For many CNC projects you may want to drive a relay from a standard 5 volt output, to work a device, such  as a suds pump, or tool changer.

 

The Ardunio range of pre-built circuit boards are ideal. They come in various shapes and sizes, with single or multiple relay combinations. You can find them listed on eBay.

 

As you see from the sample circuit below it includes fully opto isolated inputs, driven by a 5 volt supply. Ideal for connection to Pc's . Quite honestly I don’t know how they are made for the price. Purchasing even a single relay can cost may more times the cost of a dual relay board as shown. I have will be using them to switch the suds pump on and off, in a later project extension.