"Recent advancements in programmable controller development have led to the creation of a simple yet powerful device with two potential-free binary inputs and two potential-free binary outputs. To bring this innovative project to life, the developer chose to use an ESP8266-based design, which was manufactured through online PCBA services.

In designing the PCB, several key considerations were taken into account. These include using a DC-DC converter to ensure reliable relay operation, implementing bidirectional optocouplers for input isolation, and reserving space for a SMA antenna terminal for users to connect external antennas.

Additionally, the developer chose to use a housing sold in the market, which was measured and sketched out in Autocad before importing the design into Protel. This ensures that the PCB fits perfectly within the limited space of the housing.

The controller's ability to be mounted in control cabinets also influenced its design, as it does not support on-board antennas or ceramic antennas due to size constraints. Instead, an external IPEX antenna was used and attached to the housing, with a SMA antenna terminal reserved for users to connect external antennas.

Finally, a push button is included to allow users to restore factory settings in case they forget their Wi-Fi password.

Source: https://dev.to/zhangzhuyue/hardware-and-pcb-design-considerations-for-esp8266-based-programmable-controller-5ffo