Relays are electronically operated switches. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. The relays are generally used to control or switch on/off a high-power circuit using a separate low-power control signal. That is, in the simplest form, a constant voltage applied across the input pins of a relay will toggle it, that is, either close (connect) or open (disconnect) another circuit. The relays which close (connect) a circuit on application of voltage are called normally open relays and the relays which open (connect) a circuit on application of voltage are called normally closed relays. Most modern relay circuits have both a normally open and normally closed connection.

For our purposes, we want our irrigation system to run only when we detect low soil humidity and be off otherwise, so we will connect our irrigation system across the normally open connection of the relay.

We will be using a relay with 5V control circuit voltage and with a switched circuit voltage rating of up to 220V: