# Raspberry Pi Extension board

A custom PLC-like system based on the Raspberry Pi.  
With a custom interface board, a Raspberry Pi is extended with protected I/O, bus systems, analog inputs, and Relay or PWM outputs. All components, including a display, are housed in a custom-designed enclosure.  
  
The project is intended for versatile use in prototyping and project development.


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### Photo of all hardware components
<p align="center">
  <img src="doc/photos/all-components.jpg" alt="All Components (Disassembled)" width="75%"/>
</p>


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# Repository Content
- **KiCad Projects**: 3 KiCad projects with schematics and PCB layouts for the custom PCBs created.
- **Housing**: Custom enclosure design for PCBs and Raspberry Pi created in FreeCAD.
- **Software/Firmware**: Python scripts for operating and testing the PCB features (WIP).


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# Features Overview
This project provides:
- Protected GPIO I/O for Raspberry Pi.
- Various analog inputs.
- Flexible power supply options (3.3 V, 5 V, -5 V, 12 V, 24 V outputs).
- Relay and MOSFET control.
- Integrated LEDs for status indication.
- RS485, I2C, SPI, and UART bus communication.

Detailed features of each PCB are described in the respective sections below.

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# Designed PCBs

## 1. **Raspberry Pi Interface Board**
This board connects to the Raspberry Pi via a 40-pin ribbon cable and provides protected GPIO extensions and versatile input/output features.

### Photo
<img src="doc/photos/rpi-interface-board.jpg" alt="Raspberry Pi Interface Board" width="80%"/>

### Features
**Inputs**:
- **8x Digital Inputs**:
  - Wide continuous voltage range (-1.7 V to 120 V), idea: compatible with 3 V and 24 V devices.
  - TVS diodes for ESD and spike protection.
  - Reverse polarity protection.
  - Isolated with optocouplers.
  - Optional low-pass filters (toggle via DIP switches).

- **8x Analog Inputs**:
  - Different fixed range inputs:
    - 2x 0 - 3.3 V.
    - 2x 0 - 5 V.
    - 1x 0 - 12 V.
    - 1x 0 - 24 V.
    - 2x 0 - 20 mA.
  - Overvoltage protection with clamping diodes.
  - Optional low-pass filters (toggle via DIP switches).

**Outputs**:
- 1x onboard buzzer.
- 2x 16 A relays.
- 2x high-power MOSFETs (N-channel, max 55 V, 33 A continuous, 160 A pulsed).
- 8x digital outputs (via shift register):
  - Low-power (30 mA push-pull) and high-power (500 mA open-drain) outputs.
  - Buzzer and relays connected to channels 6-8, with enable/disable switches.
  - Note: Outputs are **not short-circuit proof**.

**General**:
- WAGO spring-loaded terminals for easy wiring.
- JST connectors for external LEDs for all inputs and outputs to indicate the current pin state.

**Bus Communication**:
- RS485 (TVS protection, idle pull-up/pull-down, 120 Ω terminator).
- UART (unprotected).
- I2C (TVS diodes, 2.2 kΩ pull-ups).
- SPI (unprotected).
Note: Either RS485 or UART can be used at the same time (select with jumpers).

### Schematic and Layout
<p align="center">
  <a href="pi-interface-board_v1.0/export/schematic_interface-board.pdf">
    <img src="pi-interface-board_v1.0/export/schematic_interface-board.svg" width="49%" alt="Schematic"/>
  </a>
  <img src="pi-interface-board_v1.0/export/layout_interface-board.png" width="49%" alt="PCB Layout"/>
</p>


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## 2. **Power Supply Board**
Creates different voltages from supplied 24 V. Supply for the Raspberry Pi interface board as well as several terminals for variable use (connect sensors, devices, etc., to the housing).

### Photo
<img src="doc/photos/power-supply-board.jpg" alt="Power Supply Board" width="60%"/>

### Features
**Input**:
- 24 V 5 A barrel plug.
- 5 A self-resetting polyfuse.
- Reverse polarity protection.

**Output**:
- 3.3 V, 3 A (buck converter).
- 5 V, 5 A (buck converter).
- -5 V, 20 mA (charge pump).
- 12 V, 3 A (buck converter).
- 24 V (supply voltage filtered).

**Fan Control**:
- 2x connector for 10 kΩ NTC.
- Threshold adjustable (trimmer potentiometer).
- Hysteresis adjustable (trimmer potentiometer).
- Select between 5 V or 12 V fan.

**General**:
- Spring-loaded terminals for each voltage.
- Combined internal terminals for all voltages.

### Schematic and Layout
<p align="center">
  <a href="power-supply-board_v1.0/export/schematic_power-supply.pdf">
    <img src="power-supply-board_v1.0/export/schematic_power-supply.svg" width="55%" alt="Schematic"/>
  </a>
  <img src="power-supply-board_v1.0/export/layout_power-supply.png" width="44%" alt="PCB Layout"/>
</p>

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## 3. **LED Boards**
Small PCBs with LEDs, resistors, and mounting holes for housing indicators.

### Photo
<img src="doc/photos/led-boards.jpg" alt="LED Boards" width="60%"/>

### Boards
1. 2x 5 mm LEDs for relays.
2. 2x 5 mm LEDs for PWM outputs.
3. 8x 3 mm yellow LEDs for analog inputs.
4. 8x 3 mm orange LEDs for digital inputs.
5. 8x 3 mm red LEDs for digital outputs.

### Schematic and Layout
<p align="center">
  <a href="led-boards_v0.1/export/schematic_led-boards.pdf">
    <img src="led-boards_v0.1/export/schematic_led-boards.svg" width="48%" alt="Schematic"/>
  </a>
  <img src="led-boards_v0.1/export/layout_led-boards.png" width="51%" alt="PCB Layout"/>
</p>


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# Software/Firmware
Python code for operating the extension PCBs (e.g., GPIO pins, shift registers, ADC, and bus communication). This section is a work in progress (WIP), and example scripts will be added as development continues.


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# Housing
Custom-designed enclosure includes:
- Ports for Raspberry Pi.
- Banana sockets for power outputs.
- Exposed screw terminals.
- Openings for buttons and LEDs.
- Mounts for all PCBs and wiring.

<img src="doc/graphics/3d-model_housing.png" width="80%" alt="3D Model"/>


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# Dropped Features
The following ideas were considered but not implemented:
- UI Input Elements:
  - Buttons, DIP switches, temperature sensors, encoders, or potentiometers.
- USB or battery-powered operation:
  - Battery packs with BMS and voltage measurement.
  - Li-ion cell holders.
- Analog output (DAC)
- More bus systems
  - RS232 Interface
  - CAN Interface