Build one

Build your own transmitter, receiver or dongle. Then test it safely and fly it. Everything here is free and open.

Start with the transmitter Testing before the first flight

What you can build

LockDownRadioControl has three parts you can build. To fly, you need a transmitter and a receiver. The dongle is for pilots who keep the radio they have.

The black Version 2 transmitter: two sticks, the green status light, the power button and the 5-inch touchscreen showing a front screen of nine coloured boxes

The transmitter

Two sticks, eight switches, a 5-inch touchscreen and the radio. This is Version 2.

Cost: over £120 in parts (approximate; the full total is being worked out)
Time: a few evenings, plus printing the case
Skills: soldering, crimping, 3D printing, and installing PlatformIO to type a few commands

Build the transmitter

A 3-D view of the Version 2 receiver's circuit board: the XIAO's pads, a regulator, a radio module, a status light and the lead's socket

The receiver

Goes in the model. Up to three radios for a steadier link. Talks to your phone.

Cost: about £35 of parts (approximate)
Board: files coming soon; wire it by hand for now
Time: an evening, wired by hand
Skills: fine soldering, a web browser

Build the receiver

The dongle

Rotorflight set-up from your phone, on any helicopter, whatever radio flies it.

Cost: about £7 for the XIAO; the lead comes with a Nexus
Time: under an hour
Skills: four solder joints

Build the dongle

Tools you need

A soldering iron with a fine tip, solder and flux.
A multimeter. You will use it to check every power lead before you plug it in.
Wire strippers, side cutters and heat-shrink sleeve.
A crimp tool for DuPont connectors, or ready-made DuPont leads.
A magnifier, to check small joints.
A 3D printer, or a printing service, for the transmitter's case.
A computer (Mac, Windows or Linux) with Chrome or Edge.
Software: PlatformIO (free, inside VS Code), to flash the transmitter. Some computers also need the CH340 USB driver for the screen.
Data cables: USB-C for the screen and the receiver, micro-USB for the Teensy. Many cables only charge. Use ones that carry data.

Is it for you?

Buy a shop radio if you want one that works out of the box, with a warranty and dealer support.

Build LockDownRadioControl if you enjoy making things, and want to set up and tune Rotorflight helicopters from the transmitter or your phone. It also loads the right model memory by itself when you switch the model on. That is rare: elsewhere we found it only on Multiplex's SMART SX and Mikado's VBar Control.

And it is yours to change. Its author often adds a feature within a day of a pilot asking, and everything is open, so you can change it yourself. A shop radio is almost always take it or leave it.

Many Version 2 features are new (October 2026). The radio link itself has flown since 2020.

Before you start: the law and safety

Radio rules

The radio modules that do the transmitting and receiving carry the CE mark and FCC ID 2ALPH-E01: Ebyte's E01-ML01DP5 in the transmitter and E01-ML01SP4 in the receiver.

They run at about 100 mW (20 dBm), the UK and EU limit for 2.4 GHz. An aerial cannot add power: it only shapes where the signal goes.

As with any model you build yourself, check the radio rules where you live before you fly.

Safety: blades off

Take the rotor blades or the propeller off for every test on this page. Unplug the motor too, unless the test needs it. A spinning rotor can cause serious injury.

Safety: the battery

Charge the 2S pack with a balance charger made for its type. A LiFePO4 pack needs a LiFePO4 charger (3.6 V a cell). A LiPo or Li-ion charger (4.2 V a cell) overcharges it.
Never leave it charging unattended.
If it swells or gets hot, stop charging it and stop using it.
Fitting a charger module in the case? Check it is set for two cells before the first charge.

No warranty

LockDownRadioControl is free software and open hardware. It comes with no warranty. You build it and fly it at your own risk.

The transmitter

The Version 2 transmitter has a Teensy 4.1 main board, an Ebyte radio module, two FrSky Hall-sensor sticks, eight switches, and a 5-inch touchscreen (the Elecrow CrowPanel 5.0).

The Teensy flies the model: sticks, mixing and the radio link.
The screen talks to you: the pages, WiFi, Bluetooth, sound and updates.

Already have a Version 1 transmitter?

It becomes a Version 2 when you swap its Nextion screen for the CrowPanel and update the firmware.

Your models, main board, sticks and switches stay.
Print the Version 2 case front: it holds the new screen.
Lift the Teensy out of its socket to flash it. Do not plug USB into it while it is on the board.
Keep your Teensy's own card. It holds your models. Copy it to a computer first. Then add the example's help and Images folders to it. Do not replace MODELS.DAT or mod.

Go to Printing the case, Flashing (skip step 8), Wiring the screen and First power-up. Then do every check in Testing before the first flight before the first flight.

Parts list

Prices are approximate, for parts only, in pounds, in October 2026. A dash means we have no reliable price yet.

Buy the right screen

Get the Elecrow CrowPanel 5.0" ESP32 HMI, DIS07050H, N4R8 version. Do not buy the 16 MB "IPS" model (SKU DLE06050B). Its pins are different, and today's firmware shows a black screen on it.

Easy to miss: the INA219

Older parts lists left out the INA219 voltage sensor. Without it, the transmitter cannot read its own battery.

