Relay Modules and Push-Button Switches

Coil voltage, contacts and trigger level on relay boards

A relay board lets a low-power signal switch a separate circuit. Match the coil voltage, 5 V or 12 V, to the supply you have. Count the channels, such as eight, and read the contact type: SPST-NO closes a circuit when energized, while changeover contacts switch between two. The contact rating in volts and amps must exceed the load. Optocoupler isolation separates the controller from the coil side, and high-level or low-level trigger tells you which signal switches a channel on.

Control method, line-voltage safety and push-button cables

Boards for Raspberry Pi and Pico plug onto their headers, while Ethernet modules with Modbus RTU or TCP are controlled over a network and mount on a rail for industrial panels. External PC power buttons connect to the front-panel header on a motherboard; check the cable length, listed in feet or meters. Switching line voltage needs correctly rated contacts, insulation and an enclosure, so ask a qualified person if you are unsure.

Questions about Relay Modules and Push-Button Switches

What does SPST-NO mean on a relay?

It stands for single pole, single throw, normally open. The contact stays open, with no connection, until the coil is energized, then it closes. Normally closed contacts work the opposite way, and changeover (SPDT) contacts switch between two circuits.

Can a Raspberry Pi drive a relay board directly?

Only if the board is made for it or accepts a 3.3 V logic signal at low current. Many boards use an optocoupler input or a transistor stage. Check the trigger level and power the coils from a suitable separate supply if the listing advises it.

What is optocoupler isolation for?

It uses light to pass the control signal across a gap, so the controller's circuit is electrically separated from the relay coil side. That protects the microcontroller from voltage spikes and ground problems.