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I am working on a single-digit seven-segment module. The module can take input as BCD when the ULN2803 connected to the CD4511 is grounded via the N-channel MOSFET. The other MOSFET is off in that case. Or it can toggle each segment individually by taking direct input at each pin of the second ULN2803. The BCD mode is working correctly. However, in the other mode, all the segments light up faintly, even when the ULN2803 input is connected to the ground. I assumed that the ULN2803 was bad and switched the positions of the ULN2803s. The one I thought was bad worked fine with the CD4511, while the other switched on the segments faintly as happened before. I am sure that the ULN2803 is wired correctly. I can't figure out the reason behind this weird behavior. What could it be? Circuit schematics

Hesham Hany
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    Switching the 0V causes leakage via the inputs. It won’t work as you’d like. – Kartman May 19 '23 at 23:00
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    You would probably have much more success using a pair of 74xx244 buffers to drive a single ULN2803. – brhans May 19 '23 at 23:33
  • I totally agree, but I can't do that now. I have to make some minor modifications to the PCB I have at hand to make it work. @brhans – Hesham Hany May 20 '23 at 00:10
  • I will try to fix the module by connecting the blanking test pin to the vcc in the manual input mode, this way the output of the first ULN will be high impedance, I might also connect it to the ground directly, since the mosfet will be obsolete when I use the blanking input. I think the second mosfet will be needed(although it won't make the Uln2803a function correctly as said in the previous comments). @glen_geek – Hesham Hany May 20 '23 at 00:22

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