As we are getting close to connecting components and running code now is a good time to start understanding how the PiFace Digital board is put together and which elements we will be using in this project.
The inputs are the set of terminals located at bottom left on the circuit board and is where we will be connecting our float switches. The furthermost left terminal is Pin 0 - followed by Pins 1 to 7 (left to right). The furthermost right terminal is where our 0v connection will be made.
There are eight open collector outputs located at the top of the board, with corresponding LED's, but since we dont need to physically use these outputs I am not going to go into too much detail about their use in this project. What is important about these outputs is that the first two outputs (located on Pin 0 and Pin 1) are by default configured to also trigger the output changeover relays. You will see in later sections how we use Pin 0 and Pin 1 outputs to activate the changeover relays using Python code.
Output Changeover Relays
The two output changeover relays are located on the right hand side of the board (the middle and bottom sets of terminals), we will be using both of these relays 1) to activate our mains water solenoid and 2) to activate our pump. As you look at the three terminals on each block the top terminal is our normally closed contact (n/c) - the middle terminal is our common terminal (c) and the bottom terminal is our normally open contact (n/o).
If you dont understand the terminology being used here dont worry. The relays operate no differently to how light switches are operated in our homes. We just need to tell the relays what to do .... and when.
Please note that the two output relays are only capable of switching voltages up to 20V (maximum) and loads up to 5A (maximum). In this project we will only be switching the 0v side of a 12V DC supply - with the load being limited to an external relay coil.
You will note from the picture at the top of this section that there are seven jumpers located around the PiFace Digital board. We dont need to concern ourselves too much with the functions of these jumper settings as we dont need to change these in this project - however with that said, all seven jumpers do need to be in place as shown.
In the next section we will make a few connections, fire up the PiFace Digital Emulator and see how the PiFace operates with switches connected.