I've been gradually building a control box for my home aquaponics system. It's clearly un-necessary, but I'm using it as a test bed to prepare for use in greenhouse and larger scale operations. I also want to measure various things long-term. I'm definitely taking the industrial control approach, but I think it's appropriate given the potentially harsh conditions of a greenhouse, and the potential value/risk of thousands of gallons of water and fish and produce. I definitely think these systems can be designed to be mostly passively safe and reliable (without controls), and there is clearly a breaking point below which an investment like this makes no sense. Nonetheless, I carry on for the fun of it :)

I have much of it up and running, but still have most of the operational logic to develop still. Early photo attached. More as I get things hooked up. One part I haven't completed is hooking up graphical screens on the laptop to the PLC. One big advantage of a PLC is that it can take care of all the critical functions without any attached PC or laptop. Adding the laptop just means I can graphically monitor the system, perform manual functions, and log data to disk. I'll be using an open-source control system called EPICS for that. More later. A parts list for the box follows the photo:

Parts:

• 1 Wiegmann NEMA 3R electrical enclosure - $90
• DIN rails - $10
• 2 Solid State 10A Relays - $40
• 24V power supply $30
• Terminal blocks - $20
• Used Koyo DirectLogic06 PLC from Ebay - $130
• RTD temperature sensor card for PLC - $199
• 4-20mA input card for PLC - $79
• 2 RTD temperature sensors - $100
• 2 float switch sensors - $20
• Furman MP-15 relay-controllable 110V outlet - $100
• Used laptop from Ebay - $125  (PLC programming software free for small programs)
• Relay controllable ASF-1 fish feeder - $79
• 4-20mA pH sensor transmitter - $189
• submersible pH sensor - $96

I didn't get an industrial dissolved oxygen sensor for now (way expensive).

I was going to control all 110V devices directly, but I have the control box plugged into a GFCI outlet, and what I discovered was that fluorescent grow lights consistently trip off GFCI. So I added a non-GFCI powered Furman MP-15 to control the grow lights.

Once I get the box fully wired up, and operating/monitoring everything, I'll move on to graphical screens and long-term archiving.