An SPST relay is a type of switch that has two contact terminals, one for each "throw." When a load is not present, the relay has an open circuit, called the NO state. When a current is applied to the coil, it switches the armature, which closes the contact. It is also known as a "break-before-make" relay.

The input section of an SPST relay consists of a coil and two pins. The first pin is connected to the power supply, while the other pin connects to the device's output. The output device is normally a LED. If the relay receives a 5V input, the LED will light up. If there is no power, the LED will remain off.

Solid-state relays have the advantage of switching low and medium-current signals. They are suitable for use in automation. They are also useful in automated failure-injection tests. In addition to their low amperage draw, they don't require a minimum load current or power supply. These relays also have driver circuits for Logic-Level signals.

An SPST relay has two types of contact: normally-open (NO) and normally-closed (NC). The NO contact is closed when the relay is not in use. The NC contact is a "break" contact. The NC contact remains closed until the switch or button is fully disengaged.