The IDC-12 cable from the Raspberry Pi terminates inside the switch box.
There, it needs to connect to
- The RFID reader via the serial interface.
- Switches to sense which buttons are pressed (sent to Pi).
- EStop button that is wired up as physical master-reset of the relay Set/Reset flip-flop. Also its state is sensed and sent to Pi to detect that condition in software.
- Trigger relay inputs, controlled from the Pi.
- The trigger relay switches, that are part of the Set/Reset flip-flop
- And finally: the wires to the large mains relay.
To simplify that and improve maintainability, this is a small PCB inside the switch box that provides the necessary connectors.
The IDC-12 connector has the following pinout:
| Desc | Pin | Pin | Desc |
|---|---|---|---|
| +5V | 1 | 2 | +5V (low current; for trigger relays and RFID) |
Sense EStop in |
3 | 4 | in sense relay voltage (e.g. for failure detection) |
Off-Button in |
5 | 6 | out Relay Trigger Off |
On-Button in |
7 | 8 | out Relay Trigger On |
| GND | 9 | 10 | GND |
| UART RX on Pi | 11 | 12 | UART TX on Pi (3.3V TTL) |
The connector to the On/Off/EStop switches is a 6 pos JST-XH connector, which can be cheaply acquired pre-crimped, as they are also used as battery charge balance connectors.
The connectors to the RFID reader, and the input to the trigger relay are also JST connectors. The former uses JST-XH (2.5mm pitch), the latter JST-PH (2mm pitch) to avoid wrong wirings.
As trigger relay, simple generic dual relays are used, that are super-cheap on ebay
