2-Wire

I too am very interested in 2-wire system since I have a lot of valves spread over a lot of area and don’t want to run lots of wires or multiple controllers. I have lots of trees that need irrigation, and in practical terms this means one valve per tree. The number of valves add up quickly.

First, a small amount of background on 2-wire:
2-wire is a generic term for systems that employ a single pair of wires to drive all valves in the irrigation system. The two wires carry 60 Hz 24 VAC power. Modulated onto the 24 VAC power is signal that implements a communications protocol designed to talk to a solenoid driver located at each valve station. Some implementations clip a portion of the AC waveform to indicate a digital ‘1’ or ‘0’. For example, clip one positive cycle of the 24 VAC waveform at half amplitude for a ‘1’, clip the negative half of the 24 VAC waveform for a ‘0’. The electronics in the solenoid driver demodulate the data riding on the 24 VAC power and provide switched power to the valve when that particular solenoid driver is addressed by the system. So the solenoid driver extracts a stream of command data and 24 VAC power. Each solenoid driver has to have a unique “station address” to enable each solenoid driver to be uniquely addressed. Some method must be provided to program the station address into the solenoid controller. Commercial systems uses a dedicated (expensive) programmer for this purpose. Typical systems can address hundreds of valves. Commercial systems cost many thousands of dollars.

I think the main problem with commercially available 2-wire systems is the high cost of components. It is ridiculous that a 2-wire valve adapter is more than $60 per valve (and often much more). Looking online, I see 4 valve solenoid driver for $240. That is nuts. The controllers themselves are over $1000. The solenoid driver programmer is $600.

Seems to me that if OS used an existing 2-wire protocol that would work BUT adapting an exiting serial protocol to OS plus a simple/cheap design for the valve solenoid driver would be straightforward – and avoid potential patent infringement For example:

1. Add a serial-to-modulated power interface to the OS controller hardware. Use whatever serial protocol makes sense – async serial as used in the RS-232 standard would work fine. Maybe this could simply be a serial-to-2-wire converter that could be added to the OD hardware as an option.
2. Design a solenoid valve driver. Analog electronics to demodulate the signal from the AC carrier on the 2-wire cable that runs to all valve drivers plus digital electronics to interpret valve commands. A small/cheap microcontroller with some non-volatile, reprogrammable memory plus a few analog components. Make it small, cheap, easy to assemble. Include LEDs for: 2-wire power detected, command detected, command-to-me detected, valve on/off. Users could then assemble and pot the board in a small plastic cup (or medicine bottle, or whatever). Low cost at the solenoid is key, i.e. low cost-per-valve-station.
3. Need to engineer the system for lighting protection. A 2-wire system could span many thousands of feet.
4. Add support to OS firmware.

Each valve driver would need to be programmed with a unique valve address. I think a good way to do this is to use a custom command for the driver that sets the address and have the OS firmware include a programming mode in which only a single solenoid driver is connected to the 2-wire interface. Also include a test function to check that the address was set.

I’d love to see this feature as it really makes sense for large systems spread across a lot of area.

Just found a low cost powerline model chip (TDA5051A) which looks pretty good. Now need to see how many can sit on a single 2-wire line.
perhaps with his chip plus cheap microcontroller and power driver and we’re done.

There are many 2 wire systems out there. Not for sprinklers. The problem is energy. You cannot shove enough power over a very very very long 24ga wire to actuate a solenoid even at 24v. The people who want 2 wire want to daisy chain them from valve to valve which also means if anywhere the wire is cut it takes all the solenoids after that out of production.

You want to do 2 wire then use 14 or 12ga romes and bury it. You wanna spend $200 on 200 feet of “sprinkler wire” then?

The only way to get around this is raise the voltage. But above 24v requires a licensed electrician to install.

The commercial systems have the same problem. I am sure they use remote power or power extenders (step up step down transformers) to solve this. They probably per code require licensed electricians to install.

Go ahead and try it. A couple things you might go back to Ray’s page and look at again:

1) Inrush current. Ray mentions this on the page. The solenoid needs a spike of high current to get the slug of iron in the solenoid moving. For a very long run of wire at 18ga it’s not going to be reliable because for the few microseconds that it’s pulling 20A or more the voltage is going to be way too low. Which is why I said I’m pretty sure the commercial systems are using a boost/buck system.

2) Regular cat-5 ethernet is 24ga not 18. Also, the jacket of regular ethernet will not remain intact in water. I’ve seen enough of it pulled out of pipes that had water in them to know what happens to regular PVC ethernet cable jackets. They make underground rated ethernet and it has the same tough PVC case that the 18ga sprinkler wire has. I think you will find underground rated ethernet to be the same cost as sprinkler wire. You are also ignoring that the largest cost in ANY sort of large distributed network is laying the wire. NOBODY in charge of a large system is going to put in an untested system that specs DIFFERENT cabling than all the other existing competitive 2 wire sprinkler systems on the market.

3) The reason sprinkler systems don’t use motorized ball valves is so that if power is lost the sprinkler valve shuts off. Otherwise you have your sprinkler going and there’s a power failure or the controller craps out and then the sprinkler is running all night long and you just lost $200 worth of water and caused a flood. Of course that then means that the valve has to be held open by continuous current. And if you have 5-6 valves actuated at the same time you run out of current carrying capacity on the wire.

4) Sprinkler system plumbing is organized as follows, valves are generally star topology to the water source, zones are daisy chain from the valve. In small home systems the valves are usually placed next to the water source. In large systems the valves are out in the field. Think about what happens to a daisy chain system that is mapped over a star topology – for every valve you are going to be running TWO pairs to it – one from the hub out to the valve and one from the valve back to the hub then out again to the next valve. This is because they are only going to dig 1 trench for the water pipe and the cable for the valve it’s a lot cheaper. That’s why the cable length in theses systems is so long.

You did mention the issue of powering the controller at each valve so I’ll give you that.

You are also ignoring the inherent flaw in a daisy chain config which I mentioned already which is if there’s a break in the system then all the valves after that are dead. If you had any experience in this sort of thing you would have known that this could be solved by creating the daisy chain in a loop with both ends of the loop terminating at the sprinkler controller.

If I was running a golf course for me to send a tech out to replace a failed valve in a sprinkler is 2-3 hours of labor which dwarfs the piddly amount of money I might save on a cheaper node. If I buy your cheaper nodes and they have a higher failure rate my savings will be eaten up.

I am sure that if you were to keep at this after a string of failed hardware behind you and a pile of wire pulled out from underground and a long enough time you would be able to create a running system. But in half the time a lesser schooled guy could have probably reverse engineered the most popular 2 wire commercial sprinkler system on the market and produced a compatible controller that works with all their valves, and that way the greenskeeper can buy all -their- valves and the cheapo controller and been watering his lawn by the time you get something working.

If you absolutely must design your own then why not use the cheapest wire possible – 28ga telephone wire – and run a high frequency on it like 600 Hz AC at 200volts and then you can use tiny toroid core transformers at each valve to drop it down to 20v AC, and it’s also easy to rectify that and get as much current you want to power a controller and a valve. And you can make frequency shifts to carry data to the controller. That system probably will carry the signal a few miles….

I think one obstacle to interface with a 2-wire system is I am not sure if there is any open standard for it — most companies use proprietary systems and they don’t publish any details on the inner workings of their systems. This would either require some reverse engineering (which could be very difficult) or have a way to interface with their controllers, but this sounds more like a DIY / Hack project than an elegant solution.

It’s also possible to develop our own 2-wire system that uses open standard, but it’s likely not worth the cost — each valve adapter probably needs to have a microcontroller and power circuitry to convert 24VAC to 5VDC, as is the cost is going to be quite expensive.

Regarding @tmittelstaedt’s point 3: I did recently come across U.S.Solid’s motorized ball valve, 24V AC/DC 2-wire auto return type — it’s quite interesting, it has a super capacitor inside so when the power is lost, the charge in the super capacitor will be able to drive the motor back to closed position. As a result, it does shut off the water supply when power is lost. It’s completely compatible with any 24VAC sprinkler controller, so an interesting solution to keep in mind if motorized ball valve is required.

@bzublin: very interesting. This is probably the first open design of 2-wire system I’ve ever seen. It’s very intriguing and I will take a closer look at your design and documentation soon. Thanks for sharing!

