ELECTRONICS - [designs & projects] - [I²C board] - [page 5/8]
5. SCHEMATIC DESIGN
:: 5.1 The complete circuit
:: 5.2 Explication of the circuit
The heart of the circuit: PCF8574.
The whole circuit of the I²C I/O board is build around the IC PCF8574, which provides the
I²C communication. The hardware address can be set by DIP switches on the pens A0, A1 and A2.
These inputs are connected to the ground when the switches are open via the resistors
R3, R4 and R5 (pull down). A value of 1k has been chosen for these resistors, but this value
is not critical. But they shouldn't be too low because a big current will flow in this case,
when 1 or more of the DIP switches are closed. Every board must have a unique address or
problems will occur.
The SCL and SDA lines are connected to the +Vcc line via pull-up resistors. This is a require for the I²C bus. These lines are also available on the connector J2, so other circuits can be connected to this board. The board which is connected to the 8051 evaluation board is called the "master". Any other boards are called "slaves".
The connector J3
Because the slave board aren't connected directly to the 8051 evaluation board, you have
to provide a power supply. The connector J3 allows you to power the slave boards with
the master board: you just need to make a connection between the J3 connectors of both
boards.
The I/O circuits
Every I/O port contains 3 resistors, a switch and a light emitting diode. The first resistor
has a value of 470 Ohms and is a pull-up resistor to bring the outputs of the PCF8574 to
a high level (logical "1"). The switches can be used to bring the outputs to a low level (
logical "0"), this can be detected by the PCF8574. A led and a resistor are placed in parallel
with the switches. These LEDs gives an indication of the PCF8574 outputs.
:: 5.3 The print layout
Component placing (Click picture to enlarge)
Solder side (Click picture to enlarge)
:: 5.4 Circuit Price
| Part | Amount | Price/Piece | Total Price |
| Resistor 470 | 16 | 0.05 | 0.80 |
| Resistor 1 k | 5 | 0.05 | 0.40 |
| DIP Switch 4P | 1 | 1.30 | 2.60 |
| IC Socket - DIL16 | 1 | 0.29 | 0.29 |
| IC PCF8574 | 1 | 2.89 | 2.89 |
| Leds 5 mm (Orange) | 8 | 0.18 | 1.44 |
| Switch On/Off | 8 | 0.75 | 6.0 |
| Screw Connector 2P | 2 | 0.52 | 1.04 |
| Flat cable Connector 24P | 1 | 1.00 | 1.00 |
| Photo print 160x100 | 1 | 3.25 | 3.25 |
| TOTAL | € 19.71 |
The price of a complete master print is 19.71 euro. It might occur you don't need all the components for your application, this might lower down the total price.