N7WDZ-21.

433 MHzLoRa APRS

~2.3 Wtransmit power

10,000 mAhLiPo battery

2½–4 dayswithout sun

18.5 × 8 × 6 cmwaterproof case

Video

Video coming soon I'm putting together a video of the build, from the parts on the bench to the first track on the map.

How it's wired

The LILYGO board can charge the battery through its USB port, and it powers the GPS from its 5-volt pin. The booster needs more current than the board can supply, so it gets its own boost converter that raises the battery's 3.7 volts to a steady 5 volts. At 5 volts the booster puts out about 2.3 watts, compared with 1.4 watts straight off the battery.

The solar panel started out built into the case, but it made the box much bigger than it needed to be, and most trips don't need it. Now the panel and its CN3791 MPPT charger are one separate unit, with the charger sealed in heat-shrink right on the cable, that plugs into the waterproof charge port on the side of the case.

The build

A 6-volt solar panel, a boxed 10,000 mAh battery, the LILYGO board, the booster and the boost converter laid out on a cutting mat — The main parts: solar panel, 10,000 mAh battery, LILYGO board, booster and boost converter

Close-up of the tiny boost converter, the LILYGO LoRa32 board and the AB-IOT-433 booster with its SMA connectors — Boost converter, LILYGO LoRa32 T3 v1.6.1 and the AB-IOT-433 two-way booster

Small blue CN3791 charger board with a red and black JST lead on each end — The CN3791 solar charger, before it was sealed into the panel's cable

Small black GT-U7 GPS module held in a hand, with four wires soldered to it — The GT-U7 GPS module, wired up

Open yellow case: the booster and LILYGO board on a black 3D-printed plate in the bottom, the GPS in the lid — Everything in place on a 3D-printed mounting plate, with the GPS in the lid

Close-up of the LILYGO board's screen and the booster, joined by a short coax jumper — The LILYGO board feeds the booster through a short coax jumper

Closed yellow case with a gray latch and a short black antenna on top — Closed up, with the antenna on top

End of the yellow case showing a round black waterproof connector with a cap — The waterproof M16 charge port for the solar panel

Bottom of the case with two large rubber-coated mounting magnets, next to the solar panel with mounting magnets on its frame — Rubber-coated magnets hold the case and the solar panel in place

On the map

Map of the Las Vegas valley with a green track line looping through the northwest, and red LoRa iGate markers around town — A drive around the northwest valley, drawn from the tracker's beacons. The red diamonds are LoRa iGates that can hear it. Map data © OpenStreetMap contributors.

Parts

LILYGO LoRa32 T3 v1.6.1, 433 MHz: ESP32 with a LoRa radio, OLED screen and a built-in battery charger
AirBuddy AB-IOT-433 two-way LoRa booster: about 2.3 watts transmit at 5 volts, switches between receive and transmit on its own
GT-U7 GPS module (u-blox NEO-6M compatible)
10,000 mAh 3.7 V LiPo battery with a built-in protection board
AITRIP mini boost converter, set to 5 volts, for the booster only
Master on/off switch and a push button
Waterproof case, 18.5 × 8 × 6 cm, with a 3D-printed mounting plate
Solar accessory: 6 V 5 W solar panel and CN3791 MPPT solar charger (6-volt version), with an 18 AWG M16 two-pin waterproof connector

Battery life

Estimated time on a full charge with no sun. Driving uses more power because the tracker transmits much more often.

Use	Average draw	Runtime
Parked (a beacon every 15–30 min)	about 105 mA	about 4 days
Driving (a beacon every 30–60 s)	about 145–155 mA	about 2½ days

In full sun the 5-watt panel puts back up to about 0.8 amps.

Wiring the GPS and button

The T3 v1.6.1 has no GPS or user button of its own, so both are added by hand. These pins match the firmware's "LoRa32 v2.1/1.6.1 + GPS (DIY)" board setting.

From	To LILYGO pin	Notes
GPS VCC	5V	Works on battery alone, no USB needed
GPS GND	GND
GPS TXD	GPIO 34	The board receives GPS data here
GPS RXD	GPIO 12	A boot "strapping" pin; first suspect if boot problems ever show up
Push button	GPIO 15 + GND	Select button for screens, the menu and switching profiles

Lessons learned

The board's on/off switch isn't enough. It turns off the LILYGO board and the GPS, but the booster stays powered through the boost converter. A master switch in the battery lead now turns everything off. It also cuts charging, so the switch has to be on to charge.
Turn the GPS on in the firmware. The tracker settings include a disableGPS option; it has to be false or the firmware ignores the GPS module.
Check battery polarity. Some LILYGO boards have the battery connector reversed, so check before plugging in, and label the charge port.
Add a button. The T3 v1.6.1 only has a reset button. The firmware expects a select button, so a push button goes between GPIO 15 and ground.
Slow the beacons down on long drives. A fast rate of 90–120 seconds instead of 30–60 means fewer high-power transmissions, and the track still looks good because the firmware beacons on every turn. The slow profile is there for exactly that.

Links

N7WDZ-21 on aprs.fi My LoRa APRS iGate CA2RXU LoRa APRS Tracker firmware LILYGO LoRa32 board

Credits

The LoRa APRS Tracker firmware is by Ricardo Guzman, CA2RXU, and is free and open source. Thanks to the LoRa APRS community on Discord for confirming the GPS and button pins. The map above is from the LoRa APRS map by EA2RCF Radioclub Foronda.