• Status: Draft
• Raspberry Pi 4 baseboard/HAT for freshwater planted-aquarium monitoring and control.
• The board distributes external regulated 5 V power to the Pi, hosts external I2C modules, reads wired sensors, dims three 12 V LED channels, and triggers a fish feeder through galvanic isolation.

• Personal/learning aquarium controller used indoors near freshwater equipment.
• First revision is a functional prototype; moisture exposure and cable fault protection are important.

• Mate with the Raspberry Pi 40-pin header.
• Accept regulated 5 V DC through an 8-position 5.08 mm screw terminal and supply the Pi 5 V rail.
• Connect Atlas Scientific EZO-pH carrier, Adafruit PCA9685 module, DS3231 RTC module, and a 0.96-inch OLED over I2C.
• Read one 1-Wire temperature probe and two leak detector inputs.
• Provide three low-side 12 V PWM LED channels controlled by PCA9685 outputs.
• Provide one PC817-isolated fish-feeder trigger.
• Export four Raspberry Pi GPIO signals through the power/logic umbilical terminal.

• Raspberry Pi 40-pin female stacking/header interface.
• 5 V input path rated for 3 A continuous design target.
• Shared 3.3 V I2C bus connectors.
• 1-Wire pull-up: 4.7 kOhm to 3.3 V.
• Three AO3400 N-channel MOSFET channels with gate biasing, rated for 2.5 A maximum continuous load per channel.
• One 5-position, 5.08 mm-pitch LED field terminal: +12V, 12V return/main GND, and three switched MOSFET drains.
• PC817 optocoupler input/output interface for feeder control.
• Edge-mounted 8-pin, 5.08 mm-pitch terminal block.

Diagram

• Terminal pins 1 and 2: protected board 5 V input, paralleled.
• Terminal pins 7 and 8: board ground, paralleled.
• Feed both Raspberry Pi 5 V header pins and multiple ground pins.
• Use wide copper/pours and appropriately rated terminal contacts for 3 A.
• Add input bulk/ceramic decoupling plus practical surge and overcurrent protection without excessive voltage drop.
• Do not use a conventional series diode in the 5 V input path for reverse-polarity protection.

• Raspberry Pi GPIO2/SDA1 and GPIO3/SCL1 at 3.3 V logic.
• Female sockets/headers for EZO-pH carrier, PCA9685, DS3231, plus a 4-pin OLED header.
• Avoid unintentionally excessive parallel pull-ups from breakout modules; board-level pull-ups may be optional/DNP if included.

• 1-Wire data uses Raspberry Pi GPIO4 and a 4.7 kOhm pull-up to 3.3 V.
• Leak input 1 uses GPIO22 and leak input 2 uses GPIO23. Both use Raspberry Pi internal pull-ups and external ground-contact sensing.

• Three PCA9685 channels drive AO3400 gates through series resistors; each gate has a pull-down.
• Maximum continuous LED load is 2.5 A per channel.
• MOSFET sources connect to main board GND; drains connect to external 12 V LED negative outputs through a single 5-pin field connector. The external 12 V return is tied to the same main GND net.
• PCA9685 gate drive target is approximately 5 V. The AO3400 design estimate uses RDS(ON) approximately 36 mOhm, giving about 0.225 W conduction loss at 2.5 A. Final thermal acceptance must be verified against the selected device datasheet, PCB copper, and enclosure ambient.
• PC817 LED input is current-limited from Raspberry Pi GPIO17. The transistor side is exposed as an isolated dry/open-collector-style output connector and is not tied to board ground.

• GPIO2 / physical pin 3: I2C SDA.
• GPIO3 / physical pin 5: I2C SCL.
• 3.3 V and ground supplied to external low-voltage modules.

• External source must be a regulated 5 V supply capable of at least 3 A at the board after cable loss.
• The HAT does not generate the external 12 V LED supply.

