Sounds like you need some EE courses with control theory ;>) Since you are controlling a greenhouse with a considerable amount of wiring, it sounds like your relays and A/D will be far from your main controller (BBB). Light to medium duty mechanical relays need 12VDC and a relay driver, heavy duty relays require 110VAC. If the controlled devices are far (more than a few feet) you have to worry about voltage drops across the wiring, especially if the control signals are bridging the distance (ie the relay is near the controlled device). Some sort of distributed control might be in order. By that I mean an Arduino like micro (there are a couple of small compatible boards out there) controlling one or several relays on command from the BBB and reporting status back, over serial (RS485 is multi-drop) or wireless like Zigbee. Relays are open loop type devices, there is no built-in feedback to know they have closed, usually one just assumes they closed within the time they specify. An interrupt on closing would have to be an added circuit, do you really need it? Hmm, you don't say if you are using mechanical or Solid State relays. If there are Capes with relays, I would expect they would have an external power input. Make sure to pay attention to the power required to size your supply.
www.controlanything.com has been making relay control boards for years, starting with RS232 and migrating to USB, Zigbee, and now ethernet/wifi. They may not be what you want, but they have some good application and usage notes. If you are controlling heavy loads you have to worry about noise feedback. I would start by making a block diagram of the setup, then listing all the devices controlled and monitored and their requirements (current, voltage, distance from the main controller, etc). Don't forget about enclosures. Greenhouses are pretty humid, damp and electronics does NOT like moisture. It sounds like you have an interesting and challenging project. I hope this helps, Jonathan On Saturday, September 6, 2014 7:33:29 PM UTC-4, [email protected] wrote: > > The original question was one of hardware. I can figure out the software > based on what I know and feel comfortable with. My question is: how do I > control roughly 20 relays, some that I need to set and some that I need to > 'read', ideally as an interrupt when they close? I also need to work with a > couple of A/D inputs, mainly temperature. From the documentation that I > have seen, no cape can support that many relays, so I need multiple capes. > How do I do that? Can they be stacked? Can I use I2C to select the address > to 'write' to in order to energise the relay coil? How do I organize them > to allow I2C to select an address through multiple capes? How do I get the > power to drive a relay? TTL logic can't do that. These are my fundamental > questions. Where can I go to get the answers? I really want to learn rather > than be handed answers. I can deal with the software issues well enough, it > is the hardware decisions that are stumping me. > > On Wednesday, September 3, 2014 10:24:01 PM UTC-4, [email protected] > wrote: >> >> I have a significant project that I want to accomplish this fall/winter. >> I would like to build a digital controller for my greenhouse. I have been a >> software engineer for 35 years so the programming will be easy. I don't >> have any experience with microprocessors and need to learn so that I can >> do. What introductory and intermediate sources of information would people >> recommend? I am thinking about a BBB running Ubuntu but am open to >> suggestions. >> >> Thank you, >> Chuck Crisler >> > -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
