I've been looking around this forum and the Internet, and haven't got a satisfactory answer. For some problems I read the symptoms are similar but the setup is not, so I cannot conclusively say their solution will work on my problem - so forgive me if this sounds familiar.
I'm doing a digital RPM display with two 4-digit seven segment displays, each to be displayed in two separate spaces about 25m apart. The input of the system would be an IR sensor connected to Arduino Uno (let's call this ARD1), on which the main processing code resides. The processed RPM data is then sent to two seven segment displays 25m apart, and ARD1 for all intents and purposes is situated in the middle between them. Each of the seven segment display is controlled by an Arduino Nano (let's call them ARD2 and ARD3) which is programmed to take the data from ARD1 and display it on their respective seven segment display. Communication from ARD1 to ARD2 and ARD3 are achieved via a CAT-6 cable using MAX485 to RS485 module.
During trials, each ARD is powered independently using batteries and powerbanks, and had worked wonderfully.
Here's the problem:
The only available DC supply in the operating environment where this project would be ultimately installed is 110V DC. In order to keep costs down, we decided to use a small DC-DC converter that is capable of stepping down 110V to 12V. This converter would be in the same box as ARD1, and its 12V DC output would be sent out to ARD1, ARD2 and ARD3 in parallel, obviously. For ARD2 and 3, in order to minimise cable and simplify installation, we decided to "piggyback" on the CAT6 cable going to them from within the box that contains ARD1, as the serial data going to ARD2 and ARD3 only utilises two of the six wires inside. At the other end these would branch out of the CAT6 cable with two going into the receiving MAX485 TTL module, and two going to the supply terminals of each ARD. When installation is done and we turned on the 110V DC power, both seven segments controlled by ARD2 and ARD3 start spewing random and changing high value RPM readouts, even though the rotating machinery whose RPM it was supposed to measure and display hasn't started moving yet! When we tested with independent batteries and powerbanks - it worked perfectly.
After reading around, my conjecture is that this is either a ground loop problem (should I connect the negative 110VDC input to to the negative of the 12VDC output?) or noise generated by the DC-DC converter screwing around with the data ARD1 is receiving from the IR sensor. Is a filter needed then if the latter is the case?
Any light thrown on this is greatly appreciated, as as the tree said to the lumberjack, "I'm stumped!".
Experience wise , for all intents and purposes I may be regarded as completely rookie.
Thanks in advance.