Hands-On Embedded Programming with C++17

Relays

To control the status lights and the non-permanent power outlets in the room, we use a number of relays, in this case four relays:

Relay	Function
0	Power status of non-permanent outlets
1	Green status light
2	Yellow status light
3	Red status light

The idea here is that the power status relay is connected to a switch that controls the mains power to outlets that are not powered when the club status is off. The status lights indicate the current club status. The next section provides the details on the implementation of this concept.

To simplify the design, we will use a ready-made relay board containing four relays, which are driven by an NXP PCAL9535A I/O port chip (GPIO expander) connected to the ...