I've got a rather large project that I've been working on for a few months. I've been testing each component separately, and am now at the point where I'm putting it all together. The thing I'm (re)building is a 35mm motion picture film scanner from the 90s. I was able to re-use most of the components (sensors, stepper motors, sprocket drive, etc). All of the individual pieces work fine, when I test them on their own. However, now I'm having some issues after putting the whole thing together.
My setup involves a PCB Arduino Mega 2560 shield I made. This has 3 25-pin headers, which connect to external DB25 connectors, which in turn go to breakout cables that connect to the sensors and motors. Again, these all check out fine individually. One cable is for motor and relay control, and the other cable is for the sensors.
All of the sensors (proximity sensors and photo switches) require between 5 and 24VDC external power, and I re-used a 12VDC power supply that was in the scanner. In my testing, I used a benchtop power supply, but I've tested this power supply and it works fine. It should be more than powerful enough to handle the dozen or so sensors it needs to power.
The issue I'm having is this: When I connect all of the sensors to the cable, and I connect the external 12V power to the sensors, they all work fine. Each of them powers on, and each reacts accordingly when the sensor is tripped.
HOWEVER, when I plug the DB25 connector into the Arduino, none of them will power on and the power lights on the 12VDC power supply go out. When I disconnect the Arduino from the sensor cable, I get power to the sensors and the power lights on the power supply come back on. So, something is clearly amiss.
The cable bundle consists of 5 shielded cables, with a common DB25 connector. All of the cable shields are spliced together, and the ground from the Arduino connects to these. The ground from the 12VDC power supply also connects to the shields. In some cases, there are grounding wires built into the cables I used (which are tied to the chassis at the sensor). These are also spliced into the Cable Shield/Arduino Ground/Power Supply Ground.
+12V goes to 11 sensors from a single wire spliced into a quick-release connector outside the cable bundle. I've checked that none of the ground/12V wires are shorted inside my cable. I've also tested that none of the wires that carry signals from the sensors to the Arduino are crossed, and that none of them are short circuited to ground or +12V.
A bank of 12 pins on the arduino (on my PCB) have 4.7K pullup resistors, and use the +5v out of the Arduino for power. This mirrors the setup I had on my breadboard, which worked fine. However, the breadboard didn't more than 4 sensors going at once, and I'm wondering if all those resistors hanging off of the 5V out on the Arduino are causing problems.
Any ideas?
Thanks!