Part	Exact type	How many	What it is for	Where to buy	About
The main board
Circuit board	2 layers, about 83 × 91 mm, from the Gerber files	1	Carries everything else	A PCB maker (Malcolm uses JLCPCB)	–
Teensy 4.1	PJRC Teensy 4.1 (600 MHz)	1	The transmitter's computer: reads the sticks, runs the radio link	PJRC, The Pi Hut, Cool Components	£30
Radio module	Ebyte E01-ML01DP5 (nRF24L01+ with amplifier)	1	The 2.4 GHz radio	Ebyte's own shops	–
Voltage sensor	INA219 breakout module, I2C address 0x40	1	Measures the transmitter's battery	Adafruit, SparkFun, or a compatible module	£5
Clock	"Tiny RTC" DS1307 module with its coin cell	1	Date and time for the logs. The screen sets it from the internet	Electronics shops	–
Power switch	Pololu 2808 pushbutton power switch	1	Switches the transmitter on and off. Lets the Teensy switch itself off	Pololu, or a reseller	£5
5 V regulators	Pololu S13V10F5 (5 V, 1 A), or a 7805	2	One 5 V supply for the Teensy, one for the screen	Pololu; any electronics shop	–
3.3 V regulator	AMS1117-3.3	1	3.3 V for the radio, the sticks and the sensors	Any electronics shop	–
Diodes	1N5406 × 2, 1N4001 × 2	4	Protect the regulators and the power button's circuit	Any electronics shop	–
Capacitors	Electrolytic: 330 µF, 100 µF. Tantalum: 330 µF (0E907) × 2, 47 µF (1206) × 2. Ceramic, 1206: 330 nF × 2, 100 nF × 4, 10 µF × 1	13	Keep the supplies smooth	Any electronics shop	–
The screen
Touchscreen	Elecrow CrowPanel 5.0" ESP32 HMI, DIS07050H, N4R8 (ESP32-S3, 4 MB flash, 8 MB PSRAM, 800 × 480)	1	The pages, WiFi, Bluetooth, sound and updates	Elecrow, RobotShop	£40–£43
microSD card for the screen	Any size, formatted FAT32	1	Pages, fonts, sounds and pictures. The screen fills it itself	Anywhere	–
Speaker	8 Ω, 1–2 W (a Nextion's speaker will do); a 4 Ω one is louder	1	Voice and sounds. Plugs into the screen's SPK port (2-pin PH2.0)	Electronics shops	–
Screen data lead	HY2.0 4-pin lead ("Grove" style)	1	Joins the screen's UART0 port to the main board	Electronics shops	–
Screen power lead	A short USB-C lead, its far end cut off	1	Takes 5 V from the main board into the screen's USB-C socket	Anywhere	–
Sticks, switches and lights
Sticks	FrSky M9 Hall-sensor gimbals	2	The two sticks	FrSky dealers, model shops	£40–£60 a pair
Trims	FrSky trim switches (X9D Plus spare part)	4	Digital trims beside the sticks	Model shops	–
Top-edge switches	Three-position RC switches	4	Safety, banks, rates and more: you choose each one's job	Model shops, electronics shops	–
Front switches or knobs	Three-position switches, or knobs (potentiometers)	4	Channels 5 to 8, or any job	Model shops, electronics shops	–
Power button	16 mm vandal-resistant, momentary push button	1	On and off; held at switch-on, it starts binding	Electronics shops	–
Status light	5 mm RGB LED, common cathode	1	Red: trying to connect. Green: connected. Blue: busy	Electronics shops	–
Power and aerial
Battery	2S pack (two cells): LiPo, Li-ion or LiFePO4	1	Powers the transmitter	Model shops	–
Charger Optional	A 2S balance charger module: micro-USB, or a small USB-C 2S module	1	Charges the pack inside the case. Must match the battery type: LiPo/Li-ion or LiFePO4	Electronics shops	–
Aerial	2.4 GHz RP-SMA aerial, with a 20 cm RP-SMA male-to-female extension	1	Brings the aerial outside the case. Check your radio module's socket before you buy	Electronics shops	–
Cards, wiring and case
microSD card for the Teensy	Any size, FAT32 or exFAT	1	Models, logs and the help texts	Anywhere	–
Connectors	Male and female headers, three-row headers, a 2×4 socket (for the radio), servo cable, DuPont 3- and 4-pin connectors	Various	Join the controls to the board	Electronics shops	–
Case	3D-printed, in PETG or similar	1 set	Holds everything	Print it yourself	–
Fixings	Self-tapping screws (3 mm, 2 mm flange), M3 bolts with locknuts, 2 mm cable ties, five-minute epoxy	Various	Hold the case and the parts together	Hardware shops	–
Total	Parts only, approximate. Prices change. The priced parts alone come to about £120–£143. The full total is being worked out.				Over £120

The main board

Version 2 uses the same main board as Version 1. Its Gerber files (the files a PCB maker needs) are in the repository.

Download the repository

On GitHub, press Code, then Download ZIP. Unzip it.
Zip the Gerber files

Open the folder TXV2/Circuits/Transmitter - CADCAM. Put every file in it into one zip file.
Order the boards

Upload the zip to a PCB maker. Choose 2 layers and 1.6 mm thickness. The board is about 83 × 91 mm.

Makers usually sell at least five boards. Share the spares with friends.

Check: the maker's preview looks like the picture below.

The transmitter's main board from above, drawn from its Gerber files: places for the Teensy 4.1, the radio module, the Tiny RTC clock, the INA219 sensor, the Pololu 2808 power switch, two regulators, and the headers for trims, edge switches, channels 1 to 8, the LED, the button, the battery and the screen — The top of the main board, drawn from its Gerber files. Every connector has its name printed beside it.

The underside of the transmitter's main board, drawn from its Gerber files, showing the copper tracks — The underside.

The board's design is a Proteus project (TXV2/Circuits/TransmitterMay24.pdsprj), under the CERN-OHL-S-2.0 licence. See the Gerber files on GitHub.

Printing the case

Previews of eight printed parts: the Version 2 case front with its 5-inch screen opening, the Version 2 case back, the handle, the inner support plate, the module bay, the Version 2 charger bay, a gimbal protector and the LED mount — The printed parts, drawn from their design files.

The Version 2 parts are in TXV2/STL files/V2: the case front for the 5-inch screen and the case back (STEP files: most slicers open them directly), and the charger bay (STL).
The other parts are in TXV2/STL files: handle, inner support plate, gimbal protectors, LED mount, module bay and its blank cover, and a spacer. STEP files for editing are in STL files/STEP files.
Print in PETG or a similar tough plastic.
The screen has no mounting holes. The Version 2 case front holds it.
RX.stl and RX_LID.stl in that folder are the Version 1 receiver's case. The Version 2 receiver's case is in RXV2/case.

Flashing the screen and the Teensy

Do this first, on the bench, before anything is wired in. The screen's USB-C socket will later carry its power, and the Teensy is easiest to flash on its own. After this, updates come over WiFi, and you should not need the cables again.

Install PlatformIO and get the code

About 15 minutes, the first time

Install PlatformIO (the VS Code extension, or the pio command). Then get the code:
```
git clone https://github.com/Mmessiter/LDRC_V2.git
```
Or download the ZIP: on GitHub press Code, then Download ZIP (as in step 1). It unzips to a folder called LDRC_V2-main. Rename it LDRC_V2, so the commands below work as written.

Type the commands below in a terminal, in the folder that holds LDRC_V2. The first build also downloads the compilers, so it takes a while.
Flash the screen

About 5 minutes

Put a blank microSD card, formatted FAT32, in the screen's slot before you power it. The screen only looks for its card when it starts. Cards over 32 GB come as exFAT: reformat them.

Plug the screen into your computer with a USB-C data cable. Find its port, then flash it:
```
cd LDRC_V2/TXV2-Screen
pio device list
pio run -e crowpanel50 -t upload --upload-port /dev/cu.usbserial-XXXX
```
Put your own port in place of /dev/cu.usbserial-XXXX. On Windows it looks like COM3. No buttons to press: the screen resets itself.

No port? The screen's USB chip is a CH340. Your computer may need its driver.

Check: PlatformIO ends with SUCCESS.
Give the screen WiFi and let it fill its card

About 10 minutes

The new screen says No WiFi yet. Tap WiFi setup. Choose your network and type its password on the screen's keyboard.

The screen then offers the latest release from messiter.com. Tap Install now. It fetches about 260 files (about 43 MB) in about seven minutes, then restarts as a complete screen.
- The screen's WiFi is 2.4 GHz only.
- An iPhone hotspot works if you switch on Maximise Compatibility.
- The password stays in the screen's chip. It is never written to the card.
The WiFi page. Up to eight networks are remembered.

Check: the screen fills its card and restarts by itself.
Flash the Teensy, on its own

About 5 minutes

Plug the Teensy 4.1 into your computer with a micro-USB data cable. Keep it out of the main board for now.

Teensy already in the board?

Lift it out of its socket and flash it on its own. Do not plug USB into a Teensy fitted to the board unless the pad between VUSB and VIN has been cut: USB would power the whole board.

Still in TXV2-Screen from the screen step? Type cd ../.. first, to get back to the folder that holds LDRC_V2.
```
cd LDRC_V2/TXV2/TransmitterCode
pio run -e teensy41 -t upload
```
PlatformIO fetches the libraries and runs the Teensy loader. If it sits waiting, press the small white button on the Teensy. On Linux, install PJRC's udev rules first.

Check: PlatformIO ends with SUCCESS.
Prepare the Teensy's card

Version 1 owner? Skip this step. Keep your own card.

Format a microSD card as FAT32 or exFAT. Copy everything inside TXV2/TransmitterCode/SD Examples/Teensy SD Example onto it: the folders help, Images and mod, and the file MODELS.DAT. Then make an empty folder called log.

The example models are Malcolm's. Step 21 (Reset transmitter) clears them from the model memories. Their backup files stay in the mod folder, if you want to look at them.

Check: the card's top level shows help, Images, log, mod and MODELS.DAT.

Assembly

Note

There are no step-by-step photos of the inside yet. The board's printed labels name every connector. Take your time, and check each joint with a magnifier.

Solder the small parts

Fit the diodes, the capacitors, the 3.3 V regulator (AMS1117) and the two 5 V regulators. The regulator marked 7805 feeds the Teensy. The one marked NEXTION feeds the screen.

Watch which way round the diodes and the electrolytic and tantalum capacitors go.

Check: no solder bridges between pads.
Fit the headers and sockets

Fit a socket for the Teensy, the 2×4 socket for the radio (DP1), and headers for the power switch, the INA219 and the clock. Then the three-row headers for TRIMS, EDGE SWITCHES and CHANNELS 1-8, and the headers for the LED, the button, the battery and the screen.
Test the power before the modules go in

Safety: battery the right way round

Before the first switch-on, put the meter on volts across the battery's own plug. Red probe on the pin that will meet + on the board: it must read plus 6 to 8.4 V, not minus. A battery the wrong way round can destroy the board at once.

