For the past few days I have been working on my friends irrigation system. Her pump gave out and her coffee and Hawaiian trees are really hurting. We are in a drought again, which is odd because Hilo, the town only a short drive away, is the third wettest town in the U.S. getting something like 130 inches of rain a year.
Hilo is in a rainforest and we being just a mere 45 miles or so away by air, have a volcano and desert between us. Ahh, microclimates!
It appears that since I already have a pump at my farm and was involved with the installation, that I am an "expert" on pumps. Well, I can't spec them out, but hooking one up is not that hard. They get electricity in, water in and put water out. Valves determine whether water flows and so does the addition of water or not. Electricity yes and water no, is in no-no. That part of the truth table is pretty easy.
Depending upon the design, a pump may be manually controlled or controlled by a time or environmental control. We mostly use timers here on the farms. On certain days, certain parts of the farm will get water; controlled by an electronic timer/controller. My farm is laid out in zones with each zone having a valve controlled by the controller. Applying power to a valve opens it and allows water to flow to the trees. Shutting off power will close the valve and stop the water. That master valve can make sure that if any of the other valves malfunction, water does not flow to them. Thus if it is not time to water any zone, there is no water supplied to any of the other valves.
Some people don't use a master valve, instead needing a pump to supply water to the zone valves. That pump acts like a master valve because if the pump doesn't run, there is no water in the system and thus no water at the valves.
We use a pump relay to control whether the pump gets electricity or not. The pump may run on 110 or 220 volts AC while the valves and pump controller generall run on 25 volts AC.
That master valve (if used instead of a pump) may be the same size as the zone valves or it may be larger.
It really is pretty simple and this type of system can be installed by anyone with a bit of electrical knowledge. Actually some of the diagrams and setups I have seen make it easier than programming a VCR :-)
This type of irrigation system is pretty much the same whether used on a massive golf course or at a home for shrubs. You need a supply of water, a controller, maybe a pump andn valves and some drip lines or sprinklers.
The controllers are timers with some other smarts for days of the week and perhaps the ability to change how long they enable a zone based upon the season. They also may have an input to disable watering if it just rained or if water pressure is low. Those options may sound complex but to the controller they just look like an open switch. They might all be put in series (one connected to another) and if any of them open up, the 25 volts to the master valve/pump controller opens and thus no water to the zones (if they are enabled).
You may not feel the need to understand all that but a bit of knowledge about how things in the world operate may help you in the future.
Last night a worker was installing cabinets at the farm I was doing the irrigation. He looked perplexed and I asked if I could help. He was installing am electrical outlet and was unsure which wire went where. I told him on these outlets, the smaller of the two slots (where the plug goes in) is hot and the wider one is ground, thus black here, white there and green to the top screw to ground the outlet via the round holes.
Home electric wiring is a bit like the valves and water flow. You have power at the main panel and a master switch, then a controller (either a timer or a human) that enables flow of electricity to an appliance. What is a common override to shut off electric based upon something outside the system? Getting a high electric bill and forgetting to pay it!
You might also have a light whose switch is on all the time, but in between that and the bulb is a photocell. If it is light out, the photocell is open and no electric is allowed to flow to the light.
In the case of a water sensor to keep an irrigation system from running when it is raining, it may be a simple circuit. The one I have is a little disk that gets wet and holds the water. They get heavier and press down on a switch, which then opens. That open switch tells the controller that it is raining. When the disks dry out, they get lighter and don't press down on the switch.
I am reminded of a circuit I once built to tell if it was raining or not. I took a battery and a buzzer and wired them so that the buzzer was buzzing. I then broke one wire. Touching those two ends together completed the circuit and caused the buzzer to buss again. Now it might be possible to just place those two ends close together at the bottom of the drainpipe and let the rainwater connect the circuit, but I wasn't sure if rainwater was conductive enough. I took two Popsicle sticks together with rubber bands and took two thumbtacks at put them at the ends of each Popsicle stick with a wire to each. The heads of the thumbtacks were touching each other and connecting the wires together and making the buzzer buzz. If I pulled the ends of the Popsicle stick apart, the circuit was opened and the buzzer stopped. The rubber band on that end pulled the two sticks together unless they were held apart. Then I placed a piece of (drum roll) Alka-Seltzer between the two thumbtacks and placed the thing under the downspout. When it rained, water dissolved the Alka-Seltzer tablet, causing the rubber bands to pull the Popsicle sticks together and causing the two thumbtacks to touch, which caused electricity to flow between the battery and buzzer. Note that the circuit only stays off until the first rain and then does not reset itself like the disks do for the irrigation rain sensor.
With electricity it can be the opposite. A small piece of wire in held inside a fuse. If a short or malfunction causes too much current to flow, the wire heats up and burns up, opening the circuit and permanently stopping flow.
So there you have it, a bit of science and theory to start your day. You will not be tested on this (now he tells me).