Custom pcb for controlling the electric components of a tractor with a microcontroller. During the restoration of an IHC-tractor a custom pcb was developed and built to replace the broken relay based control.

Goal

The electric system of this tractor has quite some complex logic which was originally implemented using several relays. Because the original setup was significantly damaged over 57 years of extensive use, I decided to rebuild the control by developing a custom circuit board with a microcontroller as core component for easy extendability.

Reasons for this project

Saving several separate original components, thus saving money and reducing the chance of faults:
- Flasher unit for blinker
- Flasher unit for warning lights
- Wiper control unit
- Special relay for high beam
- Special indicator switch
- 3x relay for brake light logic
- Much less wires, since all the logic is done by one pcb
Flexible logic
New features:
- Low voltage alarm
- Acoustic blinker signal
- Custom horn signals/presets
Option for more features by just upgrading the firmware

Original wiring

Course of action

Planning

Requirements

Stable and regulated 5V supply
Buzzer for acoustic signals
Connector for programming
Protected Inputs against electrical interference
Inputs:
- 6x digital input (switch to GND)
- 3x digital input (switch to 12V)
- Analog input to measure the input voltage
Outputs:
- 2x 17A Mosfet output (GND)
- 8x 16A Relay output (12V)

Schematic

After analysing the existing system and planning the required features a schematic was developed: Finished Schematic

Components

Atmega8 microcontroller
8-Channel mosfet array (ULN2803APG)
8x 12V 16A Relay
2x mosfet (IRFZ24N)
Wago pcb terminals
PCB size: 100x160 mm

Layout

According to the created Schematic the PCB-layout was designed for a single sided PCB, using as few bridges as possible. 3D model of the PCB

Finished PCB layout

The Git repository for the designed board is available on GitHub.

Production

Isolation milling

Instead of sending the files to a professional PCB manufacturer, I decided to produce it myself using the isolation milling method.

Steps

Export Gerber files from layout software (KiCad / PcbNew)
Convert Gerber files to G-code for CNC Machine (pcb2gcode)
Mill a plane slot into wood, for alignment
Fixate raw PCB using screws in free areas
Drill holes using 0.8 and 1 mm cutters
Isolation milling using a 60 degree engraving tool

Isolation milling

For more detailed instructions and experiences, refer to the PCB repository.

Assembly

Soldering

With the help of the layout and schematic the correct components can be soldered onto the pcb.

Finish

After all the components were soldered, evaluated and the first test was successful, the back side of the board was coated with a plastic layer for insulation and protection against corrosion.

Soldering