Hook: Mads Tech demonstrates a cheap hack to arm DJI O3/O4 air units without a flight controller. It restores full RF power and can auto-arm the unit using an ExpressLRS receiver.
TL:DR
ExpressLRS can output MSP DisplayPort over a UART. That lets you send an arming command to DJI air units (O3, O4, Avatar HD and similar). Flash the ELRS master (or v4.0 when public), set the receiver UART protocol to DisplayPort, wire TX↔RX, and you get full RF power without a flight controller. You can also enable a permanent arming mode in the receiver WebUI to auto-arm on power-up.
Why this matters
DJI O3 shipped with a quirk: turning off low-power mode in the menu does not give maximum RF power. The air unit requires an MSP arming command to hit full transmitter power. That blocked pilots who run ExpressLRS without an FC from getting full range.
What ExpressLRS now does
The ExpressLRS team added UART output support for MSP DisplayPort. ELRS can now send the same arming commands flight controllers used to send. You can trigger arming from the radio or let a receiver auto-arm the air unit at power-up.
Hardware you need
Parts list: an ExpressLRS receiver, the DJI O3/O4 ear unit, and a 5V BEC for the receiver. Most ELRS receivers accept max 5V. O3/O4 runs off higher pack voltages, so power the receiver via BEC.
Wiring: TX on the ELRS receiver → RX pad on the O3; RX on the receiver → TX pad on the O3. That mirrors a normal FC UART connection. Provide the receiver with stable 5V from the BEC.
Software: flash and configure
At recording time, Mads used the ExpressLRS master build. The feature will land in public ELRS 4.0. Use the ELRS Configurator, pick git branch "master" (or v4.0 once released), and flash your receiver as usual.
After flashing, connect the receiver to your radio, open the ELRS script, and go to Other Devices. Select your receiver and change the UART output protocol from CRSF to DisplayPort. Reboot the receiver for the change to take effect.
Demo: arm from your transmitter
With protocol set to DisplayPort you can arm via a mapped switch on your ELRS radio. Hit arm—O3 exits low-power mode, switches to full RF power, and can start recording if configured. That delivers the maximum FCC power that the menu toggle alone didn’t provide.
Permanent arming (no radio required)
Want the air unit to auto-arm without any transmitter? ELRS adds a receiver WebUI option: "permanently arm DJI Ear Units". Put the receiver into Wi‑Fi mode (power it without the radio until the LED flashes), connect to 10.0.0.1, go to Options and enable the permanent arming toggle. Save and reboot.
Power sequence matters. If everything powers at once the O3 may not start recording even though it reaches full RF power. Mads found it more reliable if the ELRS receiver powers a second or two after the O3. Use a separate switch or a PWM-switched feed to delay the receiver if you need consistent auto-recording.
Compatibility and caveats
Mads tested this on O3 but the MSP DisplayPort arming should work on all DJI air units that accept MSP DisplayPort (O3, O4, Avatar HD, etc.). Test before you fly; more testing across firmware and hardware variants will refine compatibility.
Also note: run the receiver off a proper 5V BEC. Don’t feed 5V receivers directly from a 6S battery. Check voltages and ground connections before powering up.
Thanks, credit and how to support
Mads thanks the ExpressLRS devs and calls out Wes Varty for implementing this. The permanent arming option was requested by Mads and added to master. If you use ELRS, consider donating to the project via Open Collective.
FAQ
Will this work with DJI O4 and other air units?
Yes—if the air unit supports MSP DisplayPort. Mads tested on O3. Expect O4 and Avatar HD to behave similarly, but always bench-test.
Do I need a flight controller?
No. That is the point. ELRS can send MSP arming over UART, removing the FC as a required intermediary.
Which firmware should I flash?
Use ELRS master branch now if you want the feature immediately. When ELRS 4.0 releases publicly, flash that stable build instead.
Any quirks I should know?
Power sequencing and auto-recording can be temperamental. Delay the receiver’s power-on slightly if auto-recording fails on simultaneous power-up.
Conclusion
This is a tidy fix for a dumb DJI quirk. ELRS adds MSP DisplayPort output and a permanent arming mode. That gives pilots full RF power and optional auto-arm without a flight controller. It’s cheap, reproducible, and useful for fixed-wing and ground rigs.
Takeaway
Tweet-sized nuggets for skimmers:
- ELRS now speaks MSP DisplayPort—arm DJI air units via UART.
- No FC required—use a single ELRS receiver and a 5V BEC.
- Permanent arming—enable via receiver WebUI (10.0.0.1) for auto-arm.
- Watch power sequencing—delay the ELRS receiver for reliable auto-recording.
This article was based from the video DJI FPV Airunit Arming Without Flight Controller On ExpressLRS- O3 & O4 No FC Needed
