I finally purchased this 8 channel relay module to control my mixer valves. This was sourced on eBay for less than $10 and is worth every penny. If you were to buy the parts individually and build it yourself it would cost much more.
This module is easily controlled by the Arduino. It only takes 5V and less than 30mA (convenient that the Arduino digital outputs are exactly this) to turn on the relays. The relay can switch up to 30V and 10A DC. Perfect for my 12V valves I have chosen.
There are many different versions of this module that you can purchase that use different types of parts to accomplish the same task. I chose a mechanical relay version with status LED’s mostly because I like to hear the clicking sound when the relays turn on, and the LED’s are nice so you can have a positive indication of when a relay is on or off.
I will be posting a video of me testing the module with my Arduino soon.