I've a problem with this circuit, it should drive 2 (or more) big 7-segment display using a MAX7219 and the relative Arduino's library. It uses ULN2003 and UDN2981 to use +12V and a series of 5 led per segment (in the image I simplify with only one led for segment).
The CD40106 is used as logic NOT adapting the inpupts for the ULN2003.
The problem is during the MAX7219 test example code, loaded into the arduino uno. The leds slightly turn on when they should stay off. A video will show better the problem.
I'm not very skilled, where I do exactly look for?
I've tested the circuit without the MAX7219, setting manually the input pins of CD40106 and UDN2981, and all works fine. I tried another MAX7219 but same problem.
CrossRoads:
Try putting a 10K pullup to 12V on the ULN2803 outputs to help bring the cathodes high.
I put 10K pullup resistors to 12V on the two outputs of the ULN2003, the result is visible in this video, when I connect and disconnect them to show the difference.
A user from another forum told me to try with a diode after the UDN2981, or a 1K pulldown resistor after the MAX7219 digits output, but they can't solve the problem. =(