Fit only the Pololu 2808 power switch, the power button and the battery lead (BATTERY header). Leave the Teensy, the radio and the other modules out.
1. Before the battery goes on, measure the resistance between + and − on the battery plug. It must not be close to zero.
2. Connect the battery and press the button. Measure:
  - 5 V on the Teensy socket's VIN pin;
  - 5 V on the middle pin (pin 3) of the screen header;
  - 3.3 V on the radio socket's VCC pin (next to its GND pin).
3. Find pin 1 of the screen header now: battery off, meter on beep. Pin 1 is the end pin that beeps to the battery's −. Mark it.
Check: all three voltages are right. If not, switch off and find the fault before you go on.
Fit the modules

Aerial on before you switch on

Screw the aerial extension and the aerial onto the radio now. The radio transmits at full power from switch-on; with no aerial its amplifier can be damaged.

Fit the Teensy (with its card), the ML01DP5 radio, the INA219 and the Tiny RTC clock. Match each one to the outline printed on the board.
- The clock: connect it by its five-pin row (DS, SCL, SDA, VCC, GND). On some newer modules the seven-pin row's "GND" hole is joined to the coin cell instead of ground.
- The clock needs 5 V. On 3.3 V it keeps time but does not answer.
Check: nothing gets warm when you switch on.

Wire the sticks

Each stick axis has three wires: 3.3 V, GND and the signal. They go to CHANNELS 1 to 4.

Wire the sticks exactly like this, whatever mode you fly. The Stick mode setting (step 22) swaps the two up-and-down axes for Mode 2.

Wire to	Stick and direction	Channel in Mode 1
CHANNELS 1 Teensy pin A0	Right stick, left and right	1 Aileron
CHANNELS 2 Teensy pin A1	Left stick, up and down	2 Elevator
CHANNELS 3 Teensy pin A2	Right stick, up and down	3 Throttle
CHANNELS 4 Teensy pin A3	Left stick, left and right	4 Rudder

Wire the trims and the switches
- Trims go to the TRIMS header, trims 1 to 4 as printed. Each trim has a common and two contacts: the common to GND, the two contacts to that trim's two pins.
- Top-edge switches go to the EDGE SWITCHES header, switches 1 to 4. Each switch is read as two inputs to ground: the centre tag to GND, the two outer tags to the switch's two pins.
- Front switches or knobs go to CHANNELS 5 to 8. A knob has three wires: 3.3 V and GND on its outer tags, the signal on its middle tag.
- A three-position switch on CHANNELS 5 to 8 needs resistors to make three voltages. That wiring is being written up: email Malcolm if you need it now.
If a trim or a top-edge switch works the wrong way round, do not re-solder it. Calibration fixes it.
Wire the light, the button, the battery and the aerial
- RGB LED to the RGB LED header: its common leg to GND; red, green and blue to the three other pins. Step 18 checks the colours.
- Power button: already fitted in step 11.
- Battery lead: already fitted in step 11.
- Aerial: already on the radio from step 12. Route the extension to the case.
- Charger, if you fit one: how it joins the battery lead and the balance plug is being written up. Email Malcolm before you fit one.

Wiring the screen

The CrowPanel goes where the Nextion screen went. Data goes to its UART0 port. Power goes into its USB-C socket.

Wiring diagram. Main board screen header pin 4 to the screen's UART0 pin 1 (RX, yellow). Pin 5 from the screen's UART0 pin 2 (TX, white). Pin 1 GND to UART0 pin 4 (GND, black) and to the USB-C lead's GND. Pin 3 +5 V to the USB-C lead's +5 V. UART0 pin 3 (3V3 out) is left unconnected. The speaker goes on the SPK port. — The screen's data lead and power lead. Tap to enlarge.

Main board, screen header	Goes to the screen
Pin 4 (Teensy pin 1, TX1)	UART0 pin 1: RX (IO44), yellow
Pin 5 (Teensy pin 0, RX1)	UART0 pin 2: TX (IO43), white
Pin 1 (GND)	UART0 pin 4: GND, black. Also the USB-C lead's GND
Pin 3 (+5 V, the screen's regulator)	The USB-C lead's +5 V
Pin 2	Not used
–	UART0 pin 3 (3V3): leave unconnected

Pin 1 is GND. Battery off, meter on beep: pin 1 is the end pin that beeps to the battery's −. (You marked it in step 11. Version 1 owners: find it now.) Check it again before you plug the screen in.

Never two power supplies

Do not plug a computer into the screen's USB-C socket while it is wired to the main board. That socket is its power input, and its UART0 port shares the USB chip. Unplug the screen leads first.

Both boards use 3.3 V logic. The link runs at 921,600 baud.
The 3V3 pins on the screen's ports are outputs. They cannot power the screen.

Wire the screen

Find the +5 V and GND wires of the cut USB-C lead with a meter: colours vary.

Wire the screen as shown above. Plug the speaker into the screen's SPK port.

Check: battery off, meter on beep: the USB-C lead's GND beeps to pin 1, its +5 V to pin 3; the screen's RX goes to pin 4, its TX to pin 5.

First power-up

On the front screen, Transmitter opens Transmitter setup and Model opens Model setup. So Transmitter → System means: press Transmitter, then System.

Check again for a short circuit

Now that everything is wired, measure the resistance between + and − on the battery plug, before the battery goes on.

Check: it is not close to zero.
Switch on

Connect the battery (the right way round, as in step 11) and press the power button. The status light turns red: it is trying to connect (no model is on). The screen starts, and the front page appears.

If the clock does not answer, the screen says so and lists the I2C addresses that did: 0x40 is the INA219, 0x68 the clock, 0x50 the clock's memory chip.

Check: the front page shows the transmitter's battery voltage. If the light is green with no model on, the red and green wires are swapped: swap them.
Bring everything up to date

Tap Transmitter → Transmitter updates. If a newer version is offered, tap Install now. Nothing is installed until you do.
- Updates need the safety on and no model connected. Your switches are not set up yet: if Transmitter updates is grey, skip this step and do it after step 26.
- Your models, settings and logs are copied to the screen's card first.
- If new firmware would read your models differently, the old firmware comes back by itself.
An update on offer.

