Ateities inžinerija · Aviamodelių kūrimas · build solve 2026-10-07

SOS drone v3

A 5-inch GPS quad that carries a tourniquet kit up to 200 m on its own, shows the scene live so a 112 dispatcher can see it, lands 5 m from the casualty facing them and plays spoken instructions.

Drone cost€288.85of €300

Take-off mass802 g704 g after delivery

Thrust / weight≈ 7estimate

Hover time≈ 7.4 min1300 mAh 6S

200 m mission≈ 16%of the pack, one way

Checks passed51 / 51model + simulator

loading 3D model…

Click a partDrag to orbit, scroll or pinch to zoom.

Rendered drone hovering over a meadow, props blurred, red payload box under the carbon frame — On the way. Camera, speaker and the box door face forward, GPS mast at the back.

Rendered drone landed in grass with the box door swung up and the tourniquet kit sliding out — Delivered. Door up, the CAT, gloves and the engraved card slide out toward the casualty.

Studio render from the front left: white head pod with camera lens, battery with strap, red box with white SOS 112 letters — Front left. White head pod, red box with the letters printed white on the AMS.

Studio render from the rear right: GPS mast with the ELRS antenna zip tied on its back, XT60 plug, box strap buckle — Rear right. GPS mast, ELRS antenna tied to its spine, XT60 and the strap buckle.

Close-up between the plates: flight controller on nylon spacers over the ESC, grommets, nyloc nuts, capacitor and wiring — Between the plates. FC on spacers over the 4-in-1 ESC, capacitor on the battery pads, twisted motor leads.

Dimensioned top and side drawing: motors 172 by 145 mm, overall 272 by 264 by 156 mm, box 175 by 66 by 54 mm, props 94 mm above the ground — All sizes in mm, generated from the same model the checks run on.

Shopping list

Prices checked live on 7 October 2026. Rotorama ships to Lithuania in 2-3 days. AliExpress prices are the cheapest listed variant, so pick the right one when ordering.

Part	Qty	€	Shop
Readytosky Mark4 5" frame	1	20.69	Rotorama
iFlight XING-E Pro 2207 1800KV	4	62.36	Rotorama
MicoAir H743 V2 + 55A AM32 stack, runs ArduPilot out of the box	1	92.79	Rotorama
MicoAir MG-903 GPS + compass	1	16.39	Rotorama
SkyGuy Nano 2.4G ELRS receiver	1	13.49	Rotorama
CNHL Ministar 1300 mAh 6S	1	21.79	Rotorama
Gemfan Hurricane 51477-3 props, sets of 4	2	6.18	Rotorama
FC buzzer, battery strap 20x250, box strap 20x550	3	9.27	Rotorama
M3 screws x8, x12, x30, M3 square nuts, foam pads, heat shrink	6	6.74	Rotorama
Shipping to Lithuania (Packeta)		7.00	Rotorama
Seeed XIAO ESP32S3 Sense, camera + WiFi + computer	1	15.49	AliExpress
DFPlayer Mini, 28 mm 1 W speaker, microSD, 1 kΩ resistor	4	7.09	AliExpress
M3 male-female hex standoffs 6+6, shipping buffer		4.17	AliExpress
PETG (≈125 g), 3 mm plywood offcut, foam tape, zip ties		5.40	materials
Drone total		288.85	€11.15 left

Before ordering: on the TX16S open Model › Internal RF. CRSF/ELRS means buy as listed. MULTI (4-in-1) means swap the ELRS receiver for a FrSky D16 one such as the RadioMaster R81 (≈€10). Not in the budget: tools, charger, and the payload itself (genuine CAT Gen 7 tourniquet ≈€32, gloves). First buy once money allows: a second battery (€21.79).

The video is WiFi rather than analog because the analog chain came to about €315 once a USB 5.8 GHz receiver was included. The XIAO streams to a laptop, draws a dashboard with the GPS position for 112, bridges MAVLink for Mission Planner and drives the voice player, all for €15.49.

Print and cut

3D print · Bambu Lab X1 Carbon

Open plate_x1c.3mf in Bambu Studio: all four parts on one plate, already standing right. Bambu PETG HF, 0.20 mm Standard, 4 wall loops, 20% infill, supports off, brim on the box and door. The mast and pod can go in PETG-CF for stiffness. With the AMS, paint the “SOS 112” letters white.

