The OWL Driver Board is a solid-state relay HAT for the Raspberry Pi. It was developed for use with the Open Weed Locator but is suitable for any automotive or agricultural application.

It allows a Raspberry Pi to drive 4x 2A outputs in any 12V or 24V system. It is able to tolerate high levels of electrical noise, reverse polarity, short circuit conditions and over voltage.

• Working voltage 7-26V
• 4x fused outputs, each max. 2A, high-side switched
• 1x fused input max. 7A
• Integrated PCA9685 PWM driver allows optional PWM control, compatible with Adafruit PCA9675 Python libraries
• Outputs can be driven in multiple ways, configurable by a header
  • From Raspberry PI GPIOs
  • From an on-board PCA9685 PWM driver
  • Directly from external control, via wires to a header. Control inputs are tolerant up to 26V.
• Buck converter generates 5V @ 3A / 5A burst to power the Raspberry Pi. Powers the Raspberry Pi 5.
• Input and output connectors use high-quality Phoenix Contact screw terminals
• Input and outputs are fused, with protection against over-voltage, reverse polarity, and inductive spikes
• LED indication of blown fuses

The board aims to be difficult to misconfigure and hard to damage. Self-resetting polyfuses on the input and outputs limit current, and TVS+Schottky diodes provide reverse polarity protection, protection against noise and inductive voltage spikes, and over-voltage protection. This protection also extends to communication buses with the Raspberry Pi.

Purchase a finished board

You can buy a finished board via the following distributors:

• paradar.co.uk (currently shipping v2.4)

Changelog

v2.4

• Add an "output enable" terminal (J10) for use with an emergency stop switch. Outputs are enabled when the terminal is shorted; they are disabled when it is disconnected.

v2.3

• Improved Pi regulator efficiency (81% -> 87%) to reduce heat generated when driving a Raspberry Pi 5

v2.2

• Change out the switching buck regulator from a 2A max limit to a 5A model. Increase the regulation voltage from 5.0V -> 5.1V to drive a Raspberry Pi 5 without undervoltage warnings.
• Add a LM73100 ideal diode on the Pi 5V output to ensure there is no backflow of current from the Pi into the board when powered off, and the Pi has a separate USB-C power supply.

v2.1

• Add a slot for the Raspberry Pi camera connector, to allow the ribbon cable to easily pass through the driver board.
• Remove stacking power connector to allow space for the cable slot. If stacking is required, board power inputs should be wired in parallel.
• Fix major error in v2.0 where MOSFET rotations were incorrect
• General manufacturability improvements (component polarity in silkscreen)

v2.0

• Significantly rework layout to allow space for instructions on the top. Move most components to the bottom layer.
• Change MOSFETs from TO-220 to a compact SMD footprint.
• Add indicator LED to each output and the power input.
• Switch to generic spring connectors that are PTH rather than SMD; the Wago SMD connectors did not hold up well in use.
• Change diodes from SMA to DO-123F to save space.
• Change most resistors from 0805 to 0603 to save space.

v1.0

Initial prototype.

Schematic and layout

KiCanvas links:

• Schematic
• Layout

License

This open hardware project is Copyright (c) 2022 Guy Coleman and Patrick Coleman, and is released under an MIT license.

Stacking

The HATs may be stacked and controlled with the PCA9685 by selecting different I2C addresses for each board in the stack. In this configuation, the input power terminals should be wired in parallel. Ensure that the wires used are rated for sufficient current.

When stacked and in GPIO mode, turning on output #1 from the Raspberry Pi will turn on all #1 outputs in the stack.

For individual control of all outputs in the stack

1. Set jumpers on header J6 to control outputs via the PCA9685 (the "PWM" setting).
2. Set SW1 to select a different address for each board in the stack.
3. Use the Adafruit PCA9685 Python library on the Raspberry Pi to communicate with each board via the chosen I2C addresses and control outputs individually.

Pi pin 22 (GPIO25) is connected to the PCA9685 Output Enable pin. Driving this pin high on the Pi will disable all outputs in the stack (eg, for a software emergency stop). It must be driven low for normal operation.

Manufacturing

The project is set up for easy manufacture and assembly with JLCPCB, though it will work with other suppliers too. All components should be available at JLCPCB/LCSC and LCSC part numbers are included in KiKad fields for each part.

If you are having JLCPCB assemble the boards for you, you must manufacture in panels (two boards per panel), as the board is too small for assembly alone. If you will be assembling yourself, use pcb/gerbers/gb-owl-driver-board.zip for a single board.

1. Upload pcb/gerbers-panel-v2.4/gb-owl-driver-board-v2.4.zip to JLCPCB's order tool
2. Configure the PCB manufacture settings to your requirements. Select "Confirm production file: Yes"
3. Select SMT assembly, and configure as follows: "Standard" assembly, "Both sides", and "Confirm production file". Click Confirm.
4. When prompted, upload the BOM (pcb/owl-driver-board_bom_jlc.csv) and Component Placement/CPL (pcb/owl-driver-board_cpl_jlc.csv) files.
5. Complete the SMT assembly wizard, aligning the parts to the board if necessary.