Done, with your settings checked.

Check: the screen says Update complete, or that you already have the latest version.
Close the case

Only now close the case. Fit the screen in the case front, then the sticks, switches, trims, LED (in its printed mount), button and aerial. Tidy the wires with cable ties so nothing can rub or pull.

Check: switch on again. The front page comes up as before.

Calibration and set-up

Reset the transmitter

A few minutes

For a new transmitter, or a new owner: Transmitter → Calibrate → Reset transmitter.

It deletes every model memory (but not the backup files) and sets stick Mode 2. Do not switch off while it works.
Set the stick mode

Transmitter → System, page General (1 of 3): set Stick mode to 1 or 2. Do this before you calibrate.

On the same page, switch Automatic model selection on. Reset transmitter switches it off. Press OK to save.

Then open System again and press Next twice, to Advanced (3 of 3). Switch Keep log files on and press OK. Reset transmitter switches it off too.

General (1 of 3): stick mode, automatic model selection, and the Bind button.
Calibrate the sticks, knobs and switches

Transmitter → Calibrate, then:
1. Press Calibrate sticks etc.
2. Move every stick and knob to its ends, several times.
3. Press Next.
4. Centre the sticks and knobs. Push all four top-edge switches fully away from you.
5. Press Finish.
Calibrating: the bars show each channel as you move it.

Check: the screen says Calibration saved. Each stick moves its bar the full width.

Calibrate again whenever you change a stick or the stick mode.
Calibrate the trims

On the same page, press Calibrate trims and follow the words on the screen. Without it, trims may work backwards.
Set the battery type

Transmitter → Batteries: choose your transmitter battery (2S LiPo or Li-ion, or 2S LiFePO4). Then the percentage means something.

Check: the voltage on the front page matches a meter. If not, correct it on System → Advanced (3 of 3), in hundredths of a volt per cell.
Give each switch its job

Transmitter → Switches. Touch a switch and choose its job: Safety, Banks 1 2 3, Bank 4 & Motor, Rates, Buddy, or a channel from 5 to 16.
- Positions are near (towards you), mid and away.
- Safety: near is safety off. Away is safe.
- Bank 4 & Motor: near is bank 4 with the motor off.
Eight switches. Each shows its job and where it is now.

Check: when you move a switch, its position changes on the page.

Every page on the transmitter has a Help button. It explains that page in plain words.

The receiver

The Version 2 receiver is a Seeed XIAO ESP32-S3 with one to three nRF24L01+ radio modules. With two or three, it switches to another radio if the one in use loses the link.

It feeds a flight controller over CRSF, SBUS, IBUS or PPM.
It sends Rotorflight telemetry (battery, head speed, ESC temperature) back to the transmitter.
It talks to the phone app over Bluetooth, and updates itself over the air.
It binds to Version 1 and Version 2 transmitters.

It has no PWM servo outputs, so it needs a flight controller. For a model without one, use the Version 1 receiver from the Version 1 repository.

A 3-D view of the dual-radio receiver board, about 46 by 22 millimetres: pads for the XIAO, a regulator, one radio module on this side, a status light with its resistor and capacitors — The dual-radio receiver board, drawn from its design files. A second radio sits on the other side.

What it is made of

Part	Exact type	How many	What it is for	Where to buy	About
Every receiver
XIAO ESP32-S3	Seeed Studio XIAO ESP32-S3, with the little aerial in its box	1	The receiver's computer, WiFi and Bluetooth	Seeed Studio, The Pi Hut	£7
Radio modules	Ebyte E01-ML01SP4 (nRF24L01+ with amplifier), or any 2.4 GHz nRF24L01+ module	1–3	The radio link. Two or three give a steadier link	Ebyte's own shops	–
Radio aerials	2.4 GHz dipole aerials with IPEX (u.FL) plugs	1 per radio	Mount them about 90° apart	Electronics shops	–
Lead	Four wires to the flight controller's receiver port	1	Signals out and in, and power	Often in the flight controller's box	–
Capacitor Wired by hand	100 µF, one per radio	1–3	Keeps each radio's supply steady. The circuit board has its own	Electronics shops	–
On the circuit board (when its files are published)
5 V regulator	Pololu S7V7F5 (takes 2.7–11.8 V in)	1	Makes a steady 5 V, even from 5 V	Pololu, or a reseller	–
3.3 V regulator	AMS1117-3.3, SOT-223	1	The radios' own 3.3 V supply	Electronics shops	–
Capacitors	1206: 100 µF × 2, 22 µF, 10 µF	4	Keep the radios' supply smooth	Electronics shops	–
Status light	LED and 100 Ω resistor, both 1206	1 + 1	Off: no link. On: receiving. Flashing twice a second: binding	Electronics shops	–
Lead socket	JST-GH 4-pin, 1.25 mm, horizontal (SM04B-GHS-TB)	1	The lead to the flight controller	Electronics shops	–
Optional
Case Optional	Printed in PETG or PLA. Case and lid: STEP files in `RXV2/case`. Made for the circuit board	1 set	Protects the board. Clear heat-shrink also works	Print it yourself	–
Total	Parts only, approximate. Prices change.				about £35

The circuit board

The board files are being prepared

The receiver's circuit board files will be published in the repository. Until then, wire the receiver by hand, as shown below. The firmware is the same either way.

Soldering the XIAO

Dry joints on the XIAO's edge pads are the most common fault. Solder can sit on the pad and look fine without flowing into the half-hole.

