If you decide to use a different pin layout from what is shown below,  update the Settings.h file, look for variables ending with Pin

Physical layout

A junction box with 12 holes was used to house the components of the Main module. These boxes are very easy to work with, you just need to drill a slightly smaller hole than the cable diameter in the rubber cap covering the holes. 

Buzzer

When a module starts up or receives a command a piezoelectric buzzer provides audio feedback. To protect the buzzer from excess current make sure to have a minimum 200Ω resistor between the Arduino port and the + side of the buzzer. If you find the volume too loud use a larger resistor. I took a few sound intensity measurements using different resistance values: 220Ω - 71dB, 330Ω - 69dB, 1kΩ - 59dB, 3kΩ and above are inaudible.

+ pin: Arduino 2 (digital output). Connected through a resistor to limit current and volume: 200Ω(loud) - 1kΩ(quiet)

- pin: Arduino GND (Ground)

Test-Buzzer sketch

Expected test result: Plays the Star Wars theme song

Bypass capacitors

Bypass capacitors or decoupling capacitors are added between each power and ground line to filter out noise from the power supply and make the entire circuit more stable. Here is an excellent video from EEVblog that explains bypass capacitors. 

Be mindful that electrolytic capacitors are polarized, meaning they can only be connected one way in a circuit. The negative lead is generally marked on the capacitor with a bar or "-" symbol, connect this to Ground (or the more negative line). Do not mix up the polarity as this will heat up and destroy the capacitor in a few seconds. Ceramic capacitors have no polarity, they can be connected in any direction. The capacitance of both capacitors can be changed based on the power needs, as long they support a minimum of 16V voltage.

DC input 12V
2200 uF - 16V Electrolytic capacitor + pin: +12V DC

2200 uF - 16V Electrolytic capacitor - pin: Ground

0.1 uF - 50V Ceramic capacitor pin 1: +12V DC

0.1 uF - 50V Ceramic capacitor pin 2: Ground

DC step down output 7V
1000 uF - 16V Electrolytic capacitor + pin: DC step down Vo+ output

1000 uF - 16V Electrolytic capacitor - pin: Ground

0.1 uF - 50V Ceramic capacitor pin 1: DC step down Vo+ output

0.1 uF - 50V Ceramic capacitor pin 2: Ground

Arduino 5V

470 uF - 16V Electrolytic capacitor + pin: Arduino 5V

470 uF - 16V Electrolytic capacitor - pin: Ground

0.1 uF - 50V Ceramic capacitor pin 1: Arduino 5V 

0.1 uF - 50V Ceramic capacitor pin 2: Ground

Arduino 3.3V
10 uF - 16V Electrolytic capacitor + pin: Arduino 3.3V
10 uF - 16V Electrolytic capacitor - pin: Ground
0.1 uF - 50V Ceramic capacitor pin 1: Arduino 3.3V
0.1 uF - 50V Ceramic capacitor pin 2: Ground

DC step down power supply

To convert the 12V power input and feed the Arduino and the 8 port Relay module two DC to DC step down power supplies are used. These buck converters are cheap, power-efficient, have a small physical size, and support a wide range of DC input voltages. To adjust the output voltage you will need to have a multimeter tool to monitor the output voltage while changing the adjuster screw. Be careful: The Vin pin does not have a reverse polarity protection diode like the barrel jack, never mix up GND and Vo+ from the power supply!

Arduino: Using a step down-power supply to power the Arduino Mega2560 is optional as it already supports 12V through either the Vin pin or the barrel jack connector. The reason I opted for using the buck converter to provide 7V to the Vin pin is to reduce the heat generated by the  Arduino's voltage regulator. 

Relay module: The relay module needs 5V to operate. When a single relay is active it uses ~60 mA, when all 8 relays are active the total power consumption is ~480 mA. This exceeds the maximum 200 mA the Arduino Mega2560 can provide on its 5V output pin. This is why we need to use an individual power supply adjusted to 5V and connect it to the JD-VCC pin of the relay module.

Arduino - DC step down power supply

VO+ pin: Arduino Vin. Adjusted to 7 volts.

GND pin: Arduino GND (Ground) + Power input ground (Common ground)

In+: Positive power input (12V)

EN pin: Not connected

Relay module - DC step down power supply

VO+ pin: Relay module JD-VCC. Adjusted to 5 volts.

GND pin: Relay module GND (Ground) + Power input ground (Common ground)

In+: Positive power input (12V)

EN pin: Not connected

Dimming

The B variant of the Meanwell HLG LED driver`s dimming circuit supports 3 different dimming options: 

• 100 kΩ adjustable resistor

• 10 V PWM signal

• 1-10 V DC voltage. 

We are going to use PWM dimming, which works by varying the duty cycle of an Arduino-generated PWM signal and modulating the voltage between the LED driver`s DIM+ and DIM- cables with it. As the HLG LED drivers use 10V signals it cannot be directly controlled by the 5V Arduino,  an electrically isolated switch called an optocoupler is required to interconnect the two systems using different voltage signals.

LED driver 1

Anode(1) pin: Arduino 11 (digital output). Connected through a 680Ω resistor to limit current. 

Cathode(2) pin: Arduino GND (Ground)

Emitter(3) pin: LED driver 1 DIM-

Collector(4) pin: LED driver 1 DIM+

LED driver 2 (Optional)

Anode(1) pin: Arduino 12 (digital output). Connected through a 680Ω resistor to limit current. 

Cathode(2) pin: Arduino GND (Ground)

Emitter(3) pin: LED driver 2 DIM-

Collector(4) pin: LED driver 2 DIM+

Test-DimmingPWM sketch

Expected test result: Increase brightness of Light 1 from 0% to 100% in 20 seconds, waiting 5 seconds, and decreasing the brightness back to 0% in 20 sec. After 5 sec the cycle restarts.

Note 1: The HLG dimming circuit can measure the duty cycle of PWM signals in the 100 Hz - 3 kHz frequency range. The Arduino Mega2560 supports PWM output on ports 2 to 13 with the default frequency of 490Hz, except for ports 4 and 13  where the frequency is 980Hz. Check out this Arduino Forum entry to customize PWM frequency. The Main sketch by default depends on Timer 1 that controls ports 12 and 11 to generate the 490Hz PWM frequency for dimming.

Note 2: Most drivers do not allow dimming to 0%. For example, HLG does not recommend going below 6%. This limit can be adjusted in Settings.h by changing the DimmingLimit in the LightsSettings section. 

Note 3: When the Arduino PWM signal is HIGH the collector and emitter on the Output legs are shorted, signaling LOW. This HIGH - LOW difference is called an inverting output, meaning the HLG dimming will measure the inverse of the PWM duty cycle ( Arduino duty cycle: 90% ➞  HLG measures 10% duty cycle, Arduino duty cycle 20% ➞ HLG measures 80% duty cycle). This is compensated in the Arduino sketch. 

Relay module

An 8 channel relay module is used to control AC power to the LED driver 1 and 2, a two-speed internal fan, a two-speed exhaust fan, reservoir air pump and one other devices of your choice. Multi-channel relays tend to use negative logic, where: HIGH = relay OFF, LOW = relay ON. You can find more details of how relays work at lastminuteengineers.com.