@bzublin: I read the document, very nice and detailed write-up. One thing that I noticed is you use DC voltage (30V) and PWM to drive 24VAC solenoid. I’ve never thought about it. Is this a known method to drive 24VAC solenoid? It sounds similar to the control circuit for step motors. I find it fascinating because I’ve always disliked the AC-based system — it’s the industry standard for decades and nobody bothers to change anything, but in modern days DC adapters are way cheaper, lighter, and easier to find than AC adapters, due to their demand. So I really hope the manufacturers will move towards more DC-based controller and solenoids. That’s what motivated me to design the DC-powered OpenSprinkler, which does not use PWM, but the underlying design principle is similar to yours, that is, use DC to drive the solenoid, and making sure the current through the solenoid is roughly the holding current. In any case, I am interesting in hearing more about the PWM method, what frequency do you use, how well does it work, any potential issues you are aware of etc. Thanks.

Seems like programming the decoders could be accomplished as follows:
1) Disconnect the 2-wire cable from the controller and attach a single decoder
2) Put the controller in “programming mode” via a command
3) Run a programming cycle:
– Program decoder to specified address using decoder’s programing command protocol
– Query decoder to confirm communication at programmed address
– Report status of programming operation – and perhaps bounce an attached solenoid with a “click code” for user feedback
4) Remove decoder from controller’s 2-wire port
5) Repeat with next decoder until done with programming
6) Reattach 2-wire cable to controller’s 2-wire connector

BTW, a little circuit with a LED (or small alphanumeric display) could be substituted for a solenoid at a specific decoder station to generate diagnostics messages fro end user troubleshooting.

Finally, seems like having a controller that accepts inputs from a commercial timer would not be as useful as a controller with a serial interface and command set that would enable attachment to a computer (raspberry pi or similar). Reducing wiring for a 6 or 12 zone timer versus a 48 zone system spread out over acres.

@bzublin: thanks (and sorry to hear that your post disappeared: I did recently discover our spam filter had false positives so several posts got incorrectly flagged). About PWM control of solenoid: very clever. As I said, it reminded me of motor or stepper controller: it would be interesting to see if we can use a monolithic motor controller to do the job, releasing the microcontroller from having to carry out the sampling and control loop, though using microcontroller does provide the benefit of software-defined current control, which is handy. I am mainly thinking whether I could change the current OpenSprinkler main controller design to use this method as well, but since each controller can handle up to 72 zones, it’s probably not feasible for one microcontroller to perform that much ADC sampling (which also takes a lot of cycle time). What’s why I am thinking whether you can delegate the job to a monolithic motor controller. In contrast, the nice thing with your 2-wire decoder design is that each decoder has its own dedicated microcontroller, so it doesn’t require one central mcu to handle all valves.

Regarding setting addresses: one method is as you said, turn on each decoder one by one. The main controller can be set in ‘discovery’ mode and continuously send out probing signals, any decoder that has already received a permanent address would not respond, while a newly turned on decoder will respond and obtain the current address, then the main controller increments the address and the user turns on the next decoder. This way the user can simply turn on each decoder one by one, and they will get sequential addresses that way.

Another method is by using the ‘old-school’ dip switch: a 8-bit dip switch can set up to 256 unique addresses. This way there is no need for the controller to discover decoders, however the burden is on the user to make sure each decoder gets a unique index.

About ‘regulated low voltage (5V or 3.3V) from the 30V’ — I have plenty of experience with using switch regulator for this. You are correct that linear regulator is not efficient (unless if we are talking about very low current draw). I’ve seen commercial sprinkler controllers that simply use a zener diode based linear regulator, which really only works if the circuit draws less than 10mA, otherwise too much power will be wasted on the current limiting resistor. Using switching regulator, the old school MC34063 is super cheap and can supply up to 500mA, though it’s relatively low frequency and noisy (under small load you may hear inductor humming). The current OS 3.x design uses XL1509, which is also a low-cost chip that can supply up to 1.8A.

Regarding surge protection, the typical approach is TVS diode and/or MOV (sometimes in conjunction with each other). I’ve taken apart some motorized ball valves which have built-in circuit, and those are pretty much what they use for protection.

Just because I have stopped posting here does not mean that have stopped development. I have a day job as an engineer and lots of other tings going on as well, so time is at a premium and it’s always a question of priorities.

At this point I have a design done (RPI Hat). I’d encourage anyone who thinks that this kind of things is not possible and practical to go and do the engineering.