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Dropper
A drop module with two servo inputs, one that picks the bay and one that drops, plugs into the drone's Raspberry Pi instead of the flight controller. Aircast drives it from your radio and from the app's Drop button, over the same Starlink or 4G link as the video. It handles up to six bays.
A drop board on the flight controller
Some baskets, such as four-bay baskets, have their own board wired to a telemetry port of the flight controller instead. That board talks MAVLink, and the app finds it by itself: there is nothing to wire to the Pi and nothing to switch on. The app shows four bays, drops the one you pick with the Drop button or the radio's drop switch, and offers Drop all, which drops the four one after another, and does not offer Grab. Every holder stays droppable after a drop, because the board cannot tell the app a holder was loaded again. The rest of this page is for a module with two servo inputs.
What you need
- A Raspberry Pi 4 running Aircast.
- A drop module that reads two servo signals: a bay select that steps through six positions from 988 to 2012 µs, and a drop line that is safe at 988 µs and drops at 2012 µs.
- Three jumper wires.
- To drop from the radio: an EdgeTX radio set up for Radio over USB.
Set it up
Wire the module to the Pi
Move the module's two signal wires and its ground onto the Pi's header. Keep powering the module from its own supply, as before, never from the Pi.
| Module wire | Pi header | Signal |
|---|---|---|
| Bay select | Pin 32 | GPIO 12 |
| Drop | Pin 33 | GPIO 13 |
| Ground | Pin 34 | GND |
Pins 32 and 34 are on the outer row of the header, near the USB end; pin 33 sits between them on the inner row.
Switch it on
In the app, open the Device tab, then Telemetry. In the Dropper card, turn on Drive the dropper from this device. The badge reads bay 1 · safe once the outputs are live. It takes effect at once, with no reboot.
Set up the radio
The dropper reads the same three channels as the rest of the app's payload, in the R2D2 layout: channel 5 picks the bay, channel 11 drops, and channel 12 arms. The channel table is under Camera and payload from the radio. A model that already drops with R2D2 needs no change.
Test it on the ground with the bays empty
Props off, bays empty. Arm the payload, then move the 6POS switch through each position and flip the drop switch at each one. The holder for that position should open and the badge should read bay N · dropping while the switch is up. If the wrong holder opens, adjust that position's weight on the radio until its value lands in its own step.
Drop from the app
With the dropper on, the app shows six bays. Arm the payload in the app, pick the bay, and press Drop. The bay line moves first, and 0.3 s later the drop line goes high for one second. The app counts the bays it has dropped. Grab is not offered, because a drop module has nothing to close.
While the radio streams, its 6POS switch picks the bay and the tiles follow it. The app's lock and the radio's arm switch both arm the payload and make it safe, and the last one moved wins.
What the badge says
| Badge | Meaning |
|---|---|
| off | The switch is off. The pins carry nothing. |
| starting | The switch is on and the outputs are coming up. |
| no output | The outputs could not start. The notice under the text says why. |
| no radio | The outputs are live, but no radio has been heard for a second. The drop line is safe. |
| bay 3 · safe | Bay 3 is picked and the payload is not armed. |
| bay 3 · armed | Bay 3 is picked and the payload is armed. The next drop goes. |
| bay 3 · dropping | The drop line is high on bay 3. |
Limits
- Raspberry Pi 4 only for now.
- A mission's drop items run on the flight controller, so a mission does not drop from this module. Drop from the radio or the app.
- The pins put out 3.3 V signals, the same as a Pixhawk's servo outputs.
- Six bays, at 988, 1192, 1397, 1602, 1807 and 2012 µs.
- A drop board on the flight controller drops whatever it is asked to, with no safety of its own. The payload arm, set by the app's lock or the radio's arm switch, is the only safety.
What's verified
- On a Raspberry Pi 4 the outputs come up in 12 ms and measure 988, 1397 and 2012 µs exactly at 50 Hz, with no pulses after the switch goes off
- The radio's bay and drop reach the pins only while channel 12 is armed, and the drop line goes safe a second after the radio goes quiet
- The app's Drop reaches the device and drops the bay it names, and the app hides Grab
- On a four-bay basket, a drop sent to its board released exactly the holder it named, with clips loaded in all four
- Not yet flown, and the two-wire module not yet run on real hardware
Flash your card in about 5 minutes
The Aircast Flasher downloads the image, sets Wi-Fi and hostname, and verifies the card.
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