Hardware · drdro-powercom

The Power HAT

A Raspberry Pi HAT that powers the Pi from a wide-input DC supply and gives it a protected RS-485 link to the drDRO controller board — the power-and-comms interface of the system. Designed in EasyEDA Pro.

Hardware repo ↗ Raspberry Pi HAT 4-layer PCB Provvedo LLC · Power Hat V4
3D render of the drDRO Power HAT

Overview

On a drDRO machine the Raspberry Pi runs the touchscreen app and talks to the STM32 controller over RS-485. The Power HAT solves the two practical problems of putting a Pi in a machine cabinet:

Power

Take the machine's DC supply (e.g. 12–24 V) and step it down to a clean 5 V to run the Pi — with a resettable fuse, reverse-polarity protection and input surge/TVS.

Comms

Bridge the Pi's UART to a half-duplex RS-485 bus (the drDRO line protocol), with automatic direction control and bus-side TVS protection.

  • Wide DC input → 5 V via a synchronous buck (SCT2650, 4.5–60 V range).
  • RS-485 transceiver (SP3485, 3.3 V, ≤ 10 Mbps) with automatic TX/RX direction — no GPIO needed.
  • Status LEDs: 5 V present, 3.3 V present, TX activity.
  • Pluggable 3.81 mm terminal blocks for DC input and for the controller (5 V + A/B).
  • Raspberry Pi HAT form factor (40-pin header, four corner mounting holes); 4-layer PCB, 1 oz outer copper.

How it works

power & comms path
DC input ─▶ PPTC fuse (U3) ─▶ series Schottky D3 ─┬─ TVS D2 ─┬─ VDC ─▶ Buck U4 (SCT2650) ─▶ 5V ─▶ Pi 5V rail
(VCC/GND)                     (reverse-polarity)  │ (surge)  │        + L1 4.7µH, bulk caps
                                                  └──────────┘
Pi 3V3 ─▶ RS-485 transceiver U2 (SP3485) ─▶ series R + TVS D1 ─▶ A / B ─▶ terminal block (to controller)
Pi UART TXD/RXD ───────────▲  auto-direction (Q1 driven from TXD)
  • The 5 V the Pi runs on is generated on the HAT and fed back into the Pi through the header; the Pi's 3.3 V rail powers the RS-485 logic side.
  • Direction is automatic — Q1 drives the transceiver's DE/RE from the TX line, so the Pi only needs its normal UART.
🔧

The 5 V rail is adjustable — set it to 5.1 V. The buck's feedback divider includes a multiturn trimmer (R12). Before relying on the board, measure the output with a multimeter and trim it to 5.1 V, giving the Pi and its USB devices a little headroom above 5.0 V under load. See the calibration step under Using it.

Specifications

Form factorRaspberry Pi HAT (40-pin GPIO, 4 corner mounting holes)
PCB4-layer, 1 oz outer copper
DC inputWide DC — buck rated 4.5–60 V, bulk caps 63 V; keep the supply ≤ 36 V (bounded by the input TVS, SMDJ36CA). A 12–24 V machine supply is ideal.
Output5 V to the Raspberry Pi — adjustable via multiturn trimmer R12; set to 5.1 V before use
CommsHalf-duplex RS-485, ≤ 10 Mbps, auto-direction; A/B on a terminal block
Connectors2 × 4-pos 3.81 mm pluggable terminal blocks (DC in, controller out) + 2×20 GPIO header
Indicators5 V, 3.3 V and TX-activity LEDs
⚡

Single power source. The HAT supplies the Pi's 5 V rail — do not also power the Pi from its USB-C connector at the same time.

Connectors

ConnectorTypeSignalsPurpose
DC input4-pos 3.81 mmVCC, VCC, GND, GNDMachine DC supply in (doubled pins for current)
Controller4-pos 3.81 mmGND, 5V, A, BRS-485 + power to the drDRO board
GPIO header2×20, 2.54 mm5V, GND, TXD/RXD (GPIO14/15), 3V3Mounts on the Raspberry Pi

Key components

JLCPCB/LCSC parts throughout; the full BOM is exportable from the EasyEDA Pro project.

RefPartFunction
U4SCT2650STERWide-input (4.5–60 V) step-down regulator → 5 V
L1SRP1265A-4R7M (4.7 µH, 13.5 A)Buck power inductor
U33425L300/48QR (PPTC, 3 A hold)Resettable input fuse
D3 / D4SL10100 SchottkyReverse-polarity / freewheel
D2SMDJ36CA (bidirectional TVS, 36 V)Input surge protection
C10/12/14100 µF / 63 V (Rubycon RVT)Input bulk
U2SP3485EN-LRS-485 transceiver (3.3 V, 10 Mbps)
Q1MMBT3904RS-485 auto-direction control
D1SM712 (CDSOT23)RS-485 A/B TVS protection
R123224W 10 kΩ multiturn trimmer5 V output adjust (set to 5.1 V)
LED1–3KT-0603RTX activity / 5 V / 3.3 V indicators

Board views

3D render

3D render

Assembled board.

2D PCB layout

PCB layout

Top + bottom copper.

Schematic

Schematic

Power + RS-485.

Open & edit the project

The design is an EasyEDA Pro project (power-hat-project.epro2).

  1. Install EasyEDA Pro

    Get it from pro.easyeda.com (desktop or web).

  2. Import the project

    File → Import → EasyEDA Pro and select power-hat-project.epro2 from the repo.

  3. Everything's inside

    Schematic, PCB, footprints, symbols and 3D models are all contained in the archive; parts reference LCSC components, ready for assembly.

Manufacturing

From the opened project in EasyEDA Pro:

  • PCB: export Gerbers (4-layer, 1 oz outer copper) + drill files.
  • BOM: export the bill of materials (LCSC part numbers already assigned).
  • Assembly: export the pick-and-place (CPL); the design targets JLCPCB SMT assembly.

Using it

  1. Assemble the board

    Or order it fabricated + assembled.

  2. Calibrate the 5 V rail first — with the HAT not on the Pi

    Apply the DC input, put a multimeter on the 5 V output, and turn the multiturn trimmer (R12) until it reads 5.1 V. Then remove input power. This protects the Pi from ever seeing an un-trimmed voltage.

  3. Mount it

    Plug the HAT onto the Raspberry Pi's 40-pin header.

  4. Wire power in

    Machine DC supply (≤ 36 V) into the DC-in terminal block (VCC / GND).

  5. Wire the controller

    The controller terminal block (5V / A / B / GND) to the drDRO board's RS-485 + power.

  6. Power on

    The HAT powers the Pi and bridges its UART to the RS-485 bus. The LEDs confirm 5 V, 3.3 V and TX activity.

✅

Next: flash the Pi with the appliance image and make sure the board runs compatible firmware.