I've thought a lot about this myself. I deal with this stuff day in and day out at work and it would be ideal for managing a tank. Some of the older Allen-Bradley PLCs can be had on eBay for pretty cheap. It's cheap though like this hobby--still too much.
If you do a search for PLC on RC you'll see that others are using them too. I've not had time to look into the luxury of this fun project
You could monitor your temp, pH, Salinity, w/ some analog inputs and use digital outputs for your lights, heaters, pumps. Still would need some interposing relays though. Floats for the inputs, etc. Logging is also an option. Personally, I would love to see some trends of tank temperature and ambient temperature. Maybe look at heater run time, and furance (or A/C)run time. too.
For the most part PLCs are rock solid these days when you get the programming right. That's the trick. Well, you'd need to keep the "marine environment" out of them too. trick. Some people just do not trust them over something hardwired though. Seems to depend on the industry.
Things you may want to consider when buying the PLC:
How much can you expand it? Some of those Japanese "brick" PLCs are pretty slick and cheap, but they can't be expanded too much. You'll probably want at least few analog inputs, and a dozen or so digital ins and outs.
How much current can it source for outputs? Do you need relays to drive your loads? LED Moon Lights, probably not. Small dosing pump, probably not, your MH, probably.
How're you going to program it? Serial, Ethernet, something else? Ethernet is very nice, but you may pay more. Do you need any fancy software?
Where are you going to mount it? Light I mentioned, the marine environment isn't very friendly. Any type of enclosure, though this may be getting overkill, is going to cost more.
Those that have access to surplus industrial control equipment are an ideal source of used components.
FormanBob: Who do you work for?
Buckeye: The microLogix are not too bad. They're Logix500 though. All the rage is the newer Logix5000 systems. They're much more expensive--breeze to work with though. Stay away from FlexLogix as they won't be around for a while though. Get yourself a copy of the MicroLogix Programmable Controllers Selection Guide (AB Publication 1761-SG001A-EN-P). It's available in PDF. It'll give you a run down of what you'll need and want.
Nekomi: PIC would be a second choice, but I think the PLC is preferred because that's exactly what its designed for. It's more robust and probably requires less work out of the box.
Jason
