Hardware and software for a power and IO extension PCB with current monitoring and I2C interface.

This project has been generously supported by PCBWay. Read more about this project and also my review of the manufacturing on my blog: Design And Assembly Of An I2C Relay PCB

Use KiCad with my CustomComponents library to view and edit the hardware project, or download the fabrication files for PCB and/or PCBA manufacturing. I used KiBOM to generate the BOM. You can also download the complete schematics as PDF.

See the Arduino library documentation for usage with the Arduino framework. The library is also available on PlatformIO. Use Visual Studio Code and the PlatformIO plugin to edit the Arduino library and tests.

See the Raspberry Pi library documentation for usage with the Raspberry Pi Python 3 smbus framework.

• Power input via 5V and 3V3 pins
  • Min. 250mA @ 5V to actuate all four relays simultaneously
  • Less than 30mA @ 5V standby current
  • 3V3 voltage level is only required for I2C level shifting if the I2C signal level is 3.3V instead of 5V
• I2C bus for connection to a I2C master via JST header RPI
  • I2C bus passthrough via JST header PASS
  • Internal pullup resistors
  • Level-shifting 3.3V - 5V tolerant bus
• Four SRD-05VDC-SL-C three-lead (NC - COM - NO) magnetic relays
  • Each has a CC6902SO-10A inductive current measuring IC connected across COM
  • Galvanically isolated driver circuit using an optocoupler
  • Flyback diode protection
  • Red status LED indicates COM - NO actuation
  • 2oz thick copper traces enable high currents
  • Isolation milling reduces risk of shorting in damp environments
  • Maximum power: 10A, 250VAC, 110VDC
  • Contact load rating: resistive 7A @ 28VDC, 10A @ 125VAC, 7A @ 240VAC, inductive 3A @ 120VAC, 3A @ 28VDC
• One PCA9557 8-channel GPIO port (I2C address: 0x18), logic level 0V | 5V
  • GPIO channels 0Z - 3 connected to JST header DIGITAL
  • Relays are connected to GPIO channels 4 - 7
  • Maximum current (except channel 0Z): 25mA sink, 20mA source per channel, max. total: 100mA sink, 85mA source
  • Channel 0Z is high-impedance and open-drain (see datasheet for details)
• Two ADS1115 4-channel 16-bit ADC ports, voltage range 0V - 5V
  • All four pins of ADC 1 connected to current measuring ICs (I2C address: 0x48)
  • All four pins of ADC 2 connected to JST header ANALOG (I2C address: 0x49)
  • Configurable sample rate and gain amplifier
  • Max. 10mA continuous input current

For further specifications and ratings, all datasheets can be found in this repository.

Due to the inherent nature of magnetic relays, the inductive current measuring ICs and also the ADCs experience a measurement offset while the relays are energized. It is recommended to measure the actual load at a distance from the board, in order to obtain reference values. These reference values should then be used to compute an offset or function for each channel and amount of relays energized.

For no load (floating) using an Arduino Uno, the maximum absolute measurement offset was found to be about as follows.

Please note that these offsets are dependent on load and environmental factors.

17 unique parts, 60 SMT parts, 12 THT parts on a double-sided 90mm x 71mm PCB with 2oz copper finish.

Component cost per board: about $25 (Ignoring shipping, bulk discounts and changes in price over time).