Switching Mains Loads with Relays

I want to control the external lights around my home from a small computer to provide a bit of automation. The plan is to use an Arduino, and have it switch the lights on and off via relays. There are plenty of low cost relay boards to choose from. Most seem to be clones of the same basic designs which offer either 2, 4 or 8 relays. Here's a typical 4 way board:

4 Way Relay Board

This board has a 2.54mm pitch pin header with 6 pins - ground, +5VDC power and 4 inputs that control the coils of the 4 relays. Each relay is provided with an individual 3 way screw terminal block that is wired to its common, normally open and normally closed contacts. Note that these terminals are big enough for 1-1.5mm solid wire and the relays are rated for up to 10 Amps. My advice is to stay way below this.

The inputs are protected from the "heavy lifting" of energising the relay coils by opto-isolators. Grounding an input pin allows current to flow from the +5VDC supply through a current limiting resistor, a surface mount red LED, and then the LED within the opto-isolator. The forward current is about 2mA in this situation so it's fine to directly connect these inputs to digital I/Os on an Arduino. The relay coils are wired to the +5VDC supply and the opto-isolator phototransistor collectors, and each is provided with a usual flyback diode. (Note that the +5VDC supply might get in the way of using one of these boards with a Raspberry Pi; these use 3.3V digital I/Os and could be damaged by higher voltages.)

So far I've built a box with an 8 way relay board and connected it to 8 external lamps. A small AC-DC module provides +5VDC locally; I will locate the control system elsewhere, with the link to the relay box containing just relay control lines and ground in a multi-core cable.

8 Way Relay Board - Driving Lights

Working on electrical installations can be dangerous and is heavily regulated by law. Consult a qualified electrician!