A sprinkler controller!

technofrog

Hi all, I'm trying to design an irrigation system for my house. I have little experience in working with triacs/AC power in general, so could somebody look at/critique this output board schematic for me? Any suggestions/constructive criticism would be much appreciated. I've based the design on others found in these forum threads:

http://forums.parallax.com/forums/default.aspx?f=21&m=151374&g=152043#m152043
http://forums.parallax.com/forums/default.aspx?f=5&m=6127&g=6250#m6250

My specific questions are:

- How do I size the limiting resistor that comes through the optoisolator and triggers the triac via the 24VAC line?
- Are the smoothing capacitors found in Bullwinkle's design completely necessary?
- Is carrying the 24VAC line through edge connectors a feasable idea? If so, what sort of edge connectors/headers do you reccommend? I have no idea how to even begin to choose this sort of component.
- Does anybody have any advice/links on how to build a power supply that will give me the +5V and 24VAC lines that I need from wall power?

My house has a huge (24 station!) system, so the idea is to make three of these smaller output boards and carry the +5V, 24VAC, and control signals for the shift registers via edge connectors, and control the whole mess from a "motherboard" that has yet to be designed that contains the uController, inputs from the power supply, and COM terminal for the sprinkler valves.

The schematic is attached as an Eagle Lite file and as an image....

Thanks!

Chris Savage

This message is being moved from the Completed Projects Forum to the Sandbox Forum.

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Chris Savage
Parallax Tech Support

T Chap

You could use two of these part numbers at Digikey: 1729144 they are Phoenix 4 connector screw terminals. Your drawing shows an SSR/Triac not an optosiolator and the circuit is a little puzzling. What is the requirement to turn on the sprinkler? If it is 24AC, then why not just run one SSR off one pin from the microprocessor if you have the i/o to spare? If you don't have the i/o then run the SSR straight off the shift register. There is no need to run an optoisolator into an SSR.

I use an SSR I got from All Electronics, it isn't shown on the site, but call them and ask them if they have an SSR that will run from 3 to 32 volts, they'll find one for you. I use them with no resistor and have no problems.

All Elec (818) 997-1806


In Eagle for the Phoenix connector, use the Library part con-ptr500 AK300/4

Post Edited (originator) : 12/27/2006 6:40:27 AM GMT

technofrog

Thanks for the reply/info! I will look into those connectors.

Isn't the MOC3012 an optoisolator? The data sheet says "optocoupler." Is optocoupler != to optoisolator? Sorry I'm kind of a noob at this, im still in the middle of my EE education (:

Anyway, the reason I wanted to use one is so that the digital logic would be protected (I hear the large underground networks of wires that feed the sprinkler valves are suceptible to lightning strikes, etc.) The data sheet for the MOC3012 says that its output is not meant to drive a big load directly; hence the triac.

T Chap

If you use the microprocessor connected to an SSR(or via shrift register), the SSR uses an LED input just the same as an optocoupler. Logic devices don't drive large loads as you have said, but an LED on an SSR is not a large load, usually under 20mA. You can check the rating on the register, but I would imagine it can source an LED. The MOC may be an opto, it just looks like a triac symbol on the output. An optoisolator usually is an LED on the input and a transistor on the output. In any case, your circuit will need a lot of rethinking if you stick with that route. You have to determine if you are going to source(provide 24ac) to the sprinkler on the header pins, where the sprinkler already has it's own GND, or, you are going to sink(provide GND) via the header pins shown. I drew a simple circuit to use the shift register into the SSR, then the sprinkler unit is shown as well for illustration. It assumes there is a common ground or earth ground shared between both systems, although it may work if without common grounding. The LED in SSR the will stop some spikes, but if you have a problem with it then there are devices suited to handle lightening. Just socket the shift register and replace it if needed. I have no experience with lightening protection. A good enough strike will jump across lines anyway, so there is only limited protection with an LED.

technofrog

Do you know the part number/price for that 3-32 volt SSR from Allelectronics? If it's cheaper than the beefy ones, it would definitely be the way to go. I called them and they had no idea what I was talking about.

I looked at the datasheet for the SSR that you have in the schematic; and its internal schematic seems to be the same as my MOC/beefy triac setup that I have in the schematic. The only ones that I could find online are like 5 bucks a pop, so since I have a 26 station sprinkler system, the setup I have would save a lot of cash (if it will work) since I do not absolutely need it to be in a tiny package like that SSR.

If you're really interested (you definitely don't have to be spending your time helping me!), here's the datasheet for the MOC:

http://www.fairchildsemi.com/ds/MO/MOC3012-M.pdf

Check out figure 7 on page 5. It describes how to use its output to drive a big inductive load. Looks like I'm going to have to add some extra components.

I was just planning on sourcing the 24V to the sprinkler unit through the output of the SSR/triac/whatever, and the sprinklers are going to have a common ground on a different board that I am designing.

Now for a really stupid question: If I am using AC current, how do you define VDD and GND, and thus define whether something is being sourced or sunk? For example, on that example circuit from the MOC datasheet, does it really matter whether the load is placed above or below the output triac?

I really appreciate your advice on this!

T Chap

The schematic I drew is where it is supplyng(sourcing) 24ac to the sprinkler which is probably the case for your system.

The part number you have is an opto isolated triac(basically an SSR), so just use it eaxctly as I drew the schematic, use maybe 200ohm - 470ohm on the resistor and you'll be fine with 5 volts signal. The triac will turn on the 24ac to your system. This assumes that your system is OK with floating the input if unused, as many 24v systems are set up.

The load can go anywhere in the path, but as I have drawn it, Vss is the same as GND.

Not knowing the current rating of your sprinkler input, I assumed you have looked at it and have deternmined that the part you want to use will have the capacity to supply the current. If the parts are more than $12.50 and lot of work to build it, then get the SSR from All, I just spoke to them, call them and say 3-32vdc input, 15 amp SSR, they have them at 12.50. They can mount without using a circuit board, just run some wires to it. These are way overkill for your project, you could call digikey.com, ask for a tech and tell him what you want as far as an SSR, they will likely have a cheaper solution.

Post Edited (originator) : 12/27/2006 11:57:56 PM GMT