I'm making a sprinkler system for my house before summer gets here. So far I'm thinking of an embedded microcontroller with light and temp sensors, so when the sun goes down and it starts to cool your sprinklers turn on for 20 minutes or so. I might also add soil moisture sensors to regulate time to water per day. What other features are available that I may not be aware of?
What advanced features are available for sprinkler systems
lawnsprinkler-system
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My guess would be that there is something wrong with the backflow preventer.
Most likely, the air inlet valve is not sealing properly. When pressurized from the side marked "normal flow" (which is where your tap is), it should open the check valve, and also force the air inlet valve shut.
It's probably a rubber gasket, and might be worn out and need to be replaced, or maybe something is physically holding it open. Try pushing on it to see if you can get it to close all the way. Some silicon grease around the gasket will help it form a good seal and prolong the life of the gasket.
To answer my own question after solving the issue...
If the problem is only at the last two heads and the first ones are fine, then it is definitely a problem occurring after the first heads and unlikely to be valve-related (though not impossible). A broken line is most likely in this case, since your last two heads are dry in the risers and flex lines (it's not the heads merely being clogged). Broken pipes can usually be revealed by looking for muddy soil after running the zone for several minutes, especially if it's a break so bad it is completely stopping your last two heads from receiving any water pressure at all. If the soil has become impacted inside the broken pipe it may be difficult for the water to escape and reveal the break.
One common method to this problem is to increase the water pressure to the break and force the water out. It will not be so much pressure that damage can be done to the PVC joints and heads (as it will only be for a few minutes nor will the PSI be any higher than the output of your main water line or jet pump). This increase in pressure can be done by capping off all the sprinkler heads in the problematic zone. This will put more force on the possible break and on the soil that may be clogged in the pipe. After several minutes the leak may reveal itself. I'm sure there are several ways to cap off sprinkler heads, but the most common and simplest method is to buy PVC threaded caps (3/4" or 1/2" depending on your risers) that screw right on to the risers.
Best Answer
Some additional things to consider:
1) You could put a flow detector in the water line that feeds to the input side of the sprinkler valves. This way you could detect when one of the sprinkler valve has failed to work according to plan. I.E. if you detect flow when you expect the valve to be off that means there is a problem.
2) You could add timer / calendar features to your microcontroller so that you can setup a watering schedule. It could very well be that some of your sprinkler zones do not need watering every day. Secondly many times in summer certain localities may impose watering restrictions to certain week days or odd/even schedule according to house number. You would want your controller to be able to comprehend that.
3) A water controller can often be extended to assist in automating the process of keeping a bird bath, fountain or swimming pool properly filled to account for evaporation. Adding some level switch inputs to the microcontroller hardware can make it easy to regulate when the water fill operation should be started and completed. Such filling and monitoring can still be applied to a schedule as in 2) above.
4) You could add in circuitry to detect the current flow going to the sprinkler valves. Detecting no current when you've turned on the valve can indicate that the wires to the valve have become disconnected or broken. Detecting excessive current can indicate that the wiring has been damaged and the conductors going to the valve have become shorted together.
5) Many times home owners will have landscape accent lighting or sidewalk/driveway perimeter lighting. A controller of the type that you are proposing can easily be extended to provide automated turn on/off for such lighting according to a schedule.
6) And if you go so far as to implement lighting control you can extend that to schedule the on/off cycling of other seasonal lighting such as outside Christmas lights.