• Professional mixed SMD/through-hole PCB.
• Through-hole Raspberry Pi header and 5.08 mm screw terminals; SMD semiconductors/passives.
• Initial target: standard 2-layer prototype unless placement/current-return constraints justify 4 layers.

• I2C on GPIO2/GPIO3.
• Umbilical GPIO mapping fixed to GPIO5, GPIO6, GPIO13, GPIO19.
• 1-Wire temperature input on GPIO4.
• Leak inputs on GPIO22 and GPIO23.
• Feeder trigger on GPIO17.
• PCA9685 LED outputs use channels 0, 1, and 2 for LED channels 1, 2, and 3 respectively.
• Leak inputs configured with internal pull-ups; active state is contact closure to ground.

• PCB outline: 85 mm x 65 mm, with Raspberry Pi mounting/interface geometry retained.
• Screw terminal and field connectors accessible at board edges.
• Keep low-level pH/I2C cabling away from PWM LED current loops where practical.
• Enforce a minimum 1.5 mm trace width for the shared +12V_LED terminal line and each MOSFET drain/current-path trace. Use copper pours or wider traces where routing and thermal performance permit.

• Replace the earlier RJ45 umbilical concept with an 8-pin 5.08 mm screw terminal.
• Paralleled terminal contacts are used for both 5 V and ground.
• The Pi is powered through its 5 V header rail; therefore the external source must be accurately regulated and protected against accidental reverse/back-power conditions.
• No series diode is permitted in the 5 V path because its voltage drop would compromise Raspberry Pi supply margin.

• The 5 V source is SELV, regulated, and dedicated to this system.
• PCA9685 is an external Adafruit module rather than a bare IC.
• EZO-pH, RTC, PCA9685, and OLED connector pinouts will be matched to documented module headers during component selection.
• LED current is limited to 2.5 A continuous per channel. AO3400 thermal safety remains subject to datasheet and PCB-copper verification rather than package rating alone.

• Replaced RJ45 power/logic umbilical with an 8-pin 5.08 mm screw terminal.
• Fixed terminal GPIO mapping and established a 3 A 5 V copper requirement.

• GPIO4 is 1-Wire temperature data. GPIO22 is leak input 1. GPIO23 is leak input 2. GPIO17 drives the PC817 feeder input.
• PCA9685 logic/gate drive target is approximately 5 V. Use I2C level shifting so module-side pull-ups to 5 V cannot expose Raspberry Pi SDA/SCL to 5 V; keep PCA9685 VCC logic separate from V+ servo power, and leave V+ unused for this design.
• Maximum continuous LED load is 2.5 A per channel. AO3400 design estimate uses about 36 mOhm at approximately 5 V gate drive, giving about 0.225 W at 2.5 A. Final thermal safety depends on PCB copper and ambient and remains a layout verification item.
• Use one 5-position 5.08 mm LED terminal: pin 1 = +12V_LED input/common; pin 2 = GND / 12 V return tied to board main GND; pin 3 = MOSFET drain channel 1; pin 4 = MOSFET drain channel 2; pin 5 = MOSFET drain channel 3.
• Give the LED connector and MOSFETs current/voltage-rating metadata. In later PCB layout, use at least 1.5 mm trace width on +12V_LED and all three MOSFET drain/current-path nets.
• The 5 V input path must contain no conventional series diode. A low-loss fuse/PTC, shunt TVS, and bulk/ceramic capacitance are permitted. Omit reverse-polarity protection rather than introducing unacceptable series drop.
• Board outline target for the later layout phase is 85 mm x 65 mm.
• Added 2.5 A-per-channel LED load, consolidated 4-pin LED terminal mapping, AO3400 loss estimate, and 1.5 mm minimum LED-current trace requirement.
• Finalized GPIO4/GPIO22/GPIO23/GPIO17 sensor and feeder assignments, changed LED wiring to a 5-pin terminal with 12 V return tied to main GND, prohibited a 5 V series diode, and set the PCB outline to 85 mm x 65 mm.