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Near Intersection of I-35 & I-90 Southern Mn. | Needing diodes for solenoid type valves does not surprise me. When I first started as a Spray Tech, I ran into this when trying to use TeeJet 144s along with a basic spray monitor. If the two were wired together, the back EMF created due to the collapsing magnetic field when the solenoids were cycled off, cause the display on the spray monitor to go blank. Fortunately they came back to life after a while.
The solution there was to connect the switch box for the solenoids and the spray monitor separately to the vehicle battery. This helped since the back EMF from the collapsing magnetic field from the solenoids was then "absorbed" by the battery and the spray controller was not affected.
The diodes you mention should help reduce the back EMF in the solenoids as the back EMF then just "runs around in circles" and will not back feed the outputs to the Nano. However I would expect some other relays are involved since the Nano would not be able to directly control the solenoid spray valves. Having separate power supplies with a common ground is the usual approach although the back EMF from the solenoids cycling off at the same time would be considerable.
A better solution yet would be to switch to ball valves and eliminate the inevitable other problems with the solenoid valves altogether. I have a very low opinion of solenoid valves although some get along fine with them.
I'm interested in your observations as I fiddle with microcontrollers as a hobby and have even come up with a few actual practical uses. I'm putting the final touches on my Batcher II for mixing batches of spray. I'm using an ESP 32 connected to a Raven flow meter to display the running total gallons for a batch. I have a second ESP 32 connected to a large 4 digit display so I can observe the batch total on either. The ESP 32 in the Batcher II uses ESP NOW to "send" the running total wirelessly to the remote display. The remote display will be somewhat useful but mainly I took it as a challenge to see if I could make it work. The remote display can be thought of as a remote scoreboard of sorts.
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sprayer rate control with some boom valves off, section control
