OK, so my problem is the following: I have a commercial box with mechanical DPDT-switches for controlling valves by changing the polarity of 2 wires per valve. The box has 7 relevant switches, which I need to control further down the line with an Arduino. This Arduino gets signals from a separate computer over serial which switch should be in the on or off Position. When the Arduino is powered off the mechanical switches in the commercial box should function as without anything in between. Ideally the mechanical switches should overrule the Arduino in the off position. So the perfect states are as follows (-1 indicates no power):
I came up with the following schematic using SPDT-Relays, if you know of DPDT-Relays that work nicely with arduinos and I could order off amazon.de, that would make things a lot easier. Problem No. 1 with this approach is that the mechanical 0 signal does not overrule the arduino. I guess I would have to read the signal into the arduino and make an if case for switching the relay? Problem No. 2 is that I think the arduino can't handle the 14 Relays (I have only an 8-channel at the moment) Based on the specification I found in an amazon description it needs 20mA for every relay, with 14 coming to a total draw of 280mA which is above the 200mA rating for the output pins of an arduino I found online. I'm guessing a possible solution could be using transistors to switch the relays?
One other idea I had would be using NOT-Gates with NPN-Transistors, since the maximum power draw of the valves seems to be 500mA at 12V (fuse in the commercial box is 500mA, the real current is probably lower). This would require 14 NPN-transistors if I'm not mistaken, which I have already. There is also probably an easy way to use some logic gates to get the "overruling" of the mechanical switches.
As a quick background on why I want to do this: The equipment I want to control is an agricultural sprayer. Ours has 7 zones which can be turned on and off with mechanical switches. Earlier this year we built a diy-solution for autosteering the tractor (Cerea) and now want to control the sprayer at the same time. The arduino receives signals over the serial connection from the computer running the autosteering program. The signals determine which zones should be turned on, based on where the sprayer has already been. The manual override of individual zones is helpful at the sides of the field which the autosteering software doesn't know about.
If you have any ideas how to implement this let me know!