I2C Relay Control Board

The I2C Relay Control Board provides control for 16 relays from the Express Receiver I2C bus. Two I2C Relay Control Boards are used in the Express Receiver:

  • One board controls the relays in the RF front end, i.e., band pass filter selection, attenuator control, and RF pre-amp in/out
  • A second board controls relay selection of the 2nd IF crystal filters
I2C Relay Control Board, top view
I2C Relay Control Board, top view
I2C Relay Control Board, bottom view
I2C Relay Control Board, bottom view

The I2C Relay Control Board was originally going to mount vertically in the lower deck of the receiver, hence only two mounting holes where included on the board. However, the board was finally mounted horizontally on the top deck, so two 1/4-inch plastic stand offs were epoxied to the bottom of the board for support.

I2C Relay Control Board Schematic
I2C Relay Control Board Schematic

The heart of the I2C Relay Control Board is U3 a PCF8575C, 16-bit I2C bus I/O expander in a 24 pin SOIC SMD package, Digi-Key part number 296-17915-1-ND. The PCF8575C provides 16 open drain outputs that drive U1 and U2, two ULN2803A Darlington transistor arrays.

The PCF8575C does not include pull up resistors on its outputs, so external pull up resistors must be provided when driving the ULN2803A. R7 through R22 provide the necessary pull up to Vcc.

J1 and J2 provide daisy-chained connection to the Express Receiver I2C bus. I2C pull up resistors R5 and R6 are not installed on the I2C Relay Control Board as I2C bus pull up resistors are installed on the Main Display and Control Board.

The I2C address for the PCF8575C is set using A0, A1 and A2 on pins 21, 2 and 3, respectively. On the I2C Relay Control Board, A0, A1 and A2 are pulled up to Vcc (logic 1) if no jumpers are installed; in this configuration, the I2C address of the I2C Relay Control Board is 0x4E, with the LSB being used for R/W control. Eight I2C address are possible using the three address selection jumpers on header H1.

The PCF8575C interrupt function on pin 1 is not used, so this pin is pulled up to Vcc via R4.

The ULN2803A is a high-voltage, high-current Darlington transistor array in an 18 pin SOIC SMD package, Digi-Key part number 296-15777-1-ND. The device consists of eight NPN Darlington pairs that feature high-voltage outputs with common-cathode clamp diodes for switching inductive loads. The collector-current rating of each Darlington pair is 500 mA. With built-in clamp diodes, the ULN2803A can be used to drive relays without the need for external diode clamps on the relays.

U4 is an NJM78L05UA 3 terminal regulator in an SOT-89 SMD package that provides +5V Vcc for the PCF8575C. Mouser part number 513-NJM78L05UA-TE2.

All relay control connections and the common relay connection are filtered / isolated using L1 through L17 in conjunction with C1 through C17. L1 through L17 are 1.5µH inductors in 1206 SMD packages, Digi-Key part number 535-11663-1-ND.

All 0.1µF capacitors are SMD 1206 style chip capacitors. C19 is a 3.3µf, 16 volt tantalum capacitor in a 1206 SMD package, Digi-Key part number 399-8284-1-ND.

Resistors are 1/8W 5% SMD 1206 style chip resistors.

For J1 through J9, refer to the RF BandPass Filter Board discussion on RF/IF inter-board connections, and control and power inter-board connections.

The ExpressPCB .pcb file for the board is here: I2C Relay Control Board Final.

© 2014 – 2015 Rod Gatehouse AD5GH

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