This expansion board is designed to operate connected to the Raspberry Pi model 2 or 3 +5V pin and so it’s power by the Raspberry Pi board itself. The +5V are also required for the relays to operate. The open collector output terminal block of this board has a terminal which is used to supply power which, by default, supplies +5V.

 

   At the current state, PCB design needs to be reviewed since this is not properly implanted as the terminal supposed to provide +5V is currently connected to GND. This expansion board also requires +3.3 V to supply the integrated circuits, which is always taken from the Raspberry Pi. Also, the maximum voltage on all digital input/output pins is +5V.

 

   The expansion board itself is a double layer PCB with green solder resist and white silkscreening. The top of the board is populated with the ICs, relays, switches, optocouplers, terminal connector blocks and, in the bottom layer, there’s the IC that drives the 7- segment display along with all the needed resistors for it to work properly. This expansion board uses two Microchip MCP23S17 16-bit I/O expander with SPI Interface. This version connects using high-speed SPI, up to 10Mhz, and provides a 16-bit bidirectional I/O. One of them working to provide the eight inputs, and eight outputs. The eight inputs are connected with a series resistor to make them +5V tolerant and the eight outputs are connected to the Texas Instruments ULN2803A Darlington Transistor Array, for the open collector outputs. The transistor array as an absolute maximum collector-emitter voltage of 50 V, with up to 500mA on a single output and 2.5A total for the substrate.

 

   Another major component on this expansion board are the OJE-SH-105DM SPST Relay. We advise not using these with switch voltages greater than 20 V or currents greater than 5 A, not because they would be damaged, but to protect the other lowvoltage components close to these. The relays are specified for:

• Rated voltage 30VDC, 250VAC with a maximum switching voltage being 30VDC, 277VAC.

• Minimum permissible load 100mA, 5VDC to ensure contact corrosion is broken through.

• 10ms operate and 4ms release time.

• Electrical endurance of 5A, 250VAC, res, -30oC to +85oC 60x103 operations.

•Mechanical endurance, DC coil 10x106 operations. Finally, we have the optocouplers outputs.

 

   We use the Vishay 4N25 phototransistor output with base connection. Each optocoupler consists of gallium arsenide infrared LED and a silicon NPN phototransistor. The absolute maximum ratings are:

 

• Input:

    - Reverse voltage of 5 V.

    - Forward voltage of 1.5 V. 19

    - Reverse current of 100 uA.

   - Forward current of 60 mA.

   - Surge current of 3 A.

  - Power dissipation of 100 mW. •

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• Output:

  - VCEO = 70V.

  - VEBO = 7V.

   - Collector current range of 50-100 mA depending on test condition.

   - Power dissipation of 150 mW.