We are creating a novelty drink dispenser that controls a set of 8 pumps with a Siemens Simatic Step7 1200 PLC. The decision to use this PLC was based on our need to have a very stable prototype.
Now we are nearing a stage where we are going into mass production, and the PLC feels like overkill. It has many features that we don't use and it costs a lot. We would like to replace this with a simple relay-card.
We have limited experience with making electronics and we will most likely use a professional service for the development and production of this card. However given the importance of this card for our business, I would like to ask some general advice here first.
For example, what are the most important things to think about when selecting a relay for controlling small valves, and what features should we know about in a valve that can make our product better?
Of course, we want them to be able to open/close as many times as possible without failing, and when failing, never failing in an open state. Also if there is any way to detect wear/failure that would be awesome.
For reference, our valves are 24VDC 5W solenoids.