transistors and parallel port

[izua]

i want to drive a few 8-seg displays, from my computer. i know it's evil to do it directly from the parallel port, since i can burn it up.

so, i was thinking of making each segment driveable by a transistor. i don't know what values i should put, for limiting the current through the base, and through the collector-emitter junction.

i'll need 8 transistors to control each segment, and another 4 to give each 8-seg display a ground. by switching which 8-seg is currently grounded quickly enough, i will achieve the effect of making all 4 of the 8-seg displays seem on.

but where do i need to limit the current through these resistors, to protect the parallel port? (the display is already made, each segment trace has a 2k resistor, just to be safe).

also, can this be done this way?
someone told me it can't, since i'll need a definite 1 and 0, (like adding a circuit), otherwise the transistors will be "in air" when not on. but i guess beeing "in air" is not grounding or connecting to v+.

thanks for ideas, suggestions.
izua

 

[dipal_z]

It can be done. check out this page for more information
https://www.epanorama.net/circuits/parallel_output.html

 

[budhy]

i was thinking of making each segment driveable by a transistor.

Use ULN2803, you can drive until 300 mA each segment without any problem. Less component, more realible.