Finish every pad with the iron: flux, a little more solder, a little more time, one pad at a time.
Check each one with a magnifier.
Restarts on battery but fine on USB? Suspect a ground joint.
Fewer radios found than you fitted? Suspect a radio's wires or joints.
Sticks work but no Rotorflight or telemetry? Suspect D5.

Wiring it by hand

Wiring diagram for the XIAO ESP32-S3, seen from the top. Left side: D0 radio 2 CE, D1 radio 2 CSN, D2 radio 1 CE, D3 radio 1 CSN, D4 radio 3 CE, D5 telemetry in from the flight controller's TX, D6 channels out to the flight controller's RX. Right side: 5V from the flight controller port, GND to every part, 3V3 to every radio's VCC, D10 MOSI, D9 radio 3 CSN, D8 SCK, D7 MISO. — Every pad of the XIAO and its job. Tap to enlarge.

Every radio: GND, VCC (3.3 V), SCK (D8), MOSI (D10) and MISO (D7) are shared. Leave IRQ unconnected.
Radio 1: CE to D2, CSN to D3.
Radio 2: CE to D0, CSN to D1.
Radio 3: CE to D4, CSN to D9.
Each radio: solder a 100 µF capacitor across its VCC and GND, right at the radio, + to VCC.
Flight controller: its RX to D6, its TX to D5, +5 V to 5V, − to GND.
Go by the D-labels printed beside the pads. The full table is in Connections.txt.

Power

Wired by hand, the XIAO takes its 5 V straight from the flight controller's port. Use only a port that gives a steady 5 V, such as a Nexus receiver port. Never a servo or BEC pin: those can carry 6–8.4 V and would destroy the XIAO.

Aerials: fit a dipole to each radio and mount them about 90° apart.
Fit the XIAO's own aerial (pressed onto the tiny gold socket). Without it, Bluetooth and WiFi reach only about an arm's length. A dipole works far better than a stubby one for WiFi.

Put the firmware on

Never two power supplies

Flash the receiver before it is wired to the flight controller, or with the flight controller unplugged. Never plug a computer into the receiver's USB socket while the model's battery is connected.

Put the board into flashing mode

Plug the XIAO into a computer with a USB-C data cable. Hold B, tap R, let go of B. The two tiny buttons are beside the USB socket.

Check: the yellow light goes out. (A red light may stay on: that is the charger. Ignore it.)
Click once in your browser

About a minute

Open the one-click flasher in Chrome, Edge or Opera, on a desktop or laptop. Press Flash this chip. Leave it alone until it says it has finished.

It erases the chip first, so the same button also resets a used board.
Unplug it and plug it in again

The chip does not restart by itself after flashing.

Check: the board appears in the phone app as Untitled.

Open the flasher

Prefer the command line? The five files and the esptool command are on the dongle page. To build from source, see the receiver's README.

First set-up on the phone

Install the app and connect

Get the free app from the Phone Apps page (iPhone, iPad and Android). Open it, tap Connect, and choose Untitled.
Rename it at once

Its front page asks you to. Two boards with the same name cannot be told apart.
Check the radios

The Home page shows each radio it found.

Check: it lists every radio you fitted. One missing? Check that radio's wires and joints.
Give it your WiFi

Setup → WiFi settings. With WiFi it can update itself. At the field it needs only Bluetooth.
Choose the output

Setup → Output protocol. CRSF (the default) for Rotorflight. SBUS, IBUS or PPM for other flight controllers.

The app's Home page for a receiver: the flight battery, Slot 1 and Slot 2 listening, and buttons for Fly now, Rotorflight setup and Wizard, Bind, Simulator and Setup — Home: both radios (slots 1 and 2) found.

The app's Setup page: Output protocol (CRSF), View channels, Receiver settings, WiFi settings, Update or roll back firmware, and Rename receiver — Setup: output, WiFi, name and updates.

No phone? Join the board's own WiFi network, called Untitled, from a computer, and open 192.168.4.1 in a browser.

Connect it to the flight controller

Safety: servos

Unplug the servos (or leave the BEC off) until the wizard's Servos step has set their frequency and the BEC voltage is right. A wrong setting can burn out a servo in seconds.

Check the lead with a meter before the first plug-in. The port's + must reach the receiver's + (the XIAO's 5V pad, if hand-wired) and − its GND. Hand-wired, + must read about 5 V with the flight controller powered (USB is enough). Over 5.5 V: stop.
Wire TX to RX and RX to TX, plus + and −. Use the flight controller's receiver port: Port A on a Nexus, often marked ELRS or RX on others. It already listens for CRSF.
In Rotorflight, that port must be set to Serial RX, with the receiver type CRSF and the TELEMETRY feature switched on. With a USB-C data cable from the flight controller to the receiver, the app's Ports page does all of this in one Save. Without the cable, use the Rotorflight Configurator once.
The Rotorflight button never appears? Swap TX and RX first. Crossed wires are the usual cause.
Head speed and ESC temperature on the transmitter: switch on the RPM and temperature sensors in Rotorflight's CRSF telemetry, and set the main gear ratio.
Then bind it (next section) and follow the app's New helicopter wizard. Stop before its Blades on step. First do every check in Testing before the first flight, blades off.

The app's New helicopter wizard: take the main and tail blades off, then a list of steps to tick off, starting with Before you start, Calibrate the flight controller and Wire it up — The New helicopter wizard: blades off first.

Battery voltage without telemetry: if your flight controller does not send the flight battery's voltage over CRSF, a small divider board can feed it to pad D9. Use it only with one or two radios: D9 is radio 3's CSN. The divider's files are being prepared too.

