The WMRC204 with USB relay card is a 4-channel relay card designed and manufactured by Velleman. This card is powered by 5VDC and is designed for controlling devices like computer peripherals, lights, motors, and other electrical equipment from a computer. It provides opto-isolated inputs and reed relay outputs and allows users to control up to four different devices or functions at the same time. Its USB interface makes it compatible with a variety of operating systems, including Windows, Linux, Mac OS, and Android. The card is equipped with several status-indicating LEDs, which display the operating status of the relays. The 4 independent channels can be interconnected to make up a circuit to switch devices on or off. The use of reed relays also ensures reliable, safe operation and makes them less susceptible to noise interference.