RS232 interface is already considered obsolete but it is still very popular and commonly used. It is no big deal to convert serial interface in to USB with special cables or simply by integrating USB chip in to schematic. Mark have designed handy relay board based on PIC microcontroller. It handles four relays that can be controlled from PC via serial cable or manually with push buttons.
Relay board has it own power supply providing 5V for microcontroller and 14V to drive relays. Four relays are capable of switching 10A loads. Each relay is isolated from MCU using opto couplers to ensure save operation of PIC during switching. Each relay state is indicated with LED. Mark shares his C# based program to access board from PC. Controller can be easily adapted for many purposes like gate control with RF receiver or other devices that you may want to control remotely.