Part	Grams	On the bed	Note
Payload box	91	closed back wall down	175 mm tall, brim. Raised “SOS 112” on both sides
Box door	7	on its bottom edge	hinge pin: 70 mm of 1.75 mm filament
Head pod	15	as flown	holds camera, voice player, speaker, WiFi antenna
GPS mast	12	upright	ELRS antenna clips on its back

Laser · red cut, blue score, black engrave

Part	Material	Qty
Skid bars 140 × 14 mm, widen the stance to 40°	3 mm birch ply	2
Instruction card 85 × 54 mm, Lithuanian, goes in the box	3 mm ply	1
Landing pad tiles 300 × 300, tape into 600 mm	box cardboard	4

Wires to cut

Measured on the model plus slack: motors ≈135 mm, battery lead ≈120 mm, XIAO cable ≈135 mm, voice player power ≈130 mm, buzzer ≈85 mm, receiver ≈70 mm.

Assembly

Frame. Build the Mark4, but where the 4 standoffs meet the bottom plate use M3 male-female 6 mm standoffs from below. Their open ends point down and carry the box.
Motors. Front right and rear left spin CCW, front left and rear right spin CW, props in.
ESC. Softmounts, ESC arrow forward. Solder the motors, the XT60 lead and the capacitor (stripe to minus). M3×30 stack screws from below.
Flight controller. 8-pin cable to the ESC, FC on the nylon spacers, arrow forward, USB to the side.
Receiver and buzzer behind the stack, on the SBUS/CRSF and LED/Buzzer plugs. Receiver TX and RX cross over.
Mast on the 2 rear top-plate screws (M3×12). GPS on top, arrow forward, its cable down the channel and out of the notch at the mast's foot, round the left edge of the top plate to the GPS port. ELRS antenna on its back, element vertical, 3 zip ties.
Head pod. Flash the XIAO first. XIAO D0 through 1 kΩ to the voice player RX, speaker on SPK1/SPK2. Cables to UART4 (XIAO) and UART5 (voice player power). Pod on the 2 front screws (M3×12).
Battery on top with its centre about 5 mm behind the motor centre, lead to the back.
Box. Skids with M3×8 and square nuts, foam pads under the ends, door on its pin. Screw the box to the hanging standoffs from inside (M3×8). Kit in, strap lengthwise over the roof, round the hinge, down the door, under the skids.
Balance. Lift by the front and rear motors and slide the battery until it hangs level.

Wiring

Block diagram: battery to ESC, ESC to motors and flight controller; GPS, ELRS receiver, buzzer, XIAO and DFPlayer on the flight controller ports; XIAO to DFPlayer to speaker; XIAO to laptop over WiFi

FC plug (JST-SH)	Goes to	ArduPilot
GPS · GND 5V TX3 RX3 SCL SDA	MG-903 GPS + compass	SERIAL3 GPS
UART4 · GND 5V TX4 RX4	XIAO: TX4→D7, RX4→D6	SERIAL4 MAVLink2 115200
UART5 · GND 5V	voice player power only	none
SBUS/CRSF · GND 5V TX6 RX6	ELRS receiver, TX/RX crossed	SERIAL6 RC in
LED/Buzzer · 5V Buzz−	buzzer	built in
ESC 8-pin	55A AM32 ESC	motors 1-4, current, ESC telemetry

Software

Flight controller

Flash ArduCopter 4.7.1 for MicoAir743v2 (hold BOOT, plug USB).
Mission Planner › Initial Tune Parameters: prop 5.1 in, 6S.
Load ardupilot/sos.param, write, reboot. All 66 values checked against the official 4.7.1 list. It caps the takeoff throttle: the default made the simulator climb to 30 m instead of 15.
Calibrate accelerometer, the GPS compass (untick the internal one), radio. Motor test, swap two wires on any motor that spins backwards.
TX16S: ELRS, AETR, ch5 three-position LOITER / AUTO / RTL, ch7 motor emergency stop, ch8 arm.

Head unit

Arduino, board XIAO_ESP32S3 with OPI PSRAM, upload firmware/sos_cam. It makes the WiFi SOS-DRONE. The laptop opens http://192.168.4.1/ for the camera, position with a Google Maps link, height, distance, battery and voice buttons. Mission Planner connects on UDP 14550. Share the dashboard tab in the video call with 112, internet through the phone over USB.

