Hello everyone,
I am doing a project -Digital Clock- on 68000 Motorolla Microprocessor. I have completed the code, I am left with the circuit, as you can see from the picture, 4 seven segments common anode type are to be used, I want to know what is the reason of putting the transistors there?
I have found two types of transistors in the lab 2N3702 amd 2N4401, which one is the suitable?

thanks ;-)

 

You can use Google to find data sheets. The 3702 is unsuitable, as it is PNP. The 4401 is an NPN switch, and should work.

 

This circuit would work better if the NPNs were swapped for PNPs. Less power would be wasted in the transistors. It would be simple to change the code to invert the pins.

The transistors are needed because if all 7 segments are on then the PORT A pins wouldn't be able to provide enough current. PORT B is fine because each pin only has to power 1 segment.