Binding the transmitter and the receiver

Binding pairs a receiver with your transmitter. You do it once per receiver. One transmitter can bind any number of receivers.

Every receiver bound to it listens to it, so switch on one model at a time.

Safety: before you bind

Blades or propeller off, and the safety on (switch away), before you switch the model on.

Choose a model memory

On the transmitter, Model → Models: choose an empty model memory for this model and give it a name.
Put the transmitter into bind mode

Switch on and keep holding the power button for about three seconds (more than two, less than five). You hear a chirp and "binding enabled", and the light blinks.

Or: Transmitter → System → Bind. The button then reads Stop binding.
Switch the receiver on

A new receiver is already waiting to bind. On the circuit board its light flashes twice a second.

Was it bound before? Open it in the app, tap Bind, then Enter bind mode. Changed your mind? The same button now reads Cancel bind: it puts the old pairing back.

The transmitter then asks whether to save this receiver's ID with the model memory you chose. Press OK. Cancel saves nothing, and the model does not connect.

Check: the transmitter's light turns green and it says it is connected.
Check the model's ID

Model → Model IDs shows the stored ID. If it says No ID stored, press Store this model's ID.

From then on, Automatic model selection (switched on in step 22) loads this model memory by itself whenever you switch this model on.

Check: switch everything off and on again. The right model name appears on the front page.

The dongle

The dongle is a bare XIAO ESP32-S3 with four wires, on a spare port of the flight controller. It brings the phone app's Rotorflight set-up to any helicopter, whatever radio flies it.

It can also turn a receiver of almost any make into a USB joystick for a simulator.

Solder four wires: the flight controller's RX to D6, its TX to D5, +5 V to 5V, − to GND. The lead in the Nexus box is the easiest. Check its colours with a meter.
Flash it with the same one-click flasher.
Plug it into a spare port: Port B or C on a Nexus, never Port A (the receiver's).
Then fit a USB-C data cable from the flight controller to the dongle. Or, to use the lead alone, choose dongle on a wire for that port on the app's Ports page (it sets MSP, 115200).

A finished dongle: the XIAO board with the Nexus lead's four wires soldered on and its plug at the other end — A finished dongle: the board and the Nexus's own lead.

Build it with four wires, not the carrier board

The repository holds a dongle carrier board (RXV2/kicad/dongle). Do not order it. Its connector is the wrong way round: it put the flight controller's 5 V onto a signal pin and destroyed two chips.

Never two power supplies at once

Once the dongle is wired to a flight controller, power it only from the flight controller. Never plug a computer, charger or power bank into its USB socket while the flight controller is connected: the dongle drives the USB cable, so two supplies would meet.

Flash it, or use it as a simulator interface, with the flight controller unplugged.

The dongle guide: pins, ports and set-up

Testing before the first flight

Do every check below before the first flight. Do them again after a crash, a repair, or a change of receiver or settings.

Safety: blades off

Take the rotor blades or the propeller off for every test. Unplug the motor too, unless the test needs it. Hold or strap the model down.

On the bench

Transmitter first, then the model

Always switch the transmitter on first. A receiver that hears its transmitter within a second of power-up keeps its WiFi off.

Once, switch the model on with the transmitter off. A receiver with no transmitter always starts safe: motor off, sticks centred.

Check: nothing moves and the motor cannot start.
The right model loads

Switch the transmitter on, then the model.

Check: the model's own name appears on the front page.
Each stick moves the right channel

In the app (connect within 30 seconds of switching the model on), Setup → View channels. Move each stick and switch in turn.

View channels: a live bar for every channel.

Check: each stick moves only its own bar, the right way, to both ends.
The model moves the right way
- Helicopter: right cyclic tilts the swashplate right; forward cyclic tilts it forward; up collective raises the blade pitch; right rudder changes the tail pitch to turn the nose right.
- Aeroplane: right aileron raises the right aileron; up elevator raises the elevator; right rudder moves the rudder right.
A channel the wrong way: Model → Reverse on the transmitter. A servo or the swashplate the wrong way: set it in Rotorflight (the app's New helicopter wizard does this step by step).

Then move every stick slowly to its ends.

Check: everything moves the right way, and nothing binds or touches.
Safety switch and motor switch

Blades off, motor connected, model held down.
- Safety on (switch away): the motor must not run, whatever you do with the sticks and switches. A Rotorflight model stays disarmed. The transmitter's Rotorflight menu says why it will not arm.
- Safety off: check the motor starts and stops the way you expect, on the throttle and on the Bank 4 & Motor switch (near is motor off).
- The screen's WiFi and Bluetooth switch themselves off when the safety is off.
Check: with the safety on, nothing you do starts the motor.
Failsafe: what happens if the link is lost

Set it up first:
- CRSF with Rotorflight: Rotorflight's own failsafe is in charge. Check it once in the Rotorflight Configurator's Failsafe tab, on a computer (neither the app nor the transmitter can change Rotorflight's failsafe). Stage 2 should stop the motor.
- SBUS, IBUS or PPM: on the transmitter, Model → Failsafe (or in the phone app: Setup → Receiver settings → Failsafe positions). Tick the channels, hold the sticks where the model should go (motor off), and press Save while connected. The positions are saved in the receiver.
  With a Version 2 receiver every channel goes to its saved position, ticked or not. Hold every stick and switch where it should be, motor off, before you press Save. (A Version 1 receiver moves only the ticked channels.)
Then test it, blades off and model held down: safety off, run the motor slowly, then switch the transmitter off (hold the button through the countdown).

