I am using NPN transistors to select which of the common cathode digits should show the output of the 8 data I/O ports (7 segments + dot).
The problem is that even though the base of only one transistor gets turned on, all the digits show the output of the 8 I/O ports (7 segments information). Heck even if I disconnect the base of all transistors, all the digits still show the data.
I even simulated the circuit using Proteus and the simulation exhibits the same problem.
In the photo the output digit is “1” and it is supposed to be shown on the first digit only. Therefore two data lines get turned on + the line which determines the digit (i.e. connected to the base of the first transistor).
Could someone tell me why all transistors are conducting electricity (enough to turn on all of the digits)?
Oh, by the way this is copied from this page: