8bit-quad

Flight controller PCB for a custom-built quadcopter based on the ATmega328 microcontroller. An IMU, an nRF24L01+ radio and four ESC outputs on a 113 mm octagon shaped to the top plate of an F450 frame.

It was etched at home, populated, flown, and retired. This repository is an archive of it, not a design anyone is expected to build. The firmware that ran on it is in its own repository.

_{The board on the frame with an FTDI adapter attached. The remote is on the right, on protoboard. It never had a PCB; what it had to provide is in rc/.}

schematic, click for the PDF	board

fc/          KiCad 10 project, symbols and footprints included
  _export/   schematic and board images
  bom.txt    62 components
rc/          what the remote had to provide
gallery/     photographs

Everything the project needs is inside it. There are no dependencies on installed libraries.

The board

The outline is a 113 mm octagon because the board was originally meant to be the F450's top plate rather than sit on it. A load-bearing flight controller: the arms bolt through it, every landing is absorbed by the PCB, the frame's vibration is piped straight into the IMU with nothing in between, and any crash that would have cracked a plate worth a couple of euros instead cracks the one part carrying all the electronics. Obvious in hindsight. Also in foresight, to be fair.

The shape stayed regardless, and the board ended up bolted above the plate like every other flight controller ever made.

Pin names below are the ones the firmware uses.

The microcontroller is an ATmega328P-PU in a DIP-28 socket with a 16 MHz crystal, programmed over either an AVR ISP header or an FTDI header, with a restart button next to it.

The IMU is an MPU6050 or MPU9250 breakout on I2C. There are two header footprints for it, 1x8 and 1x5, so either of the two common module shapes fits. Its interrupt lands on PD2. The outline drawn around it is an anti-vibration plate, drawn when it was still meant to carry the IMU module and nothing else. It ended up carrying the whole flight controller. So the vibration problem was understood, and then solved by moving it one level up.

The radio is an nRF24L01+ PA/LNA module on hardware SPI, with CE on PD7, CSN on PB0 and the interrupt on PC0. The firmware's last revision abandoned it for a PPM receiver, which takes over PD2.

The ESC outputs are four 3-pin headers labelled by frame position: TL on PD5, TR on PB2, BL on PD6, BR on PB1. All four are hardware PWM outputs, two on timer 0 and two on timer 1. Freeing timer 0 for that is why the firmware ships its own Arduino core, which moves millis() onto timer 2.

Power arrived on a JST connector behind a 2 A PPTC and a MOSFET for reverse polarity, then a 7805 and a TS1117 gave 5 V and 3.3 V. Battery voltage reached PC3 through 110k and 20k over 9k1, which the schematic notes as Vout = Vbatt * 0.06542056.

Indication is a signal LED on PC2 and a warning LED on PC1, plus two low side MOSFETs switching an arm-LED header on PD3 and a lamp header on PD4.

Two measurements from a PicoScope in March 2022, on the built board: the ESC drive edges rose in 11.2 ns at 5.065 V, and the IMU's I2C lines in 125 ns at 3.542 V.

Etched at home

There are no gerbers and never were. The copper layers were printed onto transparency and the board was etched by photoresist, so its rules are generous: 18 mil clearance, 16 mil minimum track, 0.8 mm minimum drill. There is a video of the etching.

_{Etched, still purple from the photoresist, before drilling. The clearest record of the artwork that exists.}

[!NOTE] DRC reports three errors, all on J1P0, the JST battery connector, whose pads and holes break the board's own rules. That is inherent to the footprint and predates the KiCad conversion; the board was built and flown that way. The rest is cosmetic silkscreen and thermal relief warnings. Connectivity is clean, with zero unconnected items once the zones are filled.

History

This board is v2. The first flight controller was an Arduino Nano and an nRF24L01+ hand wired on protoboard, and it is the reason this one is lettered Flight Controller v2.0.

_{v1, the protoboard flight controller this board replaced. Its IMU module is missing here.}

It was drawn in Eagle. Autodesk stopped developing standalone Eagle in 2024 and folded it into Fusion, so the design was converted to KiCad in 2026. The original Eagle files are in the first commit of this repository and nowhere else.

Licence

Copyright Vasil Kalchev 2017

Licensed under CERN-OHL-P v2 or later. You may redistribute and modify this source and make products using it under the terms of the CERN-OHL-P v2. This source is distributed WITHOUT ANY EXPRESS OR IMPLIED WARRANTY, INCLUDING OF MERCHANTABILITY, SATISFACTORY QUALITY AND FITNESS FOR A PARTICULAR PURPOSE. Please see the CERN-OHL-P v2 for applicable conditions.