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I have a countdown timer and want to drive a large 4 digit display. I tried using a NPN transistor for the display driver but I have to put the load from positive to the emitter. I want to put the load from the digit collector to a segment selector (NPN transistor). This does not work and I can't figure out why. I will send a pdf of the drawing if you email me at
By n1ist
In general, you would use a PNP (or P-channel FET) for your anode drivers, and an NPN (or N-channel FET) for your cathode drivers. I did one design using a UDN2981 for the anodes, and ULN2803 for the cathodes. Depending on your display, you may want to use a TLC5916 constant current sink for the cathodes instead.

As for the schematic, just post it as an attachment.