Check: within a few seconds the motor stops and stays stopped. When you switch the transmitter on again, the motor does not start by itself. Then put the safety on.
Battery warnings
- Model → Options: set the Stop flying voltage per cell: about 3.6 V for LiPo. The voltage recovers a little when the motor stops.
- Transmitter → System: set the transmitter's low-battery warning high enough that you can land with time to spare.
The power button is guarded

Check: while connected, a quick press does not switch the transmitter off. Holding it starts a countdown.

Check the link

About 500 packets a second

With the model connected: Model → Telemetry.
- Frame rate (packets a second) should be close to 500. A little lower is normal while the receiver's WiFi is on.
- Longest comms gap should be below about 250 ms, and is normally well below 100 ms.
Measured on the first Version 2 flight (5 October 2026): an average of 491 packets a second, 0.21 % lost, and a longest gap of 15 ms.

The Telemetry page (sample figures).

Check: frame rate close to 500; longest gap well below 250 ms. If not, move the receiver's aerials (dipoles, 90° apart) and check them for damage.
Reconnection times

On the Telemetry page, press Gaps. The chart shows how long each reconnection took. Look at it after landing, not in flight.

Reconnection times (sample figures).

Check: most reconnections take under 8 ms. Many over 100 ms mean the aerials need a look.
Look at the 2.4 GHz band (optional)

Transmitter → Scan RF shows how busy each channel is, and names the quietest and the noisiest. Useful at a new field.

The interference scan.

Range test

Walk away with the transmitter

The transmitter has no low-power range-check mode. It always runs at full power, so walk further than you will ever fly.
1. Put the model on the ground in an open space. Blades off, and the motor unplugged. Safety on.
2. A helper stays with the model.
3. Put a Frame rate box on your front screen (Transmitter → Appearance), so you can read it as you walk.
4. Walk away. Every so often, stop and move the sticks. The helper checks that the servos follow smoothly.
5. Keep going well past your usual flying distance.
6. Back at the model, look at the Telemetry and Gaps pages.
Check: the servos follow all the way out and back, the frame rate stays up, and the longest gap stays short.

First flight

A short first hover
- Choose a calm day and an open space, away from people.
- Charge the transmitter and the flight battery.
- Flight battery unplugged: fit the blades (or propeller). Check they are tight and the pitch is right (the wizard's Blades on step).
- Safety on (switch away). Transmitter on first, then plug in the flight battery. Check the model name.
- Step well back. Only then switch the safety off.
- Hover briefly, low. Land.
- Safety on, then unplug the flight battery, before you touch the model.
- Check the trims, the temperatures and the Telemetry page.
Rescue mode: set it up on the bench. Test it later, high in the air, before you rely on it.

Check: after landing, the frame rate, the longest gap and the reconnection times look like they did on the ground.

The short version, for the field

Transmitter and flight battery charged
Blades or propeller on and tight
Safety on until you are ready to fly
Transmitter on first, then the model
Only one of your models switched on
The right model name on the front page
Controls move the right way; nothing binds
Frame rate close to 500 a second
Failsafe tested since the last change
Range tested since the last change
Open space; people well away
Safety on and battery out before you touch the model

Getting help

Email Malcolm: mmessiter@gmail.com. Say which transmitter, receiver and flight controller you have, what you did, what you saw, and what you expected.
From the phone app: open the Event log (Rotorflight setup and Wizard → À la carte → Black box → Receiver log → Event log; with no Rotorflight: Receiver log → Event log; on a dongle it is on the Home page) and tap Send debug data to Malcolm. It gathers the receiver's version, its event log and what the app did into one text file. No passwords are included.
On the transmitter: every page has a Help button. With Keep log files on (step 22), the logs are under Model → Log.
Updates and questions: the Help & updates page.
The source: Version 2 on GitHub (with the READMEs for each part) and Version 1 on GitHub.

If you build one, Malcolm would love to hear how it went.

Build one

What you can build

The transmitter

The receiver

The dongle

Tools you need

Before you start: the law and safety

The transmitter

Parts list

The main board

Download the repository

Zip the Gerber files

Order the boards

Printing the case

Flashing the screen and the Teensy

Install PlatformIO and get the code

Flash the screen

Give the screen WiFi and let it fill its card

Flash the Teensy, on its own

Prepare the Teensy's card

Assembly

Solder the small parts

Fit the headers and sockets

Test the power before the modules go in

Fit the modules

Wire the sticks

Wire the trims and the switches

Wire the light, the button, the battery and the aerial

Wiring the screen

Wire the screen

First power-up

Check again for a short circuit

Switch on

Bring everything up to date

Close the case

Calibration and set-up

Reset the transmitter

Set the stick mode

Calibrate the sticks, knobs and switches

Calibrate the trims

Set the battery type

Give each switch its job

The receiver

What it is made of

The circuit board

Wiring it by hand

Put the firmware on

Put the board into flashing mode

Click once in your browser

Unplug it and plug it in again

First set-up on the phone

Install the app and connect

Rename it at once

Check the radios

Give it your WiFi

Choose the output

Connect it to the flight controller

Binding the transmitter and the receiver

Choose a model memory

Put the transmitter into bind mode

Switch the receiver on

Check the model's ID

The dongle

Testing before the first flight

On the bench

Transmitter first, then the model

The right model loads

Each stick moves the right channel

The model moves the right way

Safety switch and motor switch

Failsafe: what happens if the link is lost

Battery warnings

The power button is guarded

Check the link

About 500 packets a second

Reconnection times

Look at the 2.4 GHz band (optional)

Range test

Walk away with the transmitter

First flight