What I always do:
Usually I work with groups of 8 inputs.
At the terminals where the signals enter the board I put resistor packs dil16 8x4K7 in series with the signal. The other side of the pack I connect to a 4K7 sil9 resistorpack with the common to ground. At the junctions I put a zener of 4V7 to ground for each signal and the I feed all (0..4V7)levels into a ULN2804 or ULN2803 whatever available at the moment.
ULN280x is a 8-(darl)transistor array, very cheap and ideal for switching.
Has a lay-out like:
pin1=in -> pin18=out
pin2=in -> pin17=out
etc.
Commons are ground at pin9 and power at pin10 for built in fly-back diodes.
Capable of switching 500ma per output 1,5A max(total) and 60V. Ideal for driving relais.