Near Intersection of I-35 & I-90 Southern Mn. | There are lots of approaches that could work for this. Some are a bit more complex than others.
Electric ball valves are very common with modern spray systems. Having worked as a tech, I consider them in two general classes.
I will refer to one style as "Polarity Reversing". This style uses 3 wires. When one wire is "+" and the other is "-" the motor will run until an internal limit switch is pushed by a cam on the valve shaft. This breaks the circuit and the motor stops with the ball in the fully open position. If the first wire is "-" and the second wire is "+", the motor runs in the opposite direction and runs until the other internal limit switch is pushed by the cam on the valve shaft. This breaks the circuit and the motor stops with the ball in the fully closed position. This style requires the controller or a manual switch to be able to switch the polarity on the those two wires, thus the name "Polarity Reversing".
The other style which is possibly more common has an internal relay in the motor enclosure to handle the polarity reversing arrangement to run the motor in either direction. That style typically uses 3 wires also. However with this style one wire is constant 12V (always "hot"), one wire is constant ground. The third wire is the trigger wire. Since the internal relay has 12V and ground available at all times, it can run the motor in either direction. If the trigger wire happens to be at 12V from the controller or a hand switch, the internal relay sends the correct polarity to the motor to cause it to open until it stops due to a limit switch. If the trigger wire happens to be floating or ground, the internal relay relaxes and send the opposite polarity to cause the motor to close unit it stops due to the other limit switch.
It is important to understand the above if you are going to invade the original wiring.
Let's suppose your valves are the second type. In order to control them from a different device, the 12V constant power and the ground connections need to be present and can remain as original. All that is really necessary is to provide the trigger wire.
I agree with your wiring of inserting homemade tee's in between he existing connections. This means each valve would get constant 12V and ground as before which is necessary. Either the original controller or you new receiver could provide the trigger to cause the valve to open. In normal operation, the remote receiver would be OFF so the original controller should behave as usual. For testing you would have the usual boom switches OFF so the remote could turn a section valve On. An advantage to using tees is that you do not disturb or confuse the original wiring. You can easily remove your add on system if desired.
One slight area of concern is back feeding. By back feeding, I mean that the remote system is turning a valve ON which back feeds to the sprayer controller.
This might be good, bad or indifferent. If the controller is for example a Raven 440 style with the section switches OFF, back feeding the boom sections with your wireless remote would not cause any damage and might be desirable since you could enter a self test speed and your target rate but leave all sections OFF. You walk around the sprayer and cycle some sections ON/OFF to check them. The 440 will note that some sections are ON and will adjust the flow that if feels is necessary for that target rate, ground speed etc.
Back feeding some other brands of controllers might cause damage. If this is a concern, a diode can be placed in the trigger wire in your Weather Pack Tees. This diode would prevent one system from back feeding the other. A diode is roughly the equivalent of a check valve in that it allows flow in one direction but blocks it in the other direction.
I have seen a very simple approach to your idea where the operator installed a Single Pole Double Throw switch at each valve. This type of switch has 3 terminals. The center terminal is connected to the left terminal when the handle is to the right. The center terminal is connected to the right terminal when the handle is to the left. In normal operation, the switch on each valve would be to the direction connecting the center to the controller. For testing, the operator walks to the valve and flips the switch the other way which is connected to 12V. The valve opens and the operator checks that section and then flips it back the other way. This would be very simple to do and eliminates the need for a remote controller. This could work well but the operator might get wet.
I went with a different approach with my Ag Leader Spray control system. In a nutshell would I did was to create a test configuration that pertained only to testing. This configuration was based on input from normally unused boom inputs. Those inputs were controlled by an ESP32 microcontroller that uses the Wi-Fi connection capabilities on my smartphone.
To use the test arrangement, I would choose the Spray Test Only configuration that I had created on my Ag Leader display. I would turn the spray pump ON. The usual boom switches in the sprayer had no effect since the system wasn't watching them. I would climb down and walk around the sprayer and use the app on my phone to turn ON a section can check for leaks, pattern etc.
When I was satisfied, I would shut off all sections through the phone app and return to the sprayer. I would then choose the normal spray configuration, field target rate etc. and proceed to spray normally. There was no danger that my phone could interfere with normal spraying since those inputs were not consulted.
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Remote spray section tester
