I have designed two controllers with the following features:
- It uses an Arduino Nano as controller, which I plug in on the board on female headers.
- It has an onboard 220VAC to 24VAC transformer as I drive external devices which needs 24VAC power.
- I use a common LM2596HVS power supply to supply my board with 12V
- This 12V is used to drive the on board Relays via a ULN2003A driver.
- Both controllers have water level probes, the one has 4 channels and the other 6.
The boards are to a large extend very identical and most pins on the two boards has the same function.
The 4-channel boards performs perfect and reliable.
However, the 6-channel board was immediately extremely unreliable at power-up and freezes so much that it does not even start the sketch in the Setup void of the Arduino. I have checked that via serial monitoring as well as setting outputs to high in this block.
To make it run, it is a game of luck flipping the power switch off and on but once it runs, there are no problems at all.
So this morning after a night of no sleep about the problem, I decided to try one more thing:
What I did was to connect ONLY the Vin and ground pins of the Arduino to the header and it started every single time at power up.
Right, so I took a Nano and soldered only a header rail on the side with the Vin and all the AI's. Board started up perfectly every time.
Starting at the at other side, I then soldered only pins D0, D1 GND, Reset and D4 as those pins are not used. Unit started up perfectly every time.
So I started from the other end and solered pins D9, D10, D11 and D12. Unit started up perfectly every time.
I connected D2 and D3. Perfect.
And then came the luck. As soon as I connected D5, the board was 100% back to freezing at startup. I soldered the rest of the rails and took a brand new Nano and went through the same procedure. Once again, everything worked perfect until I soldered in D5.
So I took that Nano, cut out the D5 pin and it worked perfect.
What I find so strange, is that D% runs with a short track to a ULN2003A Input and the trouble is there just because the pin is connected. I removed the ULN2003A, same trouble. As soon as I plug in the Nano with the removed D5 pin, it works, plug in the full one and the trouble is there.
I even removed the 10K bussed resistor on the input of the ULN2003A as that is the only other path for the D5 pin to anything - in this case ground. It behaves exactly the same.
To put further pains into the mystery, I have rebuilt this PCB twice on a brand new blank and it behaves exactly the same. Yet the 4-channel boards runs super sweet.
The troublesome board with the D5 track in red.
And the working board with the same D5 track in red.
Hi Andy, Could you help this?
@Andyfierman