I'm building a project that uses 5 Arduino uno boards. 4 of them control and monitor 4 electrolysis devices, while the 5th board controls and monitors some of the infrastructure such as pressure transducers and fans. The pressure transducers are 2 wire, 4-20 ma devices powered by 24v DC. One such transducer is distributed to all 5 UNOs. First by converting the 4-20 ma signal to a 1 to 5 v through a 250 ohm resistor, then the 1-5 v signal fans to (distributed to) all 5 boards, 4 are brought to A3, and one to A1 (See attached print).
This project is powered by 120vAC. The neutrals and grounds are separated from each, but each brought to its own common point. There are 4, 15 Volt power supplies that power the 4 electrolysis cells and 1, 24v supply powers various transducers and fans. The UNO boards are powered buy wall warts (AC to DC plug-in modules).
Now here is the part that I do not understand. While testing the shared transducer signal on one board the other boards were not plugged in. When I applied a test signal to one UNO all 5 UNOs powered up!
If anyone can shed some light on this I would appreciate the info. I don't have a real schematic of the UNO but it must be that the presence of a voltage on an analog input is somehow able to energize the VCC on the entire board. Could that be right? Wouldn't that mean significant "cross talk" or some other kind of interference or confusion of signals? Best regards Gene