Voice: record the instructions in Lithuanian on a phone, save as 0001.mp3 on the microSD. It plays on repeat for 3 minutes once the drone lands away from where it took off.

Flying

Props off: motor test, radio-off failsafe, emergency stop.
LOITER hover at 2 m, then Auto Tune in AltHold on a calm day.
50 m AUTO mission empty, then with 100 g of sand.
Range check: drone on a stand 250 m away with video streaming, radio link quality above 80%. This proves WiFi and ELRS get along on 2.4 GHz.
200 m with the real kit.

make_mission.py BASE_LAT BASE_LON CASUALTY_LAT CASUALTY_LON writes the mission: climb to 15 m, hover 20 s 25 m short of the casualty so 112 sees the scene, land 5 m short facing them. Wait about 30 s after the GPS fix, then arm in AUTO with the throttle down. It does not fly back: it only has to buy time until the ambulance arrives, and the crew unplugs it and drives it back.

The whole mission in ArduPilot SITL 4.7.1 with the real params, 4× speed: take-off, 20 s overview hover, landing 5 m from the casualty. It does not fly back: the ending shows the ambulance arriving and driving it back (illustrated, not simulated). Made from the simulator log by sitl/video.py.

render progress loading

The same logged flight rendered in Blender: the detailed drone model flies the real SITL trajectory over a field at a forest edge, with the camera view 112 would get during the hover. People, ambulance and the ending are illustrated. The sound follows the log too: the motor pitch comes from the logged motor outputs, heard from each camera with distance and Doppler, and the head pod plays the real Lithuanian voice message. Still rendering on two laptops: frames not done yet show a still from the shot's test render, and the video is rebuilt with the new frames every hour. Made by film/film.py and film/sound.py.

Verification

Check	Result
Collisions, 42 parts pairwise	none	pass
Spinning props to anything	3.6 mm closest, tips 15.5 mm apart	pass
Door and kit	door swings 150° free, kit slides straight out	pass
Camera view	nothing of the drone in frame. Casualty visible hovering 25 m out at 15 m, and landed 5 m away. Out of frame for ≈8 s mid-descent	pass
GPS sky view, compass distances	0% blocked above 10°, 53-113 mm from noise sources	pass
Centre of gravity	0.8 mm off loaded, 1.3 mm empty	pass
Tip-over on the skids	40° sideways, 44° fore-aft, props 94 mm off the ground	pass
Thrust, power, endurance	T/W ≈7, 7.4 min hover, mission 16% of the pack one way	estimate
5 V rail, plugs, serial ports	0.95 of 3 A, one job per port	pass
Screws	4+ mm of thread everywhere, stack screws clear the top plate	pass
Printability	4 watertight parts on one X1C plate, no supports	pass
ArduPilot parameters	66 of 66 valid in 4.7.1	pass
Head unit firmware	compiles for XIAO ESP32S3, parser and voice-player frames tested with real MAVLink	pass
Mission in ArduPilot SITL 4.7.1	armed in AUTO, cruise 15.1 m, 26 s overview, landed 8.0 m from the casualty facing them (1° off). Voice code replayed on the telemetry fired once, after the landing	pass
Budget	€288.85 of €300	pass

Measure first

The frame and a few parts are modelled from shop data. When they arrive, measure these, set them at the top of sos.scad and run ./make.sh. If every check still passes, print.

motor-to-motor distances (shop: 145 × 172 mm)
where the 4 standoffs sit and how tall they are (25 mm assumed)
bottom and top plate size
the folded CAT with gloves and card (165 × 61 × 44 mm assumed)
the battery's real weight

Limits

No thrust table is published for these motors, so power and flight time are estimates. The first flights learn the real hover throttle.
WiFi video and ELRS share 2.4 GHz. The antennas sit 120 mm apart and the camera sends at 15 dBm. The range check decides.
The phone-class camera is fine in daylight, weak at dusk, blind at night.
Land on ground flatter than about 15°.
802 g and privately built: operator registration, A1/A3 certificate, A3 subcategory (away from people, 150 m from houses), below 120 m, in sight. Under 16 flies only under direct supervision. A real service near people needs the Specific category. Check with the TKA.
Not medical advice. Have a first aid instructor check the kit, the card and the voice script.

Everything above is generated from one parametric model in ~/sos-drone/v3 and checked by verify.py. Prices from Rotorama and AliExpress on 7 October 2026.