openDome

An astronomical observatory dome controller based on two Arduino boards. This is the PlatformIO reorganization of the original ArduinoDomeController project, with build variants for the different observatories where it is installed.

Architecture

The project builds two firmwares plus a small test utility:

• src/master/ — the main controller. Drives the dome azimuth motor, reads the rotation encoder and the home sensor, and talks to the PC over USB using the MaxDome II serial protocol (19200 baud) for compatibility with existing astronomy software. When the installation has a motorized shutter, it also acts as the radio bridge to the shutter controller.
• src/slave/ — the shutter controller. Mounted on the rotating dome and powered by a 12 V battery, it drives the shutter (and optionally a flap) DC motors, reads the limit switches, monitors the battery voltage and handles a local keypad.
• src/keypad-test/ — a small utility that prints keypad events to the serial port, used to calibrate analog keypad thresholds.

Shared code lives in lib/:

• lib/MotorDriver/ — DC motor abstraction (MotorDriver), with a generic two-pin driver (DCMotor) and a Monster Moto Shield driver (MMSMotor).
• lib/Keypad/ — analog (resistor ladder on one analog pin) and digital keypads with debounce, hold and double-click events.
• lib/RadioLink/ — the framed protocol codec used on the master↔slave radio link (see below).
• lib/AzGeometry/ — pure azimuth geometry helpers (shortest path, distances, tick counter to position conversion), free of hardware dependencies so they can be unit-tested on the host.

Hardware

• The main controller connects to the PC over USB.
• Azimuth is measured with an optical encoder coupled to a friction wheel; a home sensor provides the absolute reference.
• The azimuth motor is driven either by a generic two-signal DC driver/VFD interface or by a Monster Moto Shield, depending on the build variant.
• The shutter controller communicates with the master through HC-12 serial radio modules at 9600 baud.
• Shutter and flap motors use limit switches for the closed/open positions, plus an interference switch on two-shutter installations (the flap cannot open while the shutter is closed, and the shutter cannot close while the flap is open).
• Pin assignments are defined at the top of src/master/main.cpp and src/slave/main.cpp.

Reducing encoder noise

Brushed DC motors (especially when driven with PWM, as with the Monster Moto Shield) inject noise into the encoder lines that can show up as spurious counts. The firmware decodes the encoder with a quadrature state machine that rejects illegal transitions and cancels out glitches on a single channel, so noise cannot accumulate as position error — but keeping the noise out of the inputs in the first place is still recommended:

• Add external pull-up resistors (1–2.2 kΩ to 5 V) on both encoder channels. The AVR internal pull-ups (~35 kΩ) leave the lines in high impedance, making them very sensitive to motor noise. This is the single most effective measure.
• Add an RC low-pass filter on each channel, close to the Arduino: 1 kΩ in series plus 10–100 nF to GND. Dome encoders produce pulses in the Hz range, so a cutoff of a few hundred Hz filters aggressively without losing real pulses. Optionally add a Schmitt trigger buffer (74HC14) after the filter to restore clean edges.
• Use shielded twisted-pair cable for the encoder, with the shield connected to GND only at the Arduino end, and route it away from the motor wires.
• Suppress the noise at the motor: a 100 nF ceramic capacitor across the motor terminals and, if the case is metallic, 47–100 nF from each terminal to the case. Ferrite beads on the motor leads also help.
• Keep motor and logic grounds separate, joined at a single star point; never share a ground return between the motor and the encoder.
• For long encoder cables (several meters), use a differential line driver/receiver pair (RS-422, e.g. AM26LS31/AM26LS32).

Radio protocol

Master and slave exchange ASCII frames over the HC-12 link:

$<payload>*<XX>\n

where XX is the XOR of all payload bytes, in two uppercase hex digits. Corrupt, overlong or malformed frames are silently discarded and the parser resynchronizes on the next $.

Master → slave	Slave → master	Meaning
open, open1	ack,<cmd>	Open both shutters / main shutter only
close, close1	ack,<cmd>	Close both shutters / main shutter only
abort	ack,abort	Stop shutter movement
exit	ack,exit	Close shutters and disable the watchdog
stat	stat,<n>	Query the combined shutter state (0-5)
vbat	vbat,<nnnn>	Query the battery ADC reading

The master polls stat every second and vbat every 10 seconds from a non-blocking state machine (300 ms response timeout, 3 attempts, automatic recovery), caching the results so the MaxDome II protocol is answered instantly. Action commands take priority over polls and are acknowledged.

Any valid frame refreshes the slave's watchdog: if the link stays silent for 60 s (COMMAND_TIMEOUT) the shutters close automatically, protecting the telescope if the radio link or the master fails.

Build environments

Each environment in platformio.ini corresponds to a specific observatory installation with its own hardware configuration:

Environment	Board	Configuration
master-vst, slave-vst	Arduino Uno ×2	Two interfering shutters (Monster Moto Shield), analog keypads, HC-12 link
slave-ictea	Arduino Nano (ATmega168)	Single shutter with a generic DC driver, digital keypad
master-deva	Arduino Uno	Azimuth only, generic DC driver
master-carda	Arduino Diecimila (ATmega328)	Azimuth only, Monster Moto Shield, 4:1 encoder divider
keypad-test	Arduino Nano (ATmega168)	Keypad calibration utility

Build and flash a variant with:

pio run -e <environment> -t upload

Per-installation options are set through build_flags (NSHUTTERS, ANALOG_KEYPAD, MONSTER_SHIELD, ENCODER_DIV, AZ_TIMEOUT...); see platformio.ini and the defaults in src/master/dome.h.

Note: master and slave firmwares must be flashed from the same revision — the radio protocol is not compatible with the pre-RadioLink plain-text versions.

Tests

The hardware-independent code (lib/RadioLink, lib/AzGeometry) has unit tests that run natively on the host, without any board attached:

pio test -e native

License

MIT — see the headers in the source files.