K8055 problem on the first digital output

Hi, I have bought and assembled a k8055 that seem to works ok with demo software for windows.
I have not tried yet the analog I/O connector but the digital Input (with test button) and digital Output (led) yes.

The problem happens when I connect something like a LED that I would control through the first digital output open collector.

All connections are made as in the k8055 guide:

  • External.Power.9VDC+ on k8055.CLAMP
  • External.Power.9VDC- on k8055.GDN
  • LED+ on k8055.CLAMP
  • LED- on k8055.D1

Then from the software I switch on the first digital output and the LED light on as expected but the same 9V power is also on all other digital outputs!

Instead, if I connect the LED on one output but not the first, the board works as expected and the volts are 0 on all other outputs.

I have already double-checked all solder points and those seem to be ok.

Someone? Hints?


This is very strange problem.
I made a test: There seems to be about 0.9V on the digital outputs when 9V is connected between the CLAMP and GND.
This is normal: The 0.9V is due to the leakage current of the CLAMP diodes in the IC4.
Is there 9V on the other digital outputs if there is no LED between the CLAMP and the first digital output and you turn the first digital output ON?
Is the 9V there on the other digital outputs only when the first digital output is ON but not if OFF?

First I answer your questions and then I’ll give you more info about my story.

Now I tell you about my “apparently” solution.

In order to understand what causes the problem and since the k8055 has two ULN2803, I switch the position of the ULN2803 from IC4 with the ULN2803 from IC2. In this way I get confirm that the ULN2803 that was on IC4 is damaged. In fact, with the right ULN2803 now is all working as expected.

The strange thing is that the damaged ULN2803 that now is on the IC2 socket, is not damaged at 100% because the Input channels are also working as well!

To be sure I have connected a relay board k2633 to k8055 that switches regularly all relay on the k2633 board.

The board now is working and I’m amazed.
The ULN2803 is very sensible component that has strange behaviours when is damaged (looking at my experience and your tests with 9V on CLAMP and GND).
With the k2633 I had problem to switch the second relay but, and again, changing the BC557B from T1 to T2 and BC557B form T2 to T1, the second relay now is working, and the first relay too!
The life is really weird.

Your thought is very important for me, please let me know.

Thank you.


Is it possible that you had IC4 installed 180 degrees turned?

Thanks for your hint but all notches matches correctly: drawing_on_board->socket->ULN2803.


The ULN2803 you first had in the socket IC4 seems to be defective.

Looking at the schematic you could try swapping IC2 with IC4 aince that part of IC2 is not used.

Something to try.

@Wrong Way:

yes. As I told before, that you suggest is I what I did to understand that the ULN2803 were on IC4